summary of photosynthesis in 3 steps

Microbe Notes

Photosynthesis: Equation, Steps, Process, Diagram

Photosynthesis is defined as the process, utilized by green plants and photosynthetic bacteria, where electromagnetic radiation is converted into chemical energy and uses light energy to convert carbon dioxide and water into carbohydrates and oxygen.

• The carbohydrates formed from photosynthesis provide not only the necessary energy form the energy transfer within ecosystems, but also the carbon molecules to make a wide array of biomolecules.
• Photosynthesis is a light-driven oxidation-reduction reaction where the energy from the light is used to oxidize water, releasing oxygen gas and hydrogen ions, followed by the transfer of electrons to carbon dioxide, reducing it to organic molecules.
• Photosynthetic organisms are called autotrophs because they can synthesize chemical fuels such as glucose from carbon dioxide and water by utilizing sunlight as an energy source.
• Other organisms that obtain energy from other organisms also ultimately depend on autotrophs for energy.
• One of the essential requirements for photosynthesis is the green pigment ‘chlorophyll’ which is present in the chloroplasts of green plants and some bacteria.
• The pigment is essential for ‘capturing’ sunlight which then drives the overall process of photosynthesis.

Table of Contents

Interesting Science Videos

Photosynthesis equations/reactions/formula

• The process of photosynthesis differs in green plants and sulfur bacteria.
• In plants, water is utilized along with carbon dioxide to release glucose and oxygen molecules.
• In the case of sulfur bacteria, hydrogen sulfide is utilized along with carbon dioxide to release carbohydrates, sulfur, and water molecules.

Oxygenic Photosynthesis

The overall reaction of photosynthesis in plants is as follows:

Carbon dioxide + Water  + solar energy → Glucose + Oxygen

6CO 2 + 6H 2 O  +  solar energy   →   C 6 H 12 O 6 + 6O 2

Carbon dioxide + Water  + solar energy → Glucose + Oxygen + Water

6CO 2 + 12H 2 O+ solar energy    →    C 6 H 12 O 6 + 6O 2 + 6H 2 O

Anoxygenic Photosynthesis

The overall reaction of photosynthesis in sulfur bacteria is as follows:

CO 2 + 2H 2 S + light energy   →    (CH 2 O)  + H 2 O  + 2S

Video Animation: Photosynthesis (Crash Course)

Photosynthetic pigments

• Photosynthetic pigments are the molecules involved in absorbing electromagnetic radiation, transferring the energy of the absorbed photons to the reaction center, resulting in photochemical reactions in the organisms capable of photosynthesis.
• The molecules of photosynthetic pigments are quite ubiquitous and are always composed of chlorophylls and carotenoids.
• In addition to chlorophyll, photosynthetic systems also contain another pigment, pheophytin (bacteriopheophytin in bacteria), which plays a crucial role in the transfer of electrons in photosynthetic systems.
• Moreover, other pigments can be found in particular photosynthetic systems, such as xanthophylls in plants.

Image Source: Simply Science .

Chlorophyll

• Chlorophyll is the pigment molecule, which is the principal photoreceptor in the chloroplasts of most green plants.
• Chlorophylls consist of a porphyrin ring, which is bounded to an ion Mg 2+ , attached to a phytol chain.
• Chlorophylls are very effective photoreceptors because they contain networks of alternating single and double bonds.
• In chlorophyll, the electrons are not localized to a particular atomic nucleus and consequently can more readily absorb light energy.
• In addition, chlorophylls also have solid absorption bands in the visible region of the spectrum.
• Chlorophylls are found either in the cytoplasmic membranes of photosynthetic bacteria, or thylakoid membranes inside plant chloroplasts.

Bacteriorhodopsin

• Bacteriorhodopsin is another class of photosynthetic pigment that exists only in halobacteria.
• It is composed of a protein attached to a retinal prosthetic group.
• This pigment is responsible for the absorption of light photons, leading to a conformational change in the protein, which results in the expulsion of the protons from the cell.

Phycobilins

• Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin as their light-harvesting pigments.
• These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or central Mg 2+ .
• Phycobilins are covalently linked to specific binding proteins, forming phycobiliproteins, which associate in highly ordered complexes called phycobilisomes that constitute the primary light-harvesting structures in these microorganisms.

Carotenoids

• In addition to chlorophylls, thylakoid membranes contain secondary light-absorbing pigments, or accessory pigments, called carotenoids.
• Carotenoids may be yellow, red, or purple. The most important are β -carotene, which is a red-orange isoprenoid, and the yellow carotenoid lutein.
• The carotenoid pigments absorb light at wavelengths not absorbed by the chlorophylls and thus are supplementary light receptors.

Factors affecting photosynthesis

Blackman formulated the Law of limiting factors while studying the factors affecting the rate of photosynthesis. This Law states that the rate of a physiological process will be limited by the factor which is in the shortest supply. In the same way, the rate of photosynthesis is also affected by a number of factors, which are namely;

• As the intensity of light increases, the rate of light-dependent reactions of photosynthesis and in turn, the rate of photosynthesis increases.
• With increased light intensity, the number of photons falling on a leaf also increases. As a result, more chlorophyll molecules are ionized, and more ATPs and NADH are generated.
• After a point, however, the rate of photosynthesis remains constant as the light intensity increases. At this point, photosynthesis is limited by some other factors.
• Besides, the wavelength of light also affects the rate of photosynthesis.
• Different photosynthetic systems absorb light energy more effectively at different wavelengths.

Carbon dioxide

• An increase in the concentration of carbon dioxide increases the rate at which carbon is incorporated into carbohydrates in the light-independent reactions of photosynthesis.
• Thus, increasing the concentration of carbon dioxide in the atmosphere rapidly increases the rate of photosynthesis up to a point after which it is limited by some other factors.

Temperature

• The light-independent reactions of photosynthesis are affected by changes in temperature as they are catalyzed by enzymes, whereas the light-dependent reactions are not.
• The rate of the reactions increases as the enzymes reach their optimum temperature, after which the rate begins to decrease as the enzymes tend to denature.

Process/ Steps of Photosynthesis

The overall process of photosynthesis can be objectively divided into four steps/ process:

1. Absorption of light

• The first step in photosynthesis is the absorption of light by chlorophylls that are attached to the proteins in the thylakoids of chloroplasts.
• The light energy absorbed is then used to remove electrons from an electron donor like water, forming oxygen.
• The electrons are further transferred to a primary electron acceptor, quinine (Q) which is similar to CoQ in the electron transfer chain.

2. Electron Transfer

• The electrons are now further transferred from the primary electron acceptor through a chain of electron transfer molecules present in the thylakoid membrane to the final electron acceptor, which is usually NADP + .
• As the electrons are transferred through the membrane, protons are pumped out of the membrane, resulting in the proton gradient across the membrane.

3. Generation of ATP

• The movement of protons from the thylakoid lumen to the stroma through the F 0 F 1 complex results in the generation of ATP from ADP and Pi.
• This step is identical to the step of the generation of ATP in the electron transport chain .

4. Carbon Fixation

• The NADP and ATP generated in steps 2 and 3 provide energy, and the electrons drive the process of reducing carbon into six-carbon sugar molecules.
• The first three steps of photosynthesis are directly dependent on light energy and are thus, called light reactions, whereas the reactions in this step are independent of light and thus are termed dark reactions.

Types/ Stages/ Parts of photosynthesis

Figure: Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO 2 . Image Source: OpenStax (Rice University) .

Photosynthesis is divided into two stages based on the utilization of light energy:

1. Light-dependent reactions

• The light-dependent reactions of photosynthesis only take place when the plants/ bacteria are illuminated.
• In the light-dependent reactions, chlorophyll and other pigments of photosynthetic cells absorb light energy and conserve it as ATP and NADPH while simultaneously, evolving O 2 gas.
• In the light-dependent reactions of photosynthesis, the chlorophyll absorbs high energy, short-wavelength light, which excites the electrons present inside the thylakoid membrane.
• The excitation of electrons now initiates the transformation of light energy into chemical energy.
• The light reactions take in two photosystems that are present in the thylakoid of chloroplasts.

Figure: Light-dependent reactions of photosynthesis in the thylakoid membrane of plant cells. Image Source: Wikipedia (Somepics) .

Photosystem II

• Photosystem II is a group of proteins and pigments that work together to absorb light energy and transfer electrons through a chain of molecules until it finally reaches an electron acceptor.
• Photosystem II has a pair of chlorophyll molecules, also known as P680 as the molecules best absorb light of the wavelength 680 nm.
• The P680 donates a pair of electrons after absorbing light energy, resulting in an oxidized form of P680.
• Finally, an enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ion, and oxygen molecules.
• This pair of electrons then are transferred to P680, causing it to return to its initial stage.

Photosystem I

• Photosystem I is a similar complex like photosystem II except for that photosystem I have a pair of chlorophyll molecules known as P700 as they best absorb the wavelength of 700 nm.
• As photosystem I absorb light energy, it also becomes excited and transfers electrons.
• The now oxidized form of P700 then accepts an electron from photosystem II, restring back to its initial stage.
• The electrons from photosystem I are then passed in a series of redox reactions through the protein ferredoxin.
• The electrons finally reach NADP + , reducing them to NADPH.

2 H 2 O + 2 NADP +  + 3 ADP + 3 P i  + light → 2 NADPH + 2 H +  + 3 ATP + O 2

Video Animation: The Light Reactions of Photosynthesis (Ricochet Science)

2. Light independent reactions (Calvin cycle)

Light independent reactions of photosynthesis are anabolic reactions that lead to the formation of a sex-carbon compound, glucose in plants. The reactions in this stage are also termed dark reactions as they are not directly dependent on the light energy but do require the products formed from the light reactions.

Figure: Overview of the Calvin cycle pathway. Image Source: Wikipedia (Mike Jones) .

This stage consists of 3 further steps that lead to carbon fixation/ assimilation.

Step 1: Fixation of CO 2 into 3-phosphoglycerate

• In this step, one CO 2 molecule is covalently attached to the five-carbon compound ribulose 1,5-biphosphate catalyzed by the enzyme ribulose 1,5-biphosphate carboxylase, also called rubisco.
• The attachment results in the formation of an unstable six-carbon compound that is then cleaved to form two molecules of 3-phosphoglycerate.

Step 2: Conversion of 3-phosphoglycerate to glyceraldehydes 3-phosphate

• The 3-phosphoglycerate formed in step 1 is converted to glyceraldehyde 3-phosphate by two separate reactions.
• At first, enzyme 3-phosphoglycerate kinase present in the stroma catalyzes the transfer of a phosphoryl group from ATP to 3-phosphoglycerate, yielding 1,3-bisphosphoglycerate.
• Next, NADPH donates electrons in a reaction catalyzed by the chloroplast-specific isozyme of glyceraldehyde 3-phosphate dehydrogenase, producing glyceraldehyde 3-phosphate and phosphate (Pi).
• Most of the glyceraldehyde 3-phosphate thus produced is used to regenerate ribulose 1,5-bisphosphate.
• The rest of the glyceraldehyde is either converted to starch in the chloroplast and stored for later use or is exported to the cytosol and converted to sucrose for transport to growing regions of the plant.

Step 3: Regeneration of ribulose 1,5-biphosphate from triose phosphates

• The three-carbon compounds formed in the previous steps are then converted into the five-carbon compound, ribulose 1,5-biphosphate through a series of transformations with intermediates of three-, four,-, five-, six-, and seven-carbon sugar.
• As the first molecules in the process, if regenerated, this stage of photosynthesis results in a cycle (Calvin cycle).

3 CO 2 + 9 ATP + 6 NADPH + 6 H +     →     glyceraldehyde-3-phosphate (G3P) + 9 ADP + 8 P i  + 6 NADP +  + 3 H 2 O

A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle to produce one molecule of glucose.

Video Animation: The Calvin Cycle (Ricochet Science)

Products of Photosynthesis

The outcomes of light-dependent reactions of photosynthesis are:

The products of light-independent reactions (Calvin cycle) of photosynthesis are:

• glyceraldehyde-3-phosphate (G3P) / Glucose (carbohydrates)

The overall products of photosynthesis are:

• Glucose (carbohydrates)
• Sulfur (in photosynthetic sulfur bacteria)

Photosynthesis Examples

Photosynthesis in green plants or oxygenic bacteria.

• In plants and oxygenic bacteria like cyanobacteria, photosynthesis takes place in the presence of green pigment, chlorophyll.
• It takes place in the thylakoids of the chloroplasts, resulting in products like oxygen gas, glucose, and water molecules.
• Most of the glucose units in plants are linked to form starch or fructose or even sucrose.

Photosynthesis in sulfur bacteria

• In purple sulfur bacteria, photosynthesis takes place in the presence of hydrogen sulfur rather than water.
• Some of these bacteria like green sulfur bacteria have chlorophyll whereas other purple sulfur bacteria have carotenoids as photosynthetic pigments.
• The result of photosynthesis in these bacteria are carbohydrates (not necessarily glucose), sulfur gas, and water molecules.

Importance of photosynthesis

• Photosynthesis is the primary source of energy in autotrophs where they make their food by utilizing carbon dioxide, sunlight, and photosynthetic pigments.
• Photosynthesis is equally essential for heterotrophs, as they derive their energy from the autotrophs.
• Photosynthesis in plants is necessary to maintain the oxygen levels in the atmosphere.
• Besides, the products of photosynthesis contribute to the carbon cycle occurring in the oceans, land, plants, and animals.
• Similarly, it also helps maintain a symbiotic relationship between plants, animals, and humans.
• Sunlight or solar energy is the primary source of all other forms of energy on earth, which is utilized through the process of photosynthesis.

Artificial photosynthesis

Artificial photosynthesis is a chemical process that mimics the biological process of utilization of sunlight, water and carbon dioxide to produce oxygen and carbohydrates.

Image Source: Phys .

• In artificial photosynthesis, photocatalysts are utilized that are capable of replicating the oxidation-reduction reactions taking place during natural photosynthesis.
• The main function of artificial photosynthesis is to produce solar fuel from sunlight that can be stored and used under conditions, where sunlight is not available.
• As solar fuels are prepared, artificial photosynthesis can be used to produce just oxygen from water and sunlight, resulting in clean energy production.
• The most important part of artificial photosynthesis is the photocatalytic splitting of a water molecule, resulting in oxygen and large quantities of hydrogen gas.
• Further, light-driven carbon reduction can also be performed to replicate the process of natural carbon fixation, resulting in carbohydrates molecules.
• Thus, artificial photosynthesis has applications in the production of solar fuels, photoelectrochemistry, engineering of enzymes, and photoautotrophic microorganisms for the production of microbial biofuel and biohydrogen from sunlight.

Video Animation: Learning from leaves: Going green with artificial photosynthesis

Photosynthesis vs. Cellular respiration

Image Source: Khan Academy .

Video Animation: Photosynthesis vs. Cellular Respiration Comparison (BOGObiology)

FAQs (Revision Questions)

Where does photosynthesis occur? Photosynthesis occurs in the thylakoid membrane of the chloroplasts.

What are the products of photosynthesis? The products of photosynthesis are carbohydrates (glucose), oxygen, and water molecules.

What are the reactants of photosynthesis? The reactants of photosynthesis are carbon dioxide, water, photosynthetic pigments, and sunlight.

How are photosynthesis and cellular respiration related? Photosynthesis and cellular respiration are essentially the reverses of one another where photosynthesis is an anabolic process resulting in the formation of organic molecules. In contrast, cellular respiration is a catabolic process resulting in the breaking down of organic molecules to release energy.

• Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 17.2, Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled.Available from: https://www.ncbi.nlm.nih.gov/books/NBK22347/
• Nelson DL and Cox MM. Lehninger Principles of Biochemistry. Fourth Edition.
• Montero F. (2011) Photosynthetic Pigments. In: Gargaud M. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg
• Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 16.3, Photosynthetic Stages and Light-Absorbing Pigments.Available from: https://www.ncbi.nlm.nih.gov/books/NBK21598/

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How can we say that 6 calvin cycles are needed to produce 1 glucose molecule why not 2?

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• Biology Article

Photosynthesis

Photosynthesis is a process by which phototrophs convert light energy into chemical energy, which is later used to fuel cellular activities. The chemical energy is stored in the form of sugars, which are created from water and carbon dioxide.

Table of Contents

• What is Photosynthesis?
• Site of photosynthesis

What Is Photosynthesis in Biology?

The word “ photosynthesis ” is derived from the Greek words  phōs  (pronounced: “fos”) and σύνθεσις (pronounced: “synthesis “) Phōs means “light” and σύνθεσις   means, “combining together.” This means “ combining together with the help of light .”

Photosynthesis also applies to other organisms besides green plants. These include several prokaryotes such as cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like green plants.The glucose produced during photosynthesis is then used to fuel various cellular activities. The by-product of this physio-chemical process is oxygen.

A visual representation of the photosynthesis reaction

• Photosynthesis is also used by algae to convert solar energy into chemical energy. Oxygen is liberated as a by-product and light is considered as a major factor to complete the process of photosynthesis.
• Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen. Leaves contain microscopic cellular organelles known as chloroplasts.
• Each chloroplast contains a green-coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.
• Another by-product of photosynthesis is sugars such as glucose and fructose.
• These sugars are then sent to the roots, stems, leaves, fruits, flowers and seeds. In other words, these sugars are used by the plants as an energy source, which helps them to grow. These sugar molecules then combine with each other to form more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls.

Where Does This Process Occur?

Chloroplasts are the sites of photosynthesis in plants and blue-green algae.  All green parts of a plant, including the green stems, green leaves,  and sepals – floral parts comprise of chloroplasts – green colour plastids. These cell organelles are present only in plant cells and are located within the mesophyll cells of leaves.

Also Read:  Photosynthesis Early Experiments

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis reaction is considered to be an endothermic reaction. Following is the photosynthesis formula:

Unlike plants, certain bacteria that perform photosynthesis do not produce oxygen as the by-product of photosynthesis. Such bacteria are called anoxygenic photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

Structure Of Chlorophyll

The structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium atom. A hydrocarbon tail is also present. Pictured above is chlorophyll- f,  which is more effective in near-infrared light than chlorophyll- a

Chlorophyll is a green pigment found in the chloroplasts of the  plant cell   and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll- a  and chlorophyll- b .Besides green plants, other organisms that perform photosynthesis contain various other forms of chlorophyll such as chlorophyll- c1 ,  chlorophyll- c2 ,  chlorophyll- d and chlorophyll- f .

Also Read:   Biological Pigments

Process Of Photosynthesis

At the cellular level,  the photosynthesis process takes place in cell organelles called chloroplasts. These organelles contain a green-coloured pigment called chlorophyll, which is responsible for the characteristic green colouration of the leaves.

As already stated, photosynthesis occurs in the leaves and the specialized cell organelles responsible for this process is called the chloroplast. Structurally, a leaf comprises a petiole, epidermis and a lamina. The lamina is used for absorption of sunlight and carbon dioxide during photosynthesis.

Structure of Chloroplast. Note the presence of the thylakoid

“Photosynthesis Steps:”

• During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
• The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose. Furthermore, oxygen is liberated out into the atmosphere through the leaves as a waste product.
• Glucose is a source of food for plants that provide energy for  growth and development , while the rest is stored in the roots, leaves and fruits, for their later use.
• Pigments are other fundamental cellular components of photosynthesis. They are the molecules that impart colour and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b and carotenoids which are present in the thylakoids of chloroplasts. It is primarily used to capture light energy. Chlorophyll-a is the main pigment.

The process of photosynthesis occurs in two stages:

• Light-dependent reaction or light reaction
• Light independent reaction or dark reaction

Stages of Photosynthesis in Plants depicting the two phases – Light reaction and Dark reaction

Light Reaction of Photosynthesis (or) Light-dependent Reaction

• Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts.
• The Grana, membrane-bound sacs like structures present inside the thylakoid functions by gathering light and is called photosystems.
• These photosystems have large complexes of pigment and proteins molecules present within the plant cells, which play the primary role during the process of light reactions of photosynthesis.
• There are two types of photosystems: photosystem I and photosystem II.
• Under the light-dependent reactions, the light energy is converted to ATP and NADPH, which are used in the second phase of photosynthesis.
• During the light reactions, ATP and NADPH are generated by two electron-transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction of photosynthesis can be reduced to:

2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

Dark Reaction of Photosynthesis (or) Light-independent Reaction

• Dark reaction is also called carbon-fixing reaction.
• It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules.
• The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.
• Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin photosynthesis cycle.
• In the Calvin cycle , the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO 2 + 6 NADPH + 5H 2 O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Calvin photosynthesis Cycle (Dark Reaction)

Also Read:  Cyclic And Non-Cyclic Photophosphorylation

Importance of Photosynthesis

• Photosynthesis is essential for the existence of all life on earth. It serves a crucial role in the food chain – the plants create their food using this process, thereby, forming the primary producers.
• Photosynthesis is also responsible for the production of oxygen – which is needed by most organisms for their survival.

Frequently Asked Questions

1. what is photosynthesis explain the process of photosynthesis., 2. what is the significance of photosynthesis, 3. list out the factors influencing photosynthesis., 4. what are the different stages of photosynthesis, 5. what is the calvin cycle, 6. write down the photosynthesis equation..

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Please What Is Meant By 300-400 PPM

PPM stands for Parts-Per-Million. It corresponds to saying that 300 PPM of carbon dioxide indicates that if one million gas molecules are counted, 300 out of them would be carbon dioxide. The remaining nine hundred ninety-nine thousand seven hundred are other gas molecules.

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Photosynthesis

What is photosynthesis.

It is the process by which green plants, algae, and certain bacteria convert light energy from the sun into chemical energy that is used to make glucose. The word ‘photosynthesis’ is derived from the Greek word phōs, meaning ‘light’ and synthesis meaning ‘combining together.’

Jan Ingenhousz, the Dutch-born British physician and scientist, discovered the process of photosynthesis.

Where does Photosynthesis Occur

Photosynthesis takes place mainly in the leaves of green plants and also in the stems of herbaceous plants as they also contain chlorophyll. Sometimes it also occurs in roots that contain chlorophyll like in water chestnut and Heart-leaved moonseed. Apart from plants, photosynthesis is also found to occur in blue-green algae.

What Happens During Photosynthesis

It involves a chemical reaction where water, carbon dioxide, chlorophyll, and solar energy are utilized as raw materials (inputs) to produce glucose, oxygen, and water (outputs).

Stages of the Process

Photosynthesis occurs in two stages:

1) The Light-dependent Reaction

• Takes place in the thylakoid membranes of chloroplasts only during the day in the presence of sunlight
• High-energy phosphate molecules adenosine triphosphate ( ATP ) and the reducing agent NADPH are produced with the help of electron transport chain

2) The Light-independent or Dark Reaction ( Calvin cycle )

• Takes place in the stroma of chloroplast in the absence of light that helps to fix carbon
• ATP and NADPH produced in the light reaction are utilized along with carbon dioxide to produce sugar in the form of glucose

Factors Affecting the Rate of Photosynthesis

• Intensity of Light: The higher intensity of light increases the rate of photosynthesis
• Temperature:  Warmer the temperature, higher the rate of photosynthesis. The rate is highest between the temperatures of 25° to 35° C, after which it starts to decrease
• Concentration of Carbon dioxide: Higher concentration of carbon dioxide increases the rate of photosynthesis until it reaches a certain point, beyond which no further effects are found   

Although all the above factors together interact to affect the rate of photosynthesis, each of them individually is also capable of directly influencing the process without the other factors and thus called limiting factors.

Importance of Photosynthesis

It serves two main purposes that are essential to support life on earth:

• Producing food for organisms that depend on others for their nutrition such as humans along with all other animals
• Synthesizing oxygen by replacing carbon dioxide in the atmosphere

Ans. Photosynthesis is an endothermic reaction because it absorbs the heat of the sun to carry out the process.

Ans. The oxygen in photosynthesis comes from splitting the water molecules.

Ans. Chlorophyll is the main light-absorbing pigment in photosynthesis.

Ans. The role of water is to provide oxygen in the form of oxygen gas to the atmosphere.

Ans. Sunlight is the source of energy that drives photosynthesis.

Ans. The easiest way to measure the rate of photosynthesis is to quantify the carbon dioxide or oxygen levels using a data logger. The rate of photosynthesis can also be measured by determining the increase in the plant ’s biomass (weight).

Ans. Photosynthesis is an energy-requiring process occurring only in green plants, algae, and certain bacteria that utilizes carbon dioxide and water to produce food in the form of carbohydrates. In contrast, cellular respiration is an energy-releasing process found in all living organisms where oxygen and glucose are utilized to produce carbon dioxide and water.

Ans. Glucose produced in photosynthesis is used in cellular respiration to make ATP.

Article was last reviewed on Tuesday, April 21, 2020

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Photosynthesis

Reviewed by: BD Editors

Photosynthesis Definition

Photosynthesis is the biochemical pathway which converts the energy of light into the bonds of glucose molecules. The process of photosynthesis occurs in two steps. In the first step, energy from light is stored in the bonds of adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH). These two energy-storing cofactors are then used in the second step of photosynthesis to produce organic molecules by combining carbon molecules derived from carbon dioxide (CO 2 ). The second step of photosynthesis is known as the Calvin Cycle. These organic molecules can then be used by mitochondria to produce ATP, or they can be combined to form glucose, sucrose, and other carbohydrates. The chemical equation for the entire process can be seen below.

Photosynthesis Equation

Above is the overall reaction for photosynthesis. Using the energy from light and the hydrogens and electrons from water, the plant combines the carbons found in carbon dioxide into more complex molecules. While a 3-carbon molecule is the direct result of photosynthesis, glucose is simply two of these molecules combined and is often represented as the direct result of photosynthesis due to glucose being a foundational molecule in many cellular systems. You will also notice that 6 gaseous oxygen molecules are produced, as a by-produce. The plant can use this oxygen in its mitochondria during oxidative phosphorylation . While some of the oxygen is used for this purpose, a large portion is expelled into the atmosphere and allows us to breathe and undergo our own oxidative phosphorylation, on sugar molecules derived from plants. You will also notice that this equation shows water on both sides. That is because 12 water molecules are split during the light reactions, while 6 new molecules are produced during and after the Calvin cycle. While this is the general equation for the entire process, there are many individual reactions which contribute to this pathway.

Stages of Photosynthesis

The light reactions.

The light reactions happen in the thylakoid membranes of the chloroplasts of plant cells. The thylakoids have densely packed protein and enzyme clusters known as photosystems . There are two of these systems, which work in conjunction with each other to remove electrons and hydrogens from water and transfer them to the cofactors ADP and NADP + . These photosystems were named in the order of which they were discovered, which is opposite of how electrons flow through them. As seen in the image below, electrons excited by light energy flow first through photosystem II (PSII), and then through photosystem I (PSI) as they create NADPH. ATP is created by the protein ATP synthase , which uses the build-up of hydrogen atoms to drive the addition of phosphate groups to ADP.

The entire system works as follows. A photosystem is comprised of various proteins that surround and connect a series of pigment molecules . Pigments are molecules that absorb various photons, allowing their electrons to become excited. Chlorophyll a is the main pigment used in these systems, and collects the final energy transfer before releasing an electron. Photosystem II starts this process of electrons by using the light energy to split a water molecule, which releases the hydrogen while siphoning off the electrons. The electrons are then passed through plastoquinone, an enzyme complex that releases more hydrogens into the thylakoid space . The electrons then flow through a cytochrome complex and plastocyanin to reach photosystem I. These three complexes form an electron transport chain , much like the one seen in mitochondria. Photosystem I then uses these electrons to drive the reduction of NADP + to NADPH. The additional ATP made during the light reactions comes from ATP synthase, which uses the large gradient of hydrogen molecules to drive the formation of ATP.

The Calvin Cycle

With its electron carriers NADPH and ATP all loaded up with electrons, the plant is now ready to create storable energy. This happens during the Calvin Cycle , which is very similar to the citric acid cycle seen in mitochondria. However, the citric acid cycle creates ATP other electron carriers from 3-carbon molecules, while the Calvin cycle produces these products with the use of NADPH and ATP. The cycle has 3 phases, as seen in the graphic below.

During the first phase, a carbon is added to a 5-carbon sugar, creating an unstable 6-carbon sugar. In phase two, this sugar is reduced into two stable 3-carbon sugar molecules. Some of these molecules can be used in other metabolic pathways, and are exported. The rest remain to continue cycling through the Calvin cycle. During the third phase, the five-carbon sugar is regenerated to start the process over again. The Calvin cycle occurs in the stroma of a chloroplast. While not considered part of the Calvin cycle, these products can be used to create a variety of sugars and structural molecules.

Products of Photosynthesis

The direct products of the light reactions and the Calvin cycle are 3-phosphoglycerate and G3P, two different forms of a 3-carbon sugar molecule. Two of these molecules combined equals one glucose molecule, the product seen in the photosynthesis equation. While this is the main food source for plants and animals, these 3-carbon skeletons can be combined into many different forms. A structural form worth note is cellulose , and extremely strong fibrous material made essentially of strings of glucose. Besides sugars and sugar-based molecules, oxygen is the other main product of photosynthesis. Oxygen created from photosynthesis fuels every respiring organism on the planet.

Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology 6th. ed . New York: W.H. Freeman and Company. Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry . New York: W.H. Freeman and Company.

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ENCYCLOPEDIC ENTRY

Photosynthesis.

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

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Learning materials, instructional links.

• Photosynthesis (Google doc)

Most life on Earth depends on photosynthesis .The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O 2 ) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO 2 ) and water (H 2 O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts , which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll , which is responsible for giving the plant its green color. During photosynthesis , chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent Reactions vs. Light-independent Reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light- dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH . The light-independent stage, also known as the Calvin cycle , takes place in the stroma , the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light- independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 Photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water. The National Geographic Society is making this content available under a Creative Commons CC-BY-NC-SA license . The License excludes the National Geographic Logo (meaning the words National Geographic + the Yellow Border Logo) and any images that are included as part of each content piece. For clarity the Logo and images may not be removed, altered, or changed in any way.

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Development of the idea

Overall reaction of photosynthesis.

• Basic products of photosynthesis
• Evolution of the process
• Light intensity and temperature
• Carbon dioxide
• Internal factors
• Energy efficiency of photosynthesis
• Structural features
• Light absorption and energy transfer
• The pathway of electrons
• Evidence of two light reactions
• Photosystems I and II
• Quantum requirements
• The process of photosynthesis: the conversion of light energy to ATP
• Elucidation of the carbon pathway
• Carboxylation
• Isomerization/condensation/dismutation
• Phosphorylation
• Regulation of the cycle
• Products of carbon reduction
• Photorespiration
• Carbon fixation in C 4 plants
• Carbon fixation via crassulacean acid metabolism (CAM)
• Differences in carbon fixation pathways
• The molecular biology of photosynthesis

Why is photosynthesis important?

What is the basic formula for photosynthesis, which organisms can photosynthesize.

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• Khan Academy - Photosynthesis
• Biology LibreTexts - Photosynthesis
• University of Florida - Institute of Food and Agricultural Sciences - Photosynthesis
• Milne Library - Inanimate Life - Photosynthesis
• National Center for Biotechnology Information - Chloroplasts and Photosynthesis
• Roger Williams University Pressbooks - Introduction to Molecular and Cell Biology - Photosynthesis
• BCcampus Open Publishing - Concepts of Biology – 1st Canadian Edition - Overview of Photosynthesis
• photosynthesis - Children's Encyclopedia (Ages 8-11)
• photosynthesis - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

Photosynthesis is critical for the existence of the vast majority of life on Earth. It is the way in which virtually all energy in the biosphere becomes available to living things. As primary producers, photosynthetic organisms form the base of Earth’s food webs and are consumed directly or indirectly by all higher life-forms. Additionally, almost all the oxygen in the atmosphere is due to the process of photosynthesis. If photosynthesis ceased, there would soon be little food or other organic matter on Earth, most organisms would disappear, and Earth’s atmosphere would eventually become nearly devoid of gaseous oxygen.

The process of photosynthesis is commonly written as: 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 . This means that the reactants, six carbon dioxide molecules and six water molecules, are converted by light energy captured by chlorophyll (implied by the arrow) into a sugar molecule and six oxygen molecules, the products. The sugar is used by the organism, and the oxygen is released as a by-product.

The ability to photosynthesize is found in both eukaryotic and prokaryotic organisms. The most well-known examples are plants, as all but a very few parasitic or mycoheterotrophic species contain chlorophyll and produce their own food. Algae are the other dominant group of eukaryotic photosynthetic organisms. All algae, which include massive kelps and microscopic diatoms , are important primary producers.  Cyanobacteria and certain sulfur bacteria are photosynthetic prokaryotes, in whom photosynthesis evolved. No animals are thought to be independently capable of photosynthesis, though the emerald green sea slug can temporarily incorporate algae chloroplasts in its body for food production.

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photosynthesis , the process by which green plants and certain other organisms transform light energy into chemical energy . During photosynthesis in green plants, light energy is captured and used to convert water , carbon dioxide , and minerals into oxygen and energy-rich organic compounds .

It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on Earth . If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time Earth’s atmosphere would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic bacteria , which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy.

Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the fossil fuels (i.e., coal , oil , and gas ) that power industrial society . In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in Earth’s crust by sedimentation and other geological processes. There, protected from oxidation , these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation but also serve as the raw material for plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Consequently, the carbon dioxide that has been removed from the air to make carbohydrates in photosynthesis over millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected to have major implications on Earth’s climate .

Requirements for food, materials, and energy in a world where human population is rapidly growing have created a need to increase both the amount of photosynthesis and the efficiency of converting photosynthetic output into products useful to people. One response to those needs—the so-called Green Revolution , begun in the mid-20th century—achieved enormous improvements in agricultural yield through the use of chemical fertilizers , pest and plant- disease control, plant breeding , and mechanized tilling, harvesting, and crop processing. This effort limited severe famines to a few areas of the world despite rapid population growth , but it did not eliminate widespread malnutrition . Moreover, beginning in the early 1990s, the rate at which yields of major crops increased began to decline. This was especially true for rice in Asia. Rising costs associated with sustaining high rates of agricultural production, which required ever-increasing inputs of fertilizers and pesticides and constant development of new plant varieties, also became problematic for farmers in many countries.

A second agricultural revolution , based on plant genetic engineering , was forecast to lead to increases in plant productivity and thereby partially alleviate malnutrition. Since the 1970s, molecular biologists have possessed the means to alter a plant’s genetic material (deoxyribonucleic acid, or DNA ) with the aim of achieving improvements in disease and drought resistance, product yield and quality, frost hardiness, and other desirable properties. However, such traits are inherently complex, and the process of making changes to crop plants through genetic engineering has turned out to be more complicated than anticipated. In the future such genetic engineering may result in improvements in the process of photosynthesis, but by the first decades of the 21st century, it had yet to demonstrate that it could dramatically increase crop yields.

Another intriguing area in the study of photosynthesis has been the discovery that certain animals are able to convert light energy into chemical energy. The emerald green sea slug ( Elysia chlorotica ), for example, acquires genes and chloroplasts from Vaucheria litorea , an alga it consumes, giving it a limited ability to produce chlorophyll . When enough chloroplasts are assimilated , the slug may forgo the ingestion of food. The pea aphid ( Acyrthosiphon pisum ) can harness light to manufacture the energy-rich compound adenosine triphosphate (ATP); this ability has been linked to the aphid’s manufacture of carotenoid pigments.

General characteristics

The study of photosynthesis began in 1771 with observations made by the English clergyman and scientist Joseph Priestley . Priestley had burned a candle in a closed container until the air within the container could no longer support combustion . He then placed a sprig of mint plant in the container and discovered that after several days the mint had produced some substance (later recognized as oxygen) that enabled the confined air to again support combustion. In 1779 the Dutch physician Jan Ingenhousz expanded upon Priestley’s work, showing that the plant had to be exposed to light if the combustible substance (i.e., oxygen) was to be restored. He also demonstrated that this process required the presence of the green tissues of the plant.

In 1782 it was demonstrated that the combustion-supporting gas (oxygen) was formed at the expense of another gas, or “fixed air,” which had been identified the year before as carbon dioxide. Gas-exchange experiments in 1804 showed that the gain in weight of a plant grown in a carefully weighed pot resulted from the uptake of carbon, which came entirely from absorbed carbon dioxide, and water taken up by plant roots; the balance is oxygen, released back to the atmosphere. Almost half a century passed before the concept of chemical energy had developed sufficiently to permit the discovery (in 1845) that light energy from the sun is stored as chemical energy in products formed during photosynthesis.

This equation is merely a summary statement, for the process of photosynthesis actually involves numerous reactions catalyzed by enzymes (organic catalysts ). These reactions occur in two stages: the “light” stage, consisting of photochemical (i.e., light-capturing) reactions; and the “dark” stage, comprising chemical reactions controlled by enzymes . During the first stage, the energy of light is absorbed and used to drive a series of electron transfers, resulting in the synthesis of ATP and the electron-donor-reduced nicotine adenine dinucleotide phosphate (NADPH). During the dark stage, the ATP and NADPH formed in the light-capturing reactions are used to reduce carbon dioxide to organic carbon compounds. This assimilation of inorganic carbon into organic compounds is called carbon fixation.

Van Niel’s proposal was important because the popular (but incorrect) theory had been that oxygen was removed from carbon dioxide (rather than hydrogen from water, releasing oxygen) and that carbon then combined with water to form carbohydrate (rather than the hydrogen from water combining with CO 2 to form CH 2 O).

By 1940 chemists were using heavy isotopes to follow the reactions of photosynthesis. Water marked with an isotope of oxygen ( 18 O) was used in early experiments. Plants that photosynthesized in the presence of water containing H 2 18 O produced oxygen gas containing 18 O; those that photosynthesized in the presence of normal water produced normal oxygen gas. These results provided definitive support for van Niel’s theory that the oxygen gas produced during photosynthesis is derived from water.

8.1 Overview of Photosynthesis

Learning objectives.

In this section, you will explore the following questions:

• What is the relevance of photosynthesis to living organisms?
• What are the main cellular structures involved in photosynthesis?
• What are the substrates and products of photosynthesis?

Connection for AP ® Courses

As we learned in Chapter 7, all living organisms, from simple bacteria to complex plants and animals, require free energy to carry out cellular processes, such as growth and reproduction. Organisms use various strategies to capture, store, transform, and transfer free energy, including photosynthesis. Photosynthesis allows organisms to access enormous amounts of free energy from the sun and transform it to the chemical energy of sugars. Although all organisms carry out some form of cellular respiration, only certain organisms, called photoautotrophs, can perform photosynthesis. Examples of photoautotrophs include plants, algae, some unicellular eukaryotes, and cyanobacteria. They require the presence of chlorophyll, a specialized pigment that absorbs certain wavelengths of the visible light spectrum to harness free energy from the sun. Photosynthesis is a process where components of water and carbon dioxide are used to assemble carbohydrate molecules and where oxygen waste products are released into the atmosphere. In eukaryotes, the reactions of photosynthesis occur in chloroplasts; in prokaryotes, such as cyanobacteria, the reactions are less localized and occur within membranes and in the cytoplasm. (The structural features of the chloroplast that participate in photosynthesis will be explored in more detail later in The Light-Dependent Reactions of Photosynthesis and Using Light Energy to Make Organic Molecules.) Although photosynthesis and cellular respiration evolved as independent processes—with photosynthesis creating an oxidizing atmosphere early in Earth’s history—today they are interdependent. As we studied in Cellular Respiration, aerobic cellular respiration taps into the oxidizing ability of oxygen to synthesize the organic compounds that are used to power cellular processes.

Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 1 and Big Idea 2 of the AP ® Biology Curriculum Framework, as shown in the table. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Teacher Support

Use this first part of the chapter to present an overview that will be filled out and completed in the later two portions. This will introduce the students to the biochemistry that they need to know and give them a chance to build up their understanding of the material.

Importance of Photosynthesis

Use this section to stress the importance of the interdependence between different species and the role played by photosynthesis in bringing energy to the living organisms. A number of terms, such as photoautotroph, heterotrophy, and chemoautotroph will be introduced here.

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, whose energy is then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago.

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis ( Figure 8.2 ). Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs .

The importance of photosynthesis is not just that it can capture sunlight’s energy. A lizard sunning itself on a cold day can use the sun’s energy to warm up. Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo-” part) as energy in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer ( Figure 8.3 ), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf.

Science Practice Connection for AP® Courses

Think about it.

• Why do scientists think that photosynthesis evolved before aerobic cellular respiration?
• Why do carnivores, such as lions, depend on photosynthesis to survive? What evidence supports the claim that photosynthesis and cellular respiration are interdependent processes?
• The first Think About It question is an application of Learning Objective 1.15 and Science Practice 7.2 because students are describing the evolution of two energy-procuring processes that today are present in different organisms.
• The second Think About It question is an application of Learning Objective 2.5 and Science Practice 6.2 because you are explaining how the interdependent processes of photosynthesis and cellular respiration allow organisms to capture, store, and use free energy.

Possible answers:

• Aerobic cellular respiration requires free oxygen, which was not available in the Earth’s atmosphere until photosynthetic organisms produced enough oxygen as waste to support developing aerobic respiration.
• Carnivores at the top of the food chain eat herbivores that eat photoautotrophs. So no matter where you are in the food chain, every species depends on photosynthesis to convert light energy to chemical energy. In ecosystems that lack photosynthetic organisms (such as by forests burned by forest fire), organisms on all levels of the food chain die off.

The structures, substrates and products of photosynthesis are introduced in this section. Remind them that Figure 8.5 can also be read from right to left, if cellular respiration is the subject. This should help the students to connect the two pathways of photosynthesis and cellular respiration.

Obtain diagrams of leaf structures to illustrate the content of this section. Try to bring in some leaves for students to look at. They have all seen lots of leaves, but probably never examined them for structural detail. A simple magnifying glass should allow them to see the inner structures discussed in this section.

Main Structures and Summary of Photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates ( Figure 8.4 ). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (G3P), simple carbohydrate molecules (which are high in energy) that can subsequently be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.

The following is the chemical equation for photosynthesis ( Figure 8.5 ):

Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll . The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast . For plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane). Within the chloroplast are stacked, disc-shaped structures called thylakoids . Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen . As shown in Figure 8.6 , a stack of thylakoids is called a granum , and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).

Visual Connection

• Rate of photosynthesis will be inhibited as the level of carbon dioxide decreases.
• Rate of photosynthesis will be inhibited as the level of oxygen decreases.
• The rate of photosynthesis will increase as the level of carbon dioxide increases.
• Rate of photosynthesis will increase as the level of oxygen increases.

The Two Parts of Photosynthesis

There are different terms that have been used for these reactions. Go over each pair of terms and discuss how they apply to the pathways.

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light independent-reactions. In the light-dependent reactions , energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions , the chemical energy harvested during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers. The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. Figure 8.7 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.

Link to Learning

Click the link to learn more about photosynthesis.

• The light reactions produces ATP and NADPH, which are then used in the Calvin cycle.
• The light reactions produces NADP + and ADP, which are then used in the Calvin cycle.
• The light reactions uses NADPH and ATP, which are produced by the Calvin cycle.
• The light reactions produce only NADPH, which is produced by the Calvin cycle.

Everyday Connection for AP® Courses

Photosynthesis at the grocery store.

Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth. Each aisle ( Figure 8.8 ) contains hundreds, if not thousands, of different products for customers to buy and consume.

Although there is a large variety, each item links back to photosynthesis. Meats and dairy link, because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks? All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are less obviously derived from plants: For instance, paper goods are generally plant products, and many plastics (abundant as products and packaging) are derived from algae. Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes.

• at the base
• near the top
• in the middle, but generally closer to the top
• in the middle, but generally closer to the base

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Explainer: how photosynthesis works.

Plants make sugar and oxygen with the power of water, carbon dioxide and sunlight

Green plants take in light from the sun and turn water and carbon dioxide into the oxygen we breathe and the sugars we eat.

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By Bethany Brookshire

October 28, 2020 at 6:30 am

Take a deep breath. Then thank a plant. If you eat fruit, vegetables, grains or potatoes, thank a plant too.  Plants and algae provide us with the oxygen we need to survive, as well as the carbohydrates we use for energy. They do it all through photosynthesis.

Photosynthesis is the process of creating sugar and oxygen from carbon dioxide, water and sunlight. It happens through a long series of chemical reactions. But it can be summarized like this: Carbon dioxide, water and light go in. Glucose, water and oxygen come out. (Glucose is a simple sugar.)

Photosynthesis can be split into two processes. The “photo” part refers to reactions triggered by light. “Synthesis” — the making of the sugar — is a separate process called the Calvin cycle.

Both processes happen inside a chloroplast. This is a specialized structure, or organelle, in a plant cell. The structure contains stacks of membranes called thylakoid membranes. That’s where the light reaction begins.

Let the light shine in

When light hits a plant’s leaves, it shines on chloroplasts and into their thylakoid membranes. Those membranes are filled with chlorophyll , a green pigment. This pigment absorbs light energy. Light travels as electromagnetic waves . The wavelength — distance between waves — determines energy level. Some of those wavelengths are visible to us as the colors we see . If a molecule, such as chlorophyll, has the right shape, it can absorb the energy from some wavelengths of light.

Chlorophyll can absorb light we see as blue and red. That’s why we see plants as green. Green is the wavelength plants reflect, not the color they absorb.

While light travels as a wave, it also can be a particle called a photon . Photons have no mass. They do, however, have a small amount of light energy.

When a photon of light from the sun bounces into a leaf, its energy excites a chlorophyll molecule. That photon starts a process that splits a molecule of water. The oxygen atom that splits off from the water instantly bonds with another, creating a molecule of oxygen, or O 2 . The chemical reaction also produces a molecule called ATP and another molecule called NADPH. Both of these allow a cell to store energy. The ATP and NADPH also will take part in the synthesis part of photosynthesis.

Notice that the light reaction makes no sugar. Instead, it supplies energy — stored in the ATP and NADPH — that gets plugged into the Calvin cycle. This is where sugar is made.

But the light reaction does produce something we use: oxygen. All the oxygen we breathe is the result of this step in photosynthesis, carried out by plants and algae (which are not plants ) the world over.

Give me some sugar

The next step takes the energy from the light reaction and applies it to a process called the Calvin cycle. The cycle is named for Melvin Calvin, the man who discovered it.

The Calvin cycle is sometimes also called the dark reaction because none of its steps require light. But it still happens during the day. That’s because it needs the energy produced by the light reaction that comes before it.

While the light reaction takes place in the thylakoid membranes, the ATP and NADPH it produces end up in the stroma. This is the space inside the chloroplast but outside the thylakoid membranes.

The Calvin cycle has four major steps:

• carbon fixation : Here, the plant brings in CO 2 and attaches it to another carbon molecule, using rubisco. This is an enzyme , or chemical that makes reactions move faster. This step is so important that rubisco is the most common protein in a chloroplast — and on Earth. Rubisco attaches the carbon in CO 2 to a five-carbon molecule called ribulose 1,5-bisphosphate (or RuBP). This creates a six-carbon molecule, which immediately splits into two chemicals, each with three carbons.
• reduction : The ATP and NADPH from the light reaction pop in and transform the two three-carbon molecules into two small sugar molecules. The sugar molecules are called G3P. That’s short for glyceraldehyde 3-phosphate (GLIH- sur-AAL-duh-hide 3-FOS-fayt).
• carbohydrate formation : Some of that G3P leaves the cycle to be converted into bigger sugars such as glucose (C 6 H 12 O 6 ).
• regeneration : With more ATP from the continuing light reaction, leftover G3P picks up two more carbons to become RuBP. This RuBP pairs up with rubisco again. They are now ready to start the Calvin cycle again when the next molecule of CO 2 arrives.

At the end of photosynthesis, a plant ends up with glucose (C 6 H 12 O 6 ), oxygen (O 2 ) and water (H 2 O). The glucose molecule goes on to bigger things. It can become part of a long-chain molecule, such as cellulose; that’s the chemical that makes up cell walls. Plants also can store the energy packed in a glucose molecule within larger starch molecules. They can even put the glucose into other sugars — such as fructose — to make a plant’s fruit sweet.

All of these molecules are carbohydrates — chemicals containing carbon, oxygen and hydrogen. (CarbOHydrate makes it easy to remember.) The plant uses the bonds in these chemicals to store energy. But we use the these chemicals too. Carbohydrates are an important part of the foods we eat, particularly grains, potatoes, fruits and vegetables.

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Photosynthesis Basics - Study Guide

How Plants Make Food - Key Concepts

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Learn about photosynthesis step-by-step with this quick study guide. Start with the basics:

Quick Review of the Key Concepts of Photosynthesis

• In plants, photosynthesis is used to convert light energy from sunlight into chemical energy (glucose). Carbon dioxide, water, and light are used to make glucose and oxygen.
• Photosynthesis is not a single chemical reaction, but rather a set of chemical reactions . The overall reaction is: 6CO 2 + 6H 2 O + light → C 6 H 12 O 6 + 6O 2
• The reactions of photosynthesis can be categorized as light-dependent reactions and dark reactions .
• Chlorophyll is a key molecule for photosynthesis, though other cartenoid pigments also participate. There are four (4) types of chlorophyll: a, b, c, and d. Although we normally think of plants as having chlorophyll and performing photosynthesis, many microorganisms use this molecule, including some prokaryotic cells . In plants, chlorophyll is found in a special structure, which is called a chloroplast.
• The reactions for photosynthesis take place in different areas of the chloroplast. The chloroplast has three membranes (inner, outer, thylakoid) and is divided into three compartments (stroma, thylakoid space, inter-membrane space). Dark reactions occur in the stroma. Light reactions occur the thylakoid membranes.
• There is more than one form of photosynthesis . In addition, other organisms convert energy into food using non-photosynthetic reactions (e.g. lithotroph and methanogen bacteria) Products of Photosynthesis

Steps of Photosynthesis

Here is a summary of the steps used by plants and other organisms to use solar energy to make chemical energy:

• In plants, photosynthesis usually occurs in the leaves. This is where plants can get the raw materials for photosynthesis all in one convenient location. Carbon dioxide and oxygen enter/exit the leaves through pores called stomata. Water is delivered to the leaves from the roots through a vascular system. The chlorophyll in the chloroplasts inside leaf cells  absorbs sunlight.
• The process of photosynthesis  is divided into two main parts: light dependent reactions and light independent or dark reactions. The light dependent reaction happens when solar energy is captured to make a molecule called ATP (adenosine triphosphate). The dark reaction happens when the ATP is used to make glucose (the Calvin Cycle).
• Chlorophyll and other carotenoids form what are called antenna complexes. Antenna complexes transfer light energy to one of two types of photochemical reaction centers: P700, which is part of Photosystem I, or P680, which is part of Photosystem II. The photochemical reaction centers are located on the thylakoid membrane of the chloroplast. Excited electrons are transferred to electron acceptors, leaving the reaction center in an oxidized state.
• The light-independent reactions produce carbohydrates by using ATP and NADPH that was formed from the light-dependent reactions.

Photosynthesis Light Reactions

Not all wavelengths of light are absorbed during photosynthesis. Green, the color of most plants, is actually the color that is reflected. The light that is absorbed splits water into hydrogen and oxygen:

H2O + light energy → ½ O2 + 2H+ + 2 electrons

• Excited electrons from Photosystem I can use an electron transport chain to reduce oxidized P700. This sets up a proton gradient, which can generate ATP. The end result of this looping electron flow, called cyclic phosphorylation, is the generation of ATP and P700.
• Excited electrons from Photosystem I could flow down a different electron transport chain to produce NADPH, which is used to synthesize carbohydratyes. This is a noncyclic pathway in which P700 is reduced by an exicted electron from Photosystem II.
• An excited electron from Photosystem II flows down an electron transport chain from excited P680 to the oxidized form of P700, creating a proton gradient between the stroma and thylakoids that generates ATP. The net result of this reaction is called noncyclic photophosphorylation.
• Water contributes the electron that is needed to regenerate the reduced P680. The reduction of each molecule of NADP+ to NADPH uses two electrons  and requires four photons . Two molecules  of ATP are formed.

Photosynthesis Dark Reactions

Dark reactions don't require light, but they aren't inhibited by it, either. For most plants, the dark reactions take place during daytime. The dark reaction occurs in the stroma of the chloroplast. This reaction is called carbon fixation or the  Calvin cycle . In this reaction, carbon dioxide is converted to sugar using ATP and NADPH. Carbon dioxide is combined with a 5-carbon sugar to form a 6-carbon sugar. The 6-carbon sugar is broken into two sugar molecules, glucose and fructose, which can be used to make sucrose. The reaction requires 72 photons of light.

The efficiency of photosynthesis is limited by environmental factors, including light, water, and carbon dioxide. In hot or dry weather, plants may close their stomata to conserve water. When the stomata are closed, the plants may start photorespiration. Plants called C4 plants maintain high levels of carbon dioxide inside cells that make glucose, to help avoid photorespiration. C4 plants produce carbohydrates more efficiently than normal C3 plants, provided the carbon dioxide is limiting and sufficient light is available to support the reaction. In moderate temperatures, too much of an energy burden is placed on the plants to make the C4 strategy worthwhile (named 3 and 4 because of the number of carbons in the intermediate reaction). C4 plants thrive in hot, dry climates.Study Questions

Here are some questions you can ask yourself, to help you determine if you really understand the basics of how photosynthesis works.

• Define photosynthesis.
• What materials are required for photosynthesis? What is produced?
• Write the  overall reaction  for photosynthesis.
• Describe what happens during the cyclic phosphorylation of photosystem I. How does the transfer of electrons lead to the synthesis of ATP?
• Describe the reactions of carbon fixation or the  Calvin cycle . What enzyme catalyzes the reaction? What are the products of the reaction?

Do you feel ready to test yourself? Take the  photosynthesis quiz !

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Understanding Step by Step Process of Photosynthesis in Plants

Photosynthesis is the process through which plants convert light energy from the sun to chemical energy. The chemical energy is then stored as sugar. During the process of photosynthesis, plants capture light energy and use it to convert water, carbon dioxide, and minerals into oxygen (released into the air) and glucose (stored in the plant and used as food).

The photosynthesis process occurs in all plants and algae, as well as in some bacteria species. In addition to light energy, the process also requires water and carbon dioxide. And the end product is sugar.

Photosynthesis takes place inside plant cells in small things called as chloroplasts using chlorophyll – the green pigment in plants. While it usually occurs in leaves, it can also take place in stems albeit in minute proportion.

According to Britannica ,

“ Photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds. “

Despite the fact that photosynthesis occurs in plants and algae, it is beneficial for all lifeforms on the Earth including humans.

In this article, we will have a detailed look at the process of photosynthesis and also explore its importance . Let’s get to it.

Step by Step Process of Photosynthesis

In light of the fact that photosynthesis occurs largely in plant leaves, you should understand the structure of the leaf before you learn how the process works.

A typical leaf is made up of the following parts:

• Upper and lower epidermis
• Spongy mesophyll
• Palisade mesophyll
• Vascular bundles

Photosynthesis does not occur in the upper and lower epidermis as they don’t have chloroplasts. These parts only serve as a protection of the inner cells of a leaf. The Stomata refer to the microscopic holes found mainly on the lower epidermis. They are for reverse respiration – allow carbon dioxide in and let oxygen out.

The vascular bundles are part of the plant’s transport system. They help in the movement of nutrients and water around the plant. Photosynthesis occurs in the palisade mesophyll cells as they have chloroplasts.

Now that we have that out of the way, let’s look at the 4 main steps of photosynthesis:

The plant takes in carbon dioxide in the atmosphere through the stomata on its leaves. It is worth noting that there are some stomata on the stems as well.

Water gets into the plant mainly through the roots and finds its way to the leaves, where photosynthesis occurs. Plant roots are specially designed to draw water from the ground and transport it to the plant leaves through the stem.

Chlorophyll, the green coloring matter of the leaf, traps the energy from sunlight as it shines on the leaf. It is worth noting that it is chlorophyll that gives the leaf its green color.

The solar energy is used to break water down into hydrogen and oxygen. Then hydrogen is combined with carbon dioxide to make sugar, which is food for the plant. Oxygen is released as a byproduct through the stomata.

Chemical reactions occur with the help of protons from the water molecules and electrons from the chlorophyll molecules to produce Adenosine Triphosphate (ATP). ATP provides energy for cellular reactions and Nicotinamide Adenine Dinucleotide Diphosphate (NADP), which is integral in plant metabolism.

Chemical Formula for the Process of Photosynthesis

This chemical formula summarizes the whole process of photosynthesis:

6CO2 +12H2O + Light → C6H12O6 + 6O2+ 6H2O

And there are six factors that influence the rate of photosynthesis:

• Temperature
• Light intensity
• Light wavelength
• Carbon dioxide availability
• Chlorophyll concentration
• Water availability

Namely, the higher the light intensity, temperature, carbon dioxide and chlorophyll concentration, the faster the rate at which photosynthesis occurs. But there is an optimum rate.

In the case of light intensity, photosynthesis rate is higher in red or blue light and very low in green light. Photosynthesis rate decreases with a decrease in water availability .

Now that you understand the process of photosynthesis let’s look at its importance.

Importance of Photosynthesis

Plants play an integral role in the continuity of life on the planet, all thanks to photosynthesis. They achieve that through the following:

1. Atmospheric Gases

In photosynthesis, plants take in carbon dioxide and release oxygen as a by-product. Without the process of photosynthesis, it would be difficult to replenish all the oxygen being used in processes such as combustion and respiration.

Moreover, the amount of carbon dioxide in the atmosphere would rise to dangerous levels. Therefore, it suffices to say that photosynthesis helps to balance atmospheric gases.

2. Food and Energy

Green plants produce their own food through photosynthesis. They are called producers. Conversely, animals and humans are consumers.

They get all of their food from plants, either directly or indirectly. In fact, the larger part of the world’s population gets over 80% of its food directly from plants. The remaining source is obtained from animals.

And animals are part of the food chain, which normally starts with plants. Speaking of the food chain , the energy you get from the food you eat is also a product of photosynthesis, whether the food is from plants or animals .

3. Petroleum Products

Did you know that your car runs on what was once light energy? Or that your cooking gas is a product of photosynthesis?

We get petroleum from plants that stored light energy in their system millions of years ago, thanks to photosynthesis, as well as animals that ate those plants.

The petroleum came into being due to intense pressure applied to the plants and animals over millions of years. Coal and natural gas were also produced in the same way.

4. Wood and Other Side Products

We use wood for a wide range of purposes, including construction and combustion. Paper is also a product of wood.

Moreover, cotton and other natural fibers consist of cellulose produced virtually entirely by photosynthesizing plants. And while wood comes from the sheep, the sheep gets its food from the plants. It is, therefore, suffice to say that we wear clothes courtesy of photosynthesis.

5. Medicinal Products

Most medicines are manufactured using various chemicals extracted from plants. And thousands of plants have been confirmed to have medicinal properties. Aspirin, for example, is derived from salicylic acid.

Salicylic acid comes from the back of the willow tree. Aspirin is a popular painkiller. It is also used to minimize blood clotting in heart patients.

Much stronger analgesic drugs such as codeine and morphine are products of opium. Opium is extracted from the seeds of the poppy plant.

Tests are still ongoing to ascertain the medicinal status of thousands of other plants species, especially those found in the tropical rainforests . In the light of this, it is imperative that we protect the natural habitats of these plant species.

Bottom Line

Photosynthesis is the process used by plants, algae, and some bacteria to convert solar energy into chemical energy. Besides light energy, other photosynthesis ingredients are water and carbon dioxide. It is a complex, enzyme-controlled process that is vital for the existence of all lifeforms on Planet Earth . Namely, all living things are dependent on plants, directly or indirectly.

Photo by: Khanh , pexels

I am Sonia Madaan, a mother with a passion for science, computing, and environmental issues. Motivated by my passion and education, I started a website to spread awareness about climate change and its causes, like rising greenhouse gas levels. You can read more about me here .

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Module 6: Metabolic Pathways

Photosynthesis, identify the basic components and steps of photosynthesis.

The processes in all organisms—from bacteria to humans—require energy. To get this energy, many organisms access stored energy by eating, that is, by ingesting other organisms. But where does the stored energy in food originate? All of this energy can be traced back to photosynthesis.

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis around 300 million years ago.

Figure 1. Photoautotrophs including (a) plants, (b) algae, and (c) cyanobacteria synthesize their organic compounds via photosynthesis using sunlight as an energy source. Cyanobacteria and planktonic algae can grow over enormous areas in water, at times completely covering the surface. In a (d) deep sea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds. The ecosystem surrounding the vents has a diverse array of animals, such as tubeworms, crustaceans, and octopi that derive energy from the bacteria. (credit a: modification of work by Steve Hillebrand, U.S. Fish and Wildlife Service; credit b: modification of work by “eutrophication&hypoxia”/Flickr; credit c: modification of work by NASA; credit d: University of Washington, NOAA; credit e: modification of work by Mark Amend, West Coast and Polar Regions Undersea Research Center, UAF, NOAA)

Figure 2. The energy stored in carbohydrate molecules from photosynthesis passes through the food chain. The predator that eats these deer receives a portion of the energy that originated in the photosynthetic vegetation that the deer consumed. (credit: modification of work by Steve VanRiper, U.S. Fish and Wildlife Service)

Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis (Figure 1). Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs .

The importance of photosynthesis is not just that it can capture sunlight’s energy. A lizard sunning itself on a cold day can use the sun’s energy to warm up. Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo” part) as high-energy electrons in the carbon-carbon bonds of carbohydrate molecules (the “synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer (Figure 2), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf.

Learning Objectives

• Identify the reactants and products of photosynthesis
• Describe the visible and electromagnetic spectrums of light as they applies to photosynthesis
• Describe the light-dependent reactions that take place during photosynthesis
• Identify the light-independent reactions in photosynthesis

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 3). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (GA3P), simple carbohydrate molecules (which are high in energy) that can subsequently be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive.

Figure 3. Photosynthesis uses solar energy, carbon dioxide, and water to produce energy-storing carbohydrates. Oxygen is generated as a waste product of photosynthesis.

The following is the chemical equation for photosynthesis (Figure 4):

Figure 4. The basic equation for photosynthesis is deceptively simple. In reality, the process takes place in many steps involving intermediate reactants and products. Glucose, the primary energy source in cells, is made from two three-carbon GA3Ps.

Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved.

In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the  mesophyll . The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.

In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a  chloroplast . For plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane). Within the chloroplast are stacked, disc-shaped structures called thylakoids . Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen . As shown in Figure 5, a stack of thylakoids is called a granum , and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis).

Practice Question

Figure 5. Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer.

On a hot, dry day, plants close their stomata to conserve water. What impact will this have on photosynthesis?

The Two Parts of Photosynthesis

Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light independent-reactions. In the  light-dependent reactions , energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions , the chemical energy harvested during the light-dependent reactions drive the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers. The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. Figure 6 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place.

Figure 6. Photosynthesis takes place in two stages: light dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO 2 .

Photosynthesis at the Grocery Store

Figure 7. Foods that humans consume originate from photosynthesis. (credit: Associação Brasileira de Supermercados)

Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth. Each aisle (Figure 7) contains hundreds, if not thousands, of different products for customers to buy and consume.

Although there is a large variety, each item links back to photosynthesis. Meats and dairy link because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks? All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are less obviously derived from plants: for instance, paper goods are generally plant products, and many plastics (abundant as products and packaging) can be derived from algae or from oil, the fossilized remains of photosynthetic organisms. Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes.

Spectrums of Light

How can light be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules. However, autotrophs only use a few specific components of sunlight.

What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which portion is therefore referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength, the distance between consecutive points of a wave. A single wave is measured from two consecutive points, such as from crest to crest or from trough to trough (Figure 8).

Figure 8. The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave.

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure 9). The difference between wavelengths relates to the amount of energy carried by them.

Figure 9. The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum (Figure 9) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms.

Absorption of Light

Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, called bleaching. So retinal pigments can only “see” (absorb) 700 nm to 400 nm light, which is therefore called visible light. For the same reasons, plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as photosynthetically active radiation.

The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light portion of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy (Figure 10).

Figure 10. The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. (credit: modification of work by NASA)

Understanding Pigments

Different kinds of pigments exist, and each has evolved to absorb only certain wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae; each class has multiple types of pigment molecules. There are five major chlorophylls:  a , b , c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher plant chloroplasts and will be the focus of the following discussion.

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the  absorption spectrum . The graph in Figure 11 shows the absorption spectra for chlorophyll  a , chlorophyll b , and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.

Figure 11. (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. β-carotene is responsible for the orange color in carrots. Each pigment has (d) a unique absorbance spectrum.

Figure 12. Plants that commonly grow in the shade have adapted to low levels of light by changing the relative concentrations of their chlorophyll pigments. (credit: Jason Hollinger)

Many photosynthetic organisms have a mixture of pigments; using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation (Figure 12).

When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a  spectrophotometer can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases.

Light-Dependent Reactions

The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in Figure 13. Protein complexes and pigment molecules work together to produce NADPH and ATP.

Figure 13. A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In (a) photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In (b) photosystem I, the electron comes from the chloroplast electron transport chain discussed below.

The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a  photosystem , two types of which are found embedded in the thylakoid membrane, photosystem II (PSII) and photosystem I (PSI) (Figure 14). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).

Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll  a and b molecules as well as other pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.

Figure 14. The photosystem II (PSII) reaction center and the photosystem I (PSI).

In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces NADP + to NADPH. The electron transport chain moves protons across the thylakoid membrane into the lumen. At the same time, splitting of water adds protons to the lumen, and reduction of NADPH removes protons from the stroma. The net result is a low pH in the thylakoid lumen, and a high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP. What is the initial source of electrons for the chloroplast electron transport chain?

• carbon dioxide

The reaction center contains a pair of chlorophyll  a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation; they can actually give up an electron in a process called a photoact . It is at this step in the reaction center, that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate. PSII and PSI are two major components of the photosynthetic electron transport chain , which also includes the cytochrome complex . The cytochrome complex, an enzyme composed of two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.

The reaction center of PSII (called  P680 ) delivers its high-energy electrons, one at the time, to the primary electron acceptor , and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to PSI. P680’s missing electron is replaced by extracting a low-energy electron from water; thus, water is split and PSII is re-reduced after every photoact. Splitting one H 2 O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one molecule of diatomic O 2 gas. About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration.

As electrons move through the proteins that reside between PSII and PSI, they lose energy. That energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called  P700 ). P700 is oxidized and sends a high-energy electron to NADP + to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP + into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to fine tune that ratio to exactly match the chloroplast’s constantly changing energy needs.

Generating an Energy Carrier: ATP

As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.

To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP (Figure 14). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure.

Light-Independent Reactions

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals.

In plants, carbon dioxide (CO 2 ) enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO 2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions actually have several names associated with them. Another term, the Calvin cycle , is named for the man who discovered it, and because these reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery. The most outdated name is dark reactions, because light is not directly required (Figure 15). However, the term dark reaction can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.

Figure 15. Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place.

The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.

Stage 1: Fixation

In the stroma, in addition to CO 2 , two other components are present to initiate the light-independent reactions: an enzyme called ribulose bisphosphate carboxylase (RuBisCO), and three molecules of ribulose bisphosphate (RuBP), as shown in Figure 16. RuBP has five atoms of carbon, flanked by two phosphates.

Figure 16. The Calvin cycle has three stages.

In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.

Which of the following statements is true?

• In photosynthesis, oxygen, carbon dioxide, ATP, and NADPH are reactants. GA3P and water are products.
• In photosynthesis, chlorophyll, water, and carbon dioxide are reactants. GA3P and oxygen are products.
• In photosynthesis, water, carbon dioxide, ATP, and NADPH are reactants. RuBP and oxygen are products.
• In photosynthesis, water and carbon dioxide are reactants. GA3P and oxygen are products.

RuBisCO catalyzes a reaction between CO 2 and RuBP. For each CO 2 molecule that reacts with one RuBP, two molecules of another compound (3-PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO 2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called  carbon fixation , because CO 2 is “fixed” from an inorganic form into organic molecules.

Stage 2: Reduction

ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that a  reduction is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP + . Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized.

Stage 3: Regeneration

Interestingly, at this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.

Evolution of Photosynthesis

Figure 17. The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy. (credit: Piotr Wojtkowski)

During the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern oxygenic (does generate oxygen) photosynthesis, employing two photosystems. This modern oxygenic photosynthesis is used by many organisms—from giant tropical leaves in the rainforest to tiny cyanobacterial cells—and the process and components of this photosynthesis remain largely the same. Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of ATP and NADH. The subsequent light-independent reactions then assemble carbohydrate molecules with this energy.

Photosynthesis in desert plants has evolved adaptations that conserve water. In the harsh dry heat, every drop of water must be used to survive. Because stomata must open to allow for the uptake of CO 2 , water escapes from the leaf during active photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions. A more efficient use of CO 2 allows plants to adapt to living with less water. Some plants such as cacti (Figure 17) can prepare materials for photosynthesis during the night by a temporary carbon fixation/storage process, because opening the stomata at this time conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, a mechanism to face extremely dry periods.

Now that we’ve learned about the different pieces of photosynthesis, let’s put it all together. This video walks you through the process of photosynthesis as a whole:

In Summary: An Overview of Photosynthesis

The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, photosynthesis evolved to allow living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today.

Only certain organisms, called photoautotrophs, can perform photosynthesis; they require the presence of chlorophyll, a specialized pigment that absorbs certain portions of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a waste product into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm.

The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll  a to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of ions. The ions flow through ATP synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does  not  count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

• Authored by : Shelli Carter and Lumen Learning. Provided by : Lumen Learning. License : CC BY: Attribution
• Biology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]
• Photosynthesis: Crash Course Biology #8. Authored by : CrashCourse. Located at : https://youtu.be/sQK3Yr4Sc_k . License : All Rights Reserved . License Terms : Standard YouTube License

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The Three Stages of Photosynthesis

The Effect of Darkness on Photosynthesis

Plants and algae act as the food bank of the world thanks to their amazing photosynthetic powers. In the process of photosynthesis, sunlight is collected by living organisms and used to produce glucose and other energy-rich, carbon-based compounds.

Scientists find the three stages of the process intriguing, and the Center for Bioenergy and Photosynthesis at Arizona State University even argues for photosynthesis' importance relative to other biological processes.

TL;DR (Too Long; Didn't Read)

The process of energy exchange in photosynthesis is expressed as 6H 2 O + 6CO 2 + light energy → C 6 H 12 O 6 (glucose: a simple sugar) + 6O 2 (oxygen).

What Is Photosynthesis?

Photosynthesis is a complex process that can be divided into two or more stages, such light-dependent and light-independent reactions. The three-stage model of photosynthesis starts with absorption of sunlight and ends in the production of glucose.

Plants, algae and certain bacteria are classified as autotrophs , meaning they are capable of meeting their nutritional needs through photosynthesis. Autotrophs are at the bottom of the food chain because they produce food for all other living organisms. For instance, plants are eaten by grazers who may eventually be a food source for predators and decomposers.

Food is not the only contribution of photosynthesis. Stored energy in fossil fuels and wood is used to heat homes, businesses and industries. Scientists study the stages of photosynthesis to learn more about how autotrophs use solar energy and carbon dioxide to produce organic compounds. Research findings could lead to new methods of crop production and increased yields.

The Photosynthesis Process: Stage 1: Harvesting Radiant Energy

When a beam of sunlight hits a green, leafy plant, the process of photosynthesis is set in motion.

The first step of photosynthesis occurs in the chloroplasts of plant cells. Light photons are absorbed by a pigment called chlorophyll, which is abundant in the thylakoid membrane of each chloroplast. Chlorophyll appears green to the eye because it does not absorb green waves on the light spectrum. It reflects them instead, so that's the color you see.

Plants take in carbon dioxide through their stomata (microscopic openings in tissue) for use in photosynthesis. Plants transpire and replenish oxygen in the air and ocean.

Stage 2: Converting Radiant Energy

After radiant energy from sunlight is absorbed, the plant converts light energy into a usable form of chemical energy to fuel the plant’s cells.

In light-dependent reactions occurring during the second stage of the process of photosynthesis, electrons get excited and split off from water molecules, leaving oxygen as a by-product. The hydrogen electrons of the water molecule then move to a reaction center in the chlorophyll molecule.

In the reaction center, the electron passes along a transport chain, aided by the enzyme ATP synthase. Energy is lost as the excited electron drops to lower energy levels. Energy from electrons is transferred to adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), commonly referred to as “energy currency” of cells.

Stage 3: Storing Radiant Energy

The last stage of the photosynthesis process is known as the Calvin-Benson cycle, in which the plant uses atmospheric carbon dioxide and water from soil to convert ATP and NADPH. The chemical reactions that make up the Calvin-Benson cycle occur in the stroma of the chloroplast.

This stage of the process of photosynthesis is light-independent and can happen even at night.

ATP and NADPH have a short shelf life and must be converted and stored by the plant. Energy from ATP and NADPH molecules enables the cell to use or “fix” atmospheric carbon dioxide, resulting in the production of sugar, fatty acid and glycerol in the third stage of photosynthesis. Energy that the plant doesn’t need immediately is stored for later use.

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• Basic Biology: Photosynthesis
• Arizona State University: Center for Bioenergy & Photosynthesis: Why Study Photosynthesis

About the Author

Dr. Mary Dowd studied biology in college where she worked as a lab assistant and tutored grateful students who didn't share her love of science. Her work history includes working as a naturalist in Minnesota and Wisconsin and presenting interactive science programs to groups of all ages. She enjoys writing online articles sharing information about science and education. Currently, Dr. Dowd is a dean of students at a mid-sized university.

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3.5 Photosynthesis

1 min read • june 18, 2024

Caroline Koffke

Haseung Jun

Photosynthesis is a chemical process with the following equation: H2O + CO2 → C6H12O6 + O2 . 

Both plants and photosynthetic bacteria are capable of this complex conversion process. The overall reaction is spurred by the energy from a photon of light striking a pigment in the chloroplast . It is thought that photosynthesis first evolved in prokaryotic cells . 

Photosynthesis is broken down into two major steps which are dependent on one another:  light-dependent reactions  and  light-independent reactions ( Calvin Cycle ). Both of these processes occur in the chloroplast of a photosynthetic organism.

Chloroplast

Light-dependent reactions occur in the  thylakoid membranes of the  chloroplast . These are the “pancakes” of the chloroplast, as they look like a stack of flattened disks. The thylakoid membranes possess important pigments called  chlorophyll . This pigment has electrons in it that are excited when energy is input by a photon of light. There are many pigments, but for photosynthesis, we'll focus more on chlorophyll, the main one. 

When light strikes the chloroplast, an electron from a molecule of chlorophyll is excited and travels through the electron transport chain . In the process, a concentration gradient of hydrogen ions is formed. This will be used later to produce ATP through  ATP synthase . The electron lost from chlorophyll is replaced by an electron from water. This creates more hydrogen ions and the production of  oxygen , which is released from the plant.

Light-Dependent Reactions (Electron Transport Chain)

When light hits the pigments, it'll hit  Photosystem II  first, which is embedded in the internal membrane of the chloroplast and excites electrons. This causes H+ ions to move into the thylakoid space and to replenish electrons, the light splits water, called  photolysis , into two H+ ions and 1/2 of O2 and electrons, which replaces the missing electrons in Photosystem II. Why are the electrons missing then? Well, it's because the electrons continue to jump down the thylakoid membrane, bumping into Photosystem I , and thus leaving Photosystem II. With the electrons going down the thylakoid membrane, hydrogen ions continue to be pumped into the membrane. 

Because there a lot of hydrogen ions inside the thylakoid space, it's natural for the H+ ions to want to leave the thylakoid space. But the only way for these ions to leave is to go through a transport protein called ATP (adenosine triphosphate) synthase, where ADP (adenosine diphosphate) is  phosphorylated  (add another phosphate) when H+ goes through it. 

Other electrons from Photosystem I bind to an electron carrier, such as  NADPH . Electron carriers transport electrons in the form of a hydrogen ion. These electrons can then be used in other processes. In this case, the electrons will be used to form bonds in the Calvin Cycle. 

The ATP and electron carriers produced during the light-dependent reactions are essential to the production of glucose in the light-independent reactions. The production of oxygen is toxic to the plants but provides the rest of the world with the opportunity to breathe.

Light-Independent Reactions (Calvin Cycle)

The  light-independent reactions are named due to the fact that they  do not require light in order to proceed. This set of reactions is also referred to as the Calvin Cycle. These reactions take place in the stroma of the chloroplast, or the gooey space in between the thylakoid pancakes. With the help of ATP and NADPH, CO2 is turned into sugar. 

In the Calvin Cycle, carbon dioxide is converted into an organic carbon source, most often modeled by glucose. The first step of this reaction involves the enzyme ribulose bisphosphate carboxylase, abbreviated as  rubisco . This enzyme is responsible for carbon fixation , taking carbon dioxide from the air and converting it into an organic, usable form.

After carbon dioxide has been fixed, the process begins to convert it into glucose. This involves the creation of a lot of bonds. In order to make bonds, electrons and energy are required. This is where the electron carriers and ATP from the light-dependent reactions come into play.

By using the energy from ATP and the electrons from the electron carriers, a number of enzymes are able to convert organic carbon into glyceraldehyde-3-phosphate, or  G3P . G3P is a precursor for a number of carbohydrates such as starch , cellulose , and glucose. The cell can use this to create a number of important energy and structural components.

Also important, the ATP that is used is broken down into ADP and a phosphate group which can be recycled and rebonded in the light-dependent reactions. Similarly, the electron carrier NADPH becomes NADP+ after dropping off the hydrogen. This can then be refilled with an electron in the light-dependent reactions.

Photosynthesis is hard to understand and visualize, but it's an important part of the AP curriculum. Study tips include trying to draw out the process yourself and explain it to a friend or yourself. 

Understanding the Electron Transport Chain can be the most challenging, so really make sure you understand what causes what. Remember, the sole purpose of photosynthesis is creating sugar. Also, NADPH represents a "loaded dumptruck" with electrons. 

Check out the AP Bio   Unit 3 Replays or watch the   2021 Unit 3 Cram

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EDITORIAL article

Editorial: advances in conservation and utilization of plant genetic resources.

• 1 Department of Agriculture, Inland Norway University of Applied Sciences, Elverum, Norway
• 2 World Vegetable Center, Shanhua, Taiwan
• 3 Plant Gene Resources of Canada, Agriculture and Agri-Food Canada (AAFC), Saskatoon, SK, Canada

Editorial on the Research Topic Advances in conservation and utilization of plant genetic resources

We made a broad call for perspective papers, systematic reviews, meta-analyses, or traditional research papers on topics related to conservation and use of plant genetic resources. We invited to present advances in characterization and evaluation, strategies to improve gene bank operations and collaboration, new tools for managing and sharing information, or novel knowledge of conservation gaps. We especially encouraged contributions on underutilized crops and crops wild relatives. We received 58 positive responses, whereof 20 finalized their submissions and passed the review process with a total of 125 authors included in the accepted papers. Our motivation was to assemble contributions underlining that genetic resources are essential for crop improvement, which is achieved via plant breeding and release of new varieties that farmers cand access and use. We see that future crops need to produce high and stable yields but also be of high nutritional quality. They must adapt to shifting climates and support a sustainable agricultural or horticultural production avoiding negative environmental impacts. In addition, we need to contribute to efforts to avoid the loss of biodiversity, including the genetic resources of crop plants and crop wild relatives, which are in our focus. We are aware that there is a large body of research and literature on this, because plant breeders in the late 19 th century already imitated research and conservation activities which serve the same purpose and they recognized the loss of biodiversity, albeit not using this term. However, the global biodiversity crisis has become much bigger and globally recognized and any additional step, any additional insight addressing this topic is worth to be shared. At the same time more and more genomic tools and other technologies are available to add to our understanding of diversity and can support conservation and target promising germplasm for further research and breeding. We hope this special edition will be a small additional step in a positive direction, part of the evolutionary process of coping with the grand challenges we all face.

Conservation

Six papers focused on conservation, which included conservation methods and priorities as well as core collections. Of these, one is a review paper and five are research articles.

Subramanian et al. ’s review on Brassica biodiversity conservation discusses issues related to distribution of accessions, conservation methods, technical hurdles and future avenues for research. Despite that there are more than 80,000 Brassica accessions conserved across 81 countries the authors see that the diversity of the conserved taxa is limited in most countries, which they see may lead to biodiversity loss in the longer run. In addition, they see practical challenges as taxonomic issues in the conservation system.

The research paper from Weise et al. focuses on the rapeseed ( Brassica napus L.) gene pool that is conserved in European genebanks. They highlight that most of these species are underrepresented in current collections and that many of the natural distribution areas are not covered as it stands now. By using niche modelling they further illustrate how climate change may affect the species’ distribution ranges. The authors suggest to further develop the conservation strategies for the rapeseed gene pool and propose a list of priority species that should be targeted for collecting missions.

Carvajal-Yepes et al. made a work to identify genetically redundant accessions in the world’s largest cassava ( Manihot esculenta ) collection with 5,302 accessions maintained at the International Center for Tropical Agriculture (CIAT). Am empirical distance threshold methodology was applied with two types of molecular markers (SNP and SilicoDArT). The results showed 2,776 (SNP) and 2,785 (SilicoDArT) accessions were part of accession clusters. By comparing passport and historical characterization data clusters of genetically redundant accessions the authors provided a roadmap for genebank curators to assess redundancy within collections and/or identifying subsets of genetically distinct accessions.

Almeida et al. presented a methodology for landrace threat assessment which can assist in setting priorities for conservation. The methodology of this work was in line with the IUCN Red Listing judgement for wild species and involves the collation of time series information on population range and trends. Unlike for wild species the information used here involved farmers and market actors. The authors conclude that the archived information can be compared to a standardized set of threat criteria with a set threshold level and the methodology can be applied to any crop and geographical scale.

Dos Santos et al. developed a core collection of the cassava ( Manihot esculenta Crantz) germplasm bank in Brazil based on morphological traits, agronomic traits, and genotypic data from 20k single-nucleotide polymorphisms (SNPs). Out of the 1,486 accessions in the germplasm bank a consolidated core with 204 accessions was suggested, which is 14% of the complete collection. This core collection showed less genetic variation but retained over 97% of the allelic richness compared to the complete collection. The authors see that the core approach provides a robust and representative resource for further research and breeding in cassava.

Bernad et al. examined the genetic diversity and population structure of the newly introduced two-row spring European Heritage Barley collection. This is a small collection which consists of 363 spring-barley accessions mainly from Northern Europe and include both landraces (~14%), old cultivars (~18%), elite cultivars (~67%) and accessions with unknown breeding history (~1%). The authors used 26,585 informative SNPs based on 50k iSelect array data and the results showed that the collection could be subdivided into three main clusters. The clusters were primarily based on the accession’s year of release. Furthermore, power analysis identified a core with 230 genotypically and phenotypically diverse accessions, which shows that the collection represents a high diversity and can be a resource for research and breeding.

Characterization and evaluation

Twelve research articles from recent characterization and evaluation projects are presented, which included a range of crops, methods and aims and showing how molecular markers and genomic tools can help to identify promising germplasm for further phenotyping and evaluation.

Gomez-Galvez et al. examined more than 500 olive ( Olea europaea L.) genotypes from different regions in Spain based on 96 EST-SNP markers and identified 173 new genotypes. Based on this work the number of distinct Spanish genotypes documented in the World Olive Germplasm Bank of IFAPA, Córdoba increased from 269 to 427 accessions. In addition, a new diversity hot spot was identified in the northern regions of La Rioja and Aragon. According to the authors this adds to the great diversity already described in Spanish olive germplasm. The authors highlight the risk of genetic erosion given the expansion of modern olive cultivation with only a few cultivars and argue that conserving a broad range of genotypes will be crucial to meet the future challenges of olive cultivation. To further enhance the conservation and use of these accessions, the World Olive Germplasm Bank of Córdoba was recognized in June 2024 to become an international collection following Article 15 of the International Treaty on Plant Genetic Resources for Food and Agriculture.

Adhikari et al. conducted a genomic characterization using more than 45k SNPs of the 1,041 Aegilops accessions preserved in the Wheat Genetics Resource Center (WGRC) collection. The Aegilops genus contains a range of crop wild relative species which are regarded as critical for future wheat crop improvements. The authors present phylogenetic tree and principal component analyses that showed some species overlap but also pathways of species evolution and diversification. The high genetic diversity identified among the species indicate their importance as genetic resources for future wheat breeding. For genebank curation the study found 49 misclassified and 28 sets of redundant accessions in the WGRC collection.

Berkner et al. worked on genomic prediction to identify winter wheat accessions that can be used to breed varieties with a combined high protein and high lysine content. Their point of departure was a large-scale screening that was conducted back in the 1970s at the Leibniz Institute of Plant Genetics and Crop Plant Research in Germany. Additionally, they used a genomic dataset generated in 2022. The datasets were curated, four genomic prediction approaches were compared, and the best model was used to predict the traits of interests. Out of the 7,651 accessions included in the predictions, five accessions were highlighted as combining outstanding high protein content with high lysine content.

Chao et al. examined the genetic diversity and population structure of annual medicks ( Medicago spp.) from the Crimean Peninsula of Ukraine collected in 2008. The colleting mission was done to fill gaps at National Plant Germplasm System in USA and 102 accessions from 10 species were collected. The authors present the results from characterization work, which included 24 phenotypic descriptors and a 3k SNP marker set developed for lucerne (alfalfa). The results showed a high reproducibility between single and pooled biological replicate leaf samples, which indicates that sampling individual plants for these mostly self-pollinating species is sufficient. According to the authors the phenotypic descriptors and the applied SNP marker set was useful in assessing the population structure.

Liu et al. conducted a complete mitochondrial genome characterization and phylogenetic analysis of the endangered species Prunus pedunculata in China. The authors highlight that the results provide a basis for understanding the evolution of the genetic background and genetic breeding of Prunus.

Abondano et al. compared single plants, multiple plants, and DNA pools sampling strategies for DArTseq genotyping common bean ( Phaseolus vulgaris L.) landraces from the Alliance Biodiversity and CIAT gene bank. They concluded that pooling tissue from 25 individual plants per accession was a viable approach for characterizing germplasm compared to genotyping individual plants separately by balancing genotyping effort and costs. The results add valuable insights for characterization of collections and in marker-trait association studies.

Loarca et al. evaluated shoot-growth variation in a diversity panel of 695 accessions of carrot ( Daucus carota L.) from the United States Department of Agriculture National Plant Germplasm System. They found phenotypic variability for seedling emergence and early-season canopy coverage, which is indicating quantitative inheritance and potential for improvement through plant breeding. Accessions with high emergence and vigorous canopy growth are of immediate use to breeders targeting stand establishment, weed-tolerance, or weed-suppressant carrots, which is of advantage to the organic carrot production. In a second paper Loarca et al. evaluated flowering habit trait of the same accessions. They found a high broad-sense heritability for biennial flowering habit which indicates a strong genetic component of this trait.

Li et al. conducted phylogeographic analysis of the native grass Elymus nutans using microsatellite markers and covering 361 individual plants across 35 populations from the Qinghai-Tibetan plateau. The species has pastoral and environmental importance, and the study unveiled a notable degree of genetic diversity. Correlations were established between external environmental factors and effective alleles potentially linked to glutathione S-transferases T1 or hypothetical proteins, which are affecting environmental adaptation.

An et al. analyzed the genetic diversity and structure of a perennial evergreen tree Albizia odoratissima using 16 simple sequence repeat markers and covering 280 individuals across 10 populations from Hainan Island and mainland China. The genetic diversity of Hainan population was lower than that of the mainland population Furthermore there were significant differences in the genetic structure between Hainan and mainland populations.

Lu et al. examined the diversity of an herbaceous climber Dioscorea bulbifera native to Africa and Asia and locally used as vegetable and medicine. The study included accessions from mainland China and Taiwan that were analyzed using SSR marker and phylogenetic analyses. They showed structural features across accessions and three distinct clades indicating potential genetic divergence among populations from different geographic regions in China and Taiwan.

Yang et al. present a characterization work on an unusual type of horny goat weed ( Epimedium koreanum Nakai) discovered in the Jilin Province in China. Horny goat weed is a well-known traditional Chinese medicinal herb that is collected from natural habitats. The newly discovered type had much higher number of leaflets than commonly found (27 compared to 9). By DNA barcoding this novel type was identified as E. koreanum. Parallel RNA-seq analysis showed 1171 differentially expressed genes compared to wild type. Due to a decreasing natural population cultivation could be an alternative source for utilization and this high leaf-yielding Epimedium plant could be potentially used in breeding or cultivation.

Breeding and seed systems

Two research articles are presented, which includes one article on international breeding collaboration to achieve frost tolerance in potato, and one article overviewing breeding seeds as part of the official seed system in India.

Arcos-Pineda et al. report the results from an international breeding project using a wild potato relative Solanum commersonii that resulted in two new frost-tolerant native potato cultivars for the Andes and the Altiplano. The project was a collaboration between partners from USA and Peru as well as the International Potato Center (CIP). After 8 years of breeding the two new cultivars were released. The project shows that international collaboration and the use of valuable genetic diversity can produce results of importance for food security.

Chand et al. provide a retrospective overview on the seed production system of lucerne ( Medicago sativa L.) in India. Out of 14 lucerne varieties released and notified over the past 24 years, only nine entered the seed chain. The varietal replacement rate was found to be moderate, and the authors present a holistic overview and a way forward to develop more varieties and improved production of certified seeds in the country.

The provided contributions cover a good mix of topics related to conservation and utilization of plant genetic resources from across the globe, but we miss reports from Africa. We know that more and more research is being done on neglected and underutilized species in this region. On the other hand, we note active work on these matters especially in China. We further note a strong molecular emphasis but that many challenges of managing genetic resources be in genebanks or in in situ situations are the same as they were before.

Author contributions

SS: Writing – original draft, Writing – review & editing. MZ: Writing – original draft, Writing – review & editing. AD: Writing – original draft, Writing – review & editing.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Keywords: conservation, crop wild relatives, diversity, evaluation, food security, genebank, plant genetic resources, seed bank

Citation: Solberg SØ, van Zonneveld M and Diederichsen A (2024) Editorial: Advances in conservation and utilization of plant genetic resources. Front. Plant Sci. 15:1468904. doi: 10.3389/fpls.2024.1468904

Received: 22 July 2024; Accepted: 29 July 2024; Published: 07 August 2024.

Edited and Reviewed by:

Copyright © 2024 Solberg, van Zonneveld and Diederichsen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Svein Øivind Solberg, [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Analysis of bngpat9 gene expression patterns in brassica napus and its impact on seed oil content, 1. introduction, 2. materials and methods, 2.1. materials, 2.2. cloning and bioinformatics analysis of bngpat9 genes and promoters, 2.3. plant expression vector construction and genetic transformation, 2.4. gus staining of plant tissues.

• Immerse the plant tissue samples in 1.5 mL centrifuge tubes containing 90% pre-chilled acetone and place them on ice for 10 min to fix the tissues.
• Remove the acetone and rinse 2–3 times with a staining buffer (rinse solution: 50 mM NaPO 4 , pH = 7.2; 0.5 mM K 3 Fe(CN) 6 ; 0.5 mM K 4 Fe(CN) 6 ), with each rinse lasting 2 min.
• Remove the rinse solution and add 2 mM X-Gluc staining solution (X-Gluc diluted with rinse solution, stored at −20 °C in the dark) to immerse the plant tissues. Apply a vacuum for 10 min to ensure thorough contact with the X-Gluc staining solution.
• Incubate at 37 °C for 10–20 h for adequate staining.
• Remove the staining solution and add 75% ethanol for 1 h to decolorize (if decolorization is insufficient, this step can be repeated).
• Add 95% ethanol and store at 4 °C for long-term preservation or observe and photograph directly under a stereomicroscope.

2.5. Determination of Oil Content and Fatty Acid Composition of Seeds

• Weigh 10 mg of seeds.
• Add 2 mL of 2.5% H 2 SO 4 in methanol (0.01% BHT).
• Add 100 μL of 16.2 μmol/mL (C17:0) fatty acid (FA) as an internal standard.
• For rapeseed, gently crush the seeds with a glass rod (not necessary for Arabidopsis).
• Incubate in a water bath at 85 °C for 2 h, checking every 10–20 min to ensure the lid is not loose or leaking.
• After cooling to room temperature, add 2 mL of ddH 2 O and 2 mL of n-hexane, and shake well.
• Centrifuge at 1000 rpm for 10 min.
• Take approximately 1 mL of the supernatant into the sample vial.
• Inject 1 μL of the sample into the gas chromatograph (GC), using a split ratio of 2:1 to 10:1.

2.6. RNA Extraction and qRT-PCR Analysis

2.7. transcriptome sequencing and differential expression genes screening, 3.1. characteristic analysis of bngpat9 in b. napus, 3.2. analysis of promoter cis-acting elements of bngpat9, 3.3. analysis of expression pattern of bngpat9, 3.4. tissue-specific expression analysis of bngpat9 genes and oil content analysis in seeds, 3.5. the role of the bngpat9 gene in seed oil accumulation, 3.6. transcriptome sequencing and deg analysis of developing siliques in bngpat9 transgenic arabidopsis, 3.7. kegg enrichment analysis of differentially expressed genes, 4. discussion, 5. conclusions, supplementary materials, author contributions, data availability statement, conflicts of interest.

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Xing, M.; Hong, B.; Lv, M.; Lan, X.; Zhang, D.; Shu, C.; Qi, S.; Peng, Z.; Guan, C.; Xiong, X.; et al. Analysis of BnGPAT9 Gene Expression Patterns in Brassica napus and Its Impact on Seed Oil Content. Agriculture 2024 , 14 , 1334. https://doi.org/10.3390/agriculture14081334

Xing M, Hong B, Lv M, Lan X, Zhang D, Shu C, Qi S, Peng Z, Guan C, Xiong X, et al. Analysis of BnGPAT9 Gene Expression Patterns in Brassica napus and Its Impact on Seed Oil Content. Agriculture . 2024; 14(8):1334. https://doi.org/10.3390/agriculture14081334

Xing, Man, Bo Hong, Mengjie Lv, Xueyi Lan, Danhui Zhang, Chunlei Shu, Shucheng Qi, Zechuan Peng, Chunyun Guan, Xinghua Xiong, and et al. 2024. "Analysis of BnGPAT9 Gene Expression Patterns in Brassica napus and Its Impact on Seed Oil Content" Agriculture 14, no. 8: 1334. https://doi.org/10.3390/agriculture14081334

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S. curry g - usa, b. bogdanovic sg - srb, game information, game highlights, video highlights, kerr: steph always comes up huge in big moments, kd: usa-serbia one of best games i've ever been a part of, how u.s. men's basketball stormed back against serbia.

• Bogdan Bogdanovic made Jumper. Assisted by Aleksa Avramovic.
• Filip Petrusev made Three Point Jumper. Assisted by Bogdan Bogdanovic.
• Nikola Jokic made Dunk. Assisted by Aleksa Avramovic.
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• Filip Petrusev made Jumper.
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• Aleksa Avramovic made Three Point Jumper. Assisted by Nikola Jovic.
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• Ognjen Dobric made Three Point Jumper. Assisted by Bogdan Bogdanovic.
• Vasilije Micic made Jumper.
• Vasilije Micic made Three Point Jumper. Assisted by Nikola Jokic.
• Marko Guduric made Three Point Jumper.
• Marko Guduric missed Three Point Jumper.
• Nikola Jokic made Jumper. Assisted by Vasilije Micic.
• Nikola Milutinov made Two Point Tip Shot.
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• Filip Petrusev made Dunk. Assisted by Nikola Jokic.
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• Stephen Curry made Jumper. Assisted by LeBron James.
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Fact Checking Trump’s Mar-a-Lago News Conference

The former president took questions from reporters for more than hour. We examined his claims, attacks and policy positions.

By The New York Times

• Share full article

Former President Donald J. Trump held an hourlong news conference with reporters on Thursday at his Mar-a-Lago club in Florida, during which he attacked Vice President Kamala Harris, his general election opponent, criticized the Biden administration’s policies and boasted of the crowd size at his rallies. We took a closer look at many of his claims.

Trump claims his Jan. 6 rally crowd rivaled the 1963 March on Washington. Estimates say otherwise.

“If you look at Martin Luther King, when he did his speech, his great speech. And you look at ours, same real estate, same everything, same number of people. If not, we had more.” — Former President Donald J. Trump

This lacks evidence.

Mr. Trump was talking about the crowds gathered for his speech on Jan. 6, 2021, and for the “I Have a Dream” speech the Rev. Dr. Martin Luther King Jr. delivered during the March on Washington in 1963. While it is difficult to gauge exact crowd sizes, estimates counter Mr. Trump’s claim that the numbers gathered were comparable. Dr. King’s speech drew an estimated 250,000 people . The House Select Committee responsible for investigating the events of Jan. 6 estimated that Mr. Trump’s speech drew 53,000 people.

“She wants to take away your guns.”

— Former President Donald J. Trump

Ms. Harris, in 2019, said she supports a gun buyback program for assault weapons, not all guns. Her campaign told The New York Times recently that she no longer supports a buyback program.

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Peter Baker

“They take the strategic national reserves. They’re virtually empty now. We have never had it this low.”

This is exaggerated..

President Biden has indeed tapped the Strategic Petroleum Reserve to try to mitigate gasoline price increases , drawing it down by about 40 percent from when he took office, and it is currently at the lowest level since the 1980s. But it still has 375 million barrels in it now , which is not “virtually empty” nor is it at the lowest level ever.

“The vast majority of the country does support me.”

Mr. Trump never won a majority of the popular vote in either of the elections he ran in and never had the approval of a majority of Americans in a single day of Gallup polling during his presidency. An average of polls by FiveThirtyEight.com shows that he is viewed favorably by just 43 percent of Americans today and has the same level of support in a matchup against Vice President Kamala Harris.

Alan Rappeport

“They’re going to destroy Social Security.”

President Biden and Vice President Kamala Harris have pledged not to make any cuts to America’s social safety net programs. Mr. Trump suggested this year that he was open to scaling back the programs when he said there was “a lot you can do in terms of entitlements in terms of cutting.” He later walked back those comments and pledged to protect the programs. But if changes to the programs are not made, the programs’ benefits will automatically be reduced eventually. Government reports released earlier this year projected that the Social Security and disability insurance programs, if combined, would not have enough money to pay all of their obligations in 2035. Medicare will be unable to pay all its hospital bills starting in 2036.

Coral Davenport

“Everybody is going to be forced to buy an electric car.”

While the Biden administration has enacted regulations designed to ensure that the majority of new passenger cars and light trucks sold in the United States are all-electric or hybrids by 2032, the rules do not require consumers to buy electric vehicles.

“Our tax cuts, which are the biggest in history.”

The $1.5 trillion tax cut, enacted in December 2017, ranks below at least half a dozen others by several metrics. The 1981 tax cut enacted under President Ronald Reagan is the largest as a percentage of the economy and by its reduction to federal revenue. The 2012 cut enacted under President Barack Obama amounted to the largest cut in inflation-adjusted dollars: $321 billion a year.

“They’re drilling now because they had to go back because gasoline was going up to seven, eight, nine dollars a barrel. The day after the election, if they won, you’re going to have fuel prices go through the roof.”

The price of gasoline reached a low of $1.98 per gallon in April 2020, when Mr. Trump was president, chiefly as a result of the drop in driving in the first months of the Covid pandemic. It rose to a peak of $5 per gallon in June 2022, but has since steadily dropped to $3.60 per gallon in July 2024. The United States has steadily increased its oil production over the last decade, becoming the world’s largest producer of oil in 2018, a status it still holds today .

“If you go back and check your records for 18 months, I had a talk with Abdul. Abdul was the leader of the Taliban still is, but had a strong talk with him. For 18 months. Not one American soldier was shot at or killed, but not even shot at 18 months.”

Mr. Trump spoke with a leader of the Taliban in March 2020. In the 18 months that followed, from April 2020 to October 2021, 13 soldiers died in hostile action in Afghanistan.

“Democrats are really the radical ones on this, because they’re allowed to do abortion on the eighth and ninth month, and even after birth.”

No state has passed a law allowing for the execution of a baby after it is born, which is infanticide. Moreover, abortions later in pregnancy are very rare: In 2021, less than 1 percent of abortions happened after 21 weeks’ gestation, according to a Centers for Disease Control and Prevention report based on data from state and other health agencies. More than 90 percent of abortions happened within 13 weeks of gestation.

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AP®︎/College Biology

Course: ap®︎/college biology   >   unit 3.

• Photosynthesis
• Intro to photosynthesis
• Breaking down photosynthesis stages
• Conceptual overview of light dependent reactions
• The light-dependent reactions

The Calvin cycle

• Photosynthesis evolution
• Photosynthesis review

Introduction

Overview of the calvin cycle, reactions of the calvin cycle.

• Carbon fixation. A CO 2 ‍   molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate ( RuBP ). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco . Details of this step The first stage of the Calvin cycle incorporates carbon from CO 2 ‍   into an organic molecule, a process called carbon fixation . In plants, atmospheric CO 2 ‍   enters the mesophyll layer of leaves by passing through pores on the leaf surface called stomata. It can then diffuse into mesophyll cells, and into the stroma of chloroplasts, where the Calvin cycle takes place. In the first step of the cycle, an enzyme nicknamed rubisco (RuBP carboxylase-oxygenase) catalyzes attachment of CO 2 ‍   to a five-carbon sugar called ribulose bisphosphate (RuBP ). The resulting 6-carbon molecule is unstable, however, and quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA). Thus, for each CO 2 ‍   that enters the cycle, two 3-PGA molecules are produced. The actual molecular structures are show below:
• First, each molecule of 3-PGA receives a phosphate group from ATP, turning into a doubly phosphorylated molecule called 1,3-bisphosphoglycerate (and leaving behind ADP as a by-product).
• Second, the 1,3-bisphosphoglycerate molecules are reduced (gain electrons). Each molecule receives two electrons from NADPH and loses one of its phosphate groups, turning into a three-carbon sugar called glyceraldehyde 3-phosphate (G3P) . This step produces NADP + ‍   and phosphate ( P i ‍   ) as by-products.
• Regeneration. Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions, which my college bio professor liked to call the "carbohydrate scramble." 1 ‍  

Summary of Calvin cycle reactants and products

• 1 ‍   G3P molecule exits the cycle and goes towards making glucose.
• 5 ‍   G3P molecules are recycled, regenerating 3 ‍   RuBP acceptor molecules.
• ATP. 9 ‍   ATP are converted to 9 ‍   ADP ( 6 ‍   during the reduction step, 3 ‍   during the regeneration step).
• NADPH . 6 ‍   NADPH are converted to 6 ‍   NADP + ‍   (during the reduction step).

Attribution:

• " The Calvin cycle ," by OpenStax College, Concepts of Biology, CC BY 4.0 . Download the original article for free at http://cnx.org/contents/[email protected] .
• " Using light energy to make organic molecules ," by OpenStax College, Biology, CC BY 4.0 . Download the original article for free at http://cnx.org/contents/[email protected] .

Works cited:

• Koning, R. E. (1994). Calvin cycle. In Plant physiology information website . Retrieved from http://plantphys.info/plant_physiology/calvincycle.shtml .

References:

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