biology photosynthesis essay

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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.

photosynthesis

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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.

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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Related Resources

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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Photosynthesis

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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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An overview of photosynthesis

How the photosystems work, other electron transfer chain components, abbreviations, competing interests, recommended reading and key publications, photosynthesis.

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Matthew P. Johnson; Photosynthesis. Essays Biochem 31 October 2016; 60 (3): 255–273. doi: https://doi.org/10.1042/EBC20160016

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Photosynthesis sustains virtually all life on planet Earth providing the oxygen we breathe and the food we eat; it forms the basis of global food chains and meets the majority of humankind's current energy needs through fossilized photosynthetic fuels. The process of photosynthesis in plants is based on two reactions that are carried out by separate parts of the chloroplast. The light reactions occur in the chloroplast thylakoid membrane and involve the splitting of water into oxygen, protons and electrons. The protons and electrons are then transferred through the thylakoid membrane to create the energy storage molecules adenosine triphosphate (ATP) and nicotinomide–adenine dinucleotide phosphate (NADPH). The ATP and NADPH are then utilized by the enzymes of the Calvin–Benson cycle (the dark reactions), which converts CO 2 into carbohydrate in the chloroplast stroma. The basic principles of solar energy capture, energy, electron and proton transfer and the biochemical basis of carbon fixation are explained and their significance is discussed.

Introduction

Photosynthesis is the ultimate source of all of humankind's food and oxygen, whereas fossilized photosynthetic fuels provide ∼87% of the world's energy. It is the biochemical process that sustains the biosphere as the basis for the food chain. The oxygen produced as a by-product of photosynthesis allowed the formation of the ozone layer, the evolution of aerobic respiration and thus complex multicellular life.

Oxygenic photosynthesis involves the conversion of water and CO 2 into complex organic molecules such as carbohydrates and oxygen. Photosynthesis may be split into the ‘light’ and ‘dark’ reactions. In the light reactions, water is split using light into oxygen, protons and electrons, and in the dark reactions, the protons and electrons are used to reduce CO 2 to carbohydrate (given here by the general formula CH 2 O). The two processes can be summarized thus:

Light reactions:

Dark reactions:

The positive sign of the standard free energy change of the reaction (Δ G °) given above means that the reaction requires energy ( an endergonic reaction ). The energy required is provided by absorbed solar energy, which is converted into the chemical bond energy of the products ( Box 1 ).

Photosynthesis converts ∼200 billion tonnes of CO 2 into complex organic compounds annually and produces ∼140 billion tonnes of oxygen into the atmosphere. By facilitating conversion of solar energy into chemical energy, photosynthesis acts as the primary energy input into the global food chain. Nearly all living organisms use the complex organic compounds derived from photosynthesis as a source of energy. The breakdown of these organic compounds occurs via the process of aerobic respiration, which of course also requires the oxygen produced by photosynthesis.

Unlike photosynthesis, aerobic respiration is an exergonic process (negative Δ G °) with the energy released being used by the organism to power biosynthetic processes that allow growth and renewal, mechanical work (such as muscle contraction or flagella rotation) and facilitating changes in chemical concentrations within the cell (e.g. accumulation of nutrients and expulsion of waste). The use of exergonic reactions to power endergonic ones associated with biosynthesis and housekeeping in biological organisms such that the overall free energy change is negative is known as ‘ coupling’.

Photosynthesis and respiration are thus seemingly the reverse of one another, with the important caveat that both oxygen formation during photosynthesis and its utilization during respiration result in its liberation or incorporation respectively into water rather than CO 2 . In addition, glucose is one of several possible products of photosynthesis with amino acids and lipids also being synthesized rapidly from the primary photosynthetic products.

The consideration of photosynthesis and respiration as opposing processes helps us to appreciate their role in shaping our environment. The fixation of CO 2 by photosynthesis and its release during breakdown of organic molecules during respiration, decay and combustion of organic matter and fossil fuels can be visualized as the global carbon cycle ( Figure 1 ).

The global carbon cycle

The relationship between respiration, photosynthesis and global CO 2 and O 2 levels.

At present, this cycle may be considered to be in a state of imbalance due to the burning of fossil fuels (fossilized photosynthesis), which is increasing the proportion of CO 2 entering the Earth's atmosphere, leading to the so-called ‘greenhouse effect’ and human-made climate change.

Oxygenic photosynthesis is thought to have evolved only once during Earth's history in the cyanobacteria. All other organisms, such as plants, algae and diatoms, which perform oxygenic photosynthesis actually do so via cyanobacterial endosymbionts or ‘chloroplasts’. An endosymbiotoic event between an ancestral eukaryotic cell and a cyanobacterium that gave rise to plants is estimated to have occurred ∼1.5 billion years ago. Free-living cyanobacteria still exist today and are responsible for ∼50% of the world's photosynthesis. Cyanobacteria themselves are thought to have evolved from simpler photosynthetic bacteria that use either organic or inorganic compounds such a hydrogen sulfide as a source of electrons rather than water and thus do not produce oxygen.

The site of photosynthesis in plants

In land plants, the principal organs of photosynthesis are the leaves ( Figure 2 A). Leaves have evolved to expose the largest possible area of green tissue to light and entry of CO 2 to the leaf is controlled by small holes in the lower epidermis called stomata ( Figure 2 B). The size of the stomatal openings is variable and regulated by a pair of guard cells, which respond to the turgor pressure (water content) of the leaf, thus when the leaf is hydrated, the stomata can open to allow CO 2 in. In contrast, when water is scarce, the guard cells lose turgor pressure and close, preventing the escape of water from the leaf via transpiration.

Location of the photosynthetic machinery

( A ) The model plant Arabidopsis thaliana . ( B ) Basic structure of a leaf shown in cross-section. Chloroplasts are shown as green dots within the cells. ( C ) An electron micrograph of an Arabidopsis chloroplast within the leaf. ( D ) Close-up region of the chloroplast showing the stacked structure of the thylakoid membrane.

Within the green tissue of the leaf (mainly the mesophyll) each cell (∼100 μm in length) contains ∼100 chloroplasts (2–3 μm in length), the tiny organelles where photosynthesis takes place. The chloroplast has a complex structure ( Figure 2 C, D) with two outer membranes (the envelope), which are colourless and do not participate in photosynthesis, enclosing an aqueous space (the stroma) wherein sits a third membrane known as the thylakoid, which in turn encloses a single continuous aqueous space called the lumen.

The light reactions of photosynthesis involve light-driven electron and proton transfers, which occur in the thylakoid membrane, whereas the dark reactions involve the fixation of CO 2 into carbohydrate, via the Calvin–Benson cycle, which occurs in the stroma ( Figure 3 ). The light reactions involve electron transfer from water to NADP + to form NADPH and these reactions are coupled to proton transfers that lead to the phosphorylation of adenosine diphosphate (ADP) into ATP. The Calvin–Benson cycle uses ATP and NADPH to convert CO 2 into carbohydrates ( Figure 3 ), regenerating ADP and NADP + . The light and dark reactions are therefore mutually dependent on one another.

Division of labour within the chloroplast

The light reactions of photosynthesis take place in the thylakoid membrane, whereas the dark reactions are located in the chloroplast stroma.

Photosynthetic electron and proton transfer chain

The light-driven electron transfer reactions of photosynthesis begin with the splitting of water by Photosystem II (PSII). PSII is a chlorophyll–protein complex embedded in the thylakoid membrane that uses light to oxidize water to oxygen and reduce the electron acceptor plastoquinone to plastoquinol. Plastoquinol in turn carries the electrons derived from water to another thylakoid-embedded protein complex called cytochrome b 6 f (cyt b 6 f ). cyt b 6 f oxidizes plastoquinol to plastoquinone and reduces a small water-soluble electron carrier protein plastocyanin, which resides in the lumen. A second light-driven reaction is then carried out by another chlorophyll protein complex called Photosystem I (PSI). PSI oxidizes plastocyanin and reduces another soluble electron carrier protein ferredoxin that resides in the stroma. Ferredoxin can then be used by the ferredoxin–NADP + reductase (FNR) enzyme to reduce NADP + to NADPH. This scheme is known as the linear electron transfer pathway or Z-scheme ( Figure 4 ).

The photosynthetic electron and proton transfer chain

The linear electron transfer pathway from water to NADP + to form NADPH results in the formation of a proton gradient across the thylakoid membrane that is used by the ATP synthase enzyme to make ATP.

The Z-scheme, so-called since it resembles the letter ‘Z’ when turned on its side ( Figure 5 ), thus shows how the electrons move from the water–oxygen couple (+820 mV) via a chain of redox carriers to NADP + /NADPH (−320 mV) during photosynthetic electron transfer. Generally, electrons are transferred from redox couples with low potentials (good reductants) to those with higher potentials (good oxidants) (e.g. during respiratory electron transfer in mitochondria) since this process is exergonic (see Box 2 ). However, photosynthetic electron transfer also involves two endergonic steps, which occur at PSII and at PSI and require an energy input in the form of light. The light energy is used to excite an electron within a chlorophyll molecule residing in PSII or PSI to a higher energy level; this excited chlorophyll is then able to reduce the subsequent acceptors in the chain. The oxidized chlorophyll is then reduced by water in the case of PSII and plastocyanin in the case of PSI.

Z-scheme of photosynthetic electron transfer

The main components of the linear electron transfer pathway are shown on a scale of redox potential to illustrate how two separate inputs of light energy at PSI and PSII result in the endergonic transfer of electrons from water to NADP + .

The water-splitting reaction at PSII and plastoquinol oxidation at cyt b 6 f result in the release of protons into the lumen, resulting in a build-up of protons in this compartment relative to the stroma. The difference in the proton concentration between the two sides of the membrane is called a proton gradient. The proton gradient is a store of free energy (similar to a gradient of ions in a battery) that is utilized by a molecular mechanical motor ATP synthase, which resides in the thylakoid membrane ( Figure 4 ). The ATP synthase allows the protons to move down their concentration gradient from the lumen (high H + concentration) to the stroma (low H + concentration). This exergonic reaction is used to power the endergonic synthesis of ATP from ADP and inorganic phosphate (P i ). This process of photophosphorylation is thus essentially similar to oxidative phosphorylation, which occurs in the inner mitochondrial membrane during respiration.

An alternative electron transfer pathway exists in plants and algae, known as cyclic electron flow. Cyclic electron flow involves the recycling of electrons from ferredoxin to plastoquinone, with the result that there is no net production of NADPH; however, since protons are still transferred into the lumen by oxidation of plastoquinol by cyt b 6 f , ATP can still be formed. Thus photosynthetic organisms can control the ratio of NADPH/ATP to meet metabolic need by controlling the relative amounts of cyclic and linear electron transfer.

Light absorption by pigments

Photosynthesis begins with the absorption of light by pigments molecules located in the thylakoid membrane. The most well-known of these is chlorophyll, but there are also carotenoids and, in cyanobacteria and some algae, bilins. These pigments all have in common within their chemical structures an alternating series of carbon single and double bonds, which form a conjugated system π–electron system ( Figure 6 ).

Major photosynthetic pigments in plants

The chemical structures of the chlorophyll and carotenoid pigments present in the thylakoid membrane. Note the presence in each of a conjugated system of carbon–carbon double bonds that is responsible for light absorption.

The variety of pigments present within each type of photosynthetic organism reflects the light environment in which it lives; plants on land contain chlorophylls a and b and carotenoids such as β-carotene, lutein, zeaxanthin, violaxanthin, antheraxanthin and neoxanthin ( Figure 6 ). The chlorophylls absorb blue and red light and so appear green in colour, whereas carotenoids absorb light only in the blue and so appear yellow/red ( Figure 7 ), colours more obvious in the autumn as chlorophyll is the first pigment to be broken down in decaying leaves.

Basic absorption spectra of the major chlorophyll and carotenoid pigments found in plants

Chlorophylls absorb light energy in the red and blue part of the visible spectrum, whereas carotenoids only absorb light in the blue/green.

Light, or electromagnetic radiation, has the properties of both a wave and a stream of particles (light quanta). Each quantum of light contains a discrete amount of energy that can be calculated by multiplying Planck's constant, h (6.626×10 −34 J·s) by ν, the frequency of the radiation in cycles per second (s −1 ):

The frequency (ν) of the light and so its energy varies with its colour, thus blue photons (∼450 nm) are more energetic than red photons (∼650 nm). The frequency (ν) and wavelength (λ) of light are related by:

where c is the velocity of light (3.0×10 8 m·s −1 ), and the energy of a particular wavelength (λ) of light is given by:

Thus 1 mol of 680 nm photons of red light has an energy of 176 kJ·mol −1 .

The electrons within the delocalized π system of the pigment have the ability to jump up from the lowest occupied molecular orbital (ground state) to higher unoccupied molecular electron orbitals (excited states) via the absorption of specific wavelengths of light in the visible range (400–725 nm). Chlorophyll has two excited states known as S 1 and S 2 and, upon interaction of the molecule with a photon of light, one of its π electrons is promoted from the ground state (S 0 ) to an excited state, a process taking just 10 −15 s ( Figure 8 ). The energy gap between the S 0 and S 1 states is spanned by the energy provided by a red photon (∼600–700 nm), whereas the energy gap between the S 0 and S 2 states is larger and therefore requires a more energetic (shorter wavelength, higher frequency) blue photon (∼400–500 nm) to span the energy gap.

Jablonski diagram of chlorophyll showing the possible fates of the S 1 and S 2 excited states and timescales of the transitions involved

Photons with slightly different energies (colours) excite each of the vibrational substates of each excited state (as shown by variation in the size and colour of the arrows).

Upon excitation, the electron in the S 2 state quickly undergoes losses of energy as heat through molecular vibration and undergoes conversion into the energy of the S 1 state by a process called internal conversion. Internal conversion occurs on a timescale of 10 −12 s. The energy of a blue photon is thus rapidly degraded to that of a red photon. Excitation of the molecule with a red photon would lead to promotion of an electron to the S 1 state directly. Once the electron resides in the S 1 state, it is lower in energy and thus stable on a somewhat longer timescale (10 −9 s). The energy of the excited electron in the S 1 state can have one of several fates: it could return to the ground state (S 0 ) by emission of the energy as a photon of light (fluorescence), or it could be lost as heat due to internal conversion between S 1 and S 0 . Alternatively, if another chlorophyll is nearby, a process known as excitation energy transfer (EET) can result in the non-radiative exchange of energy between the two molecules ( Figure 9 ). For this to occur, the two chlorophylls must be close by (<7 nm), have a specific orientation with respect to one another, and excited state energies that overlap (are resonant) with one another. If these conditions are met, the energy is exchanged, resulting in a mirror S 0 →S 1 transition in the acceptor molecule and a S 1 →S 0 transition in the other.

Basic mechanism of excitation energy transfer between chlorophyll molecules

Two chlorophyll molecules with resonant S 1 states undergo a mirror transition resulting in the non-radiative transfer of excitation energy between them.

Light-harvesting complexes

In photosynthetic systems, chlorophylls and carotenoids are found attached to membrane-embedded proteins known as light-harvesting complexes (LHCs). Through careful binding and orientation of the pigment molecules, absorbed energy can be transferred among them by EET. Each pigment is bound to the protein by a series of non-covalent bonding interactions (such as, hydrogen bonds, van der Waals interactions, hydrophobic interaction and co-ordination bonds between lone pair electrons of residues such as histidine in the protein and the Mg 2+ ion in chlorophyll); the protein structure is such that each bound pigment experiences a slightly different environment in terms of the surrounding amino acid side chains, lipids, etc., meaning that the S 1 and S 2 energy levels are shifted in energy with respect to that of other neighbouring pigment molecules. The effect is to create a range of pigment energies that act to ‘funnel’ the energy on to the lowest-energy pigments in the LHC by EET.

Reaction centres

A photosystem consists of numerous LHCs that form an antenna of hundreds of pigment molecules. The antenna pigments act to collect and concentrate excitation energy and transfer it towards a ‘special pair’ of chlorophyll molecules that reside in the reaction centre (RC) ( Figure 10 ). Unlike the antenna pigments, the special pair of chlorophylls are ‘redox-active’ in the sense that they can return to the ground state (S 0 ) by the transfer of the electron residing in the S 1 excited state (Chl*) to another species. This process is known as charge separation and result in formation of an oxidized special pair (Chl + ) and a reduced acceptor (A − ). The acceptor in PSII is plastoquinone and in PSI it is ferredoxin. If the RC is to go on functioning, the electron deficiency on the special pair must be made good, in PSII the electron donor is water and in PSI it is plastocyanin.

Basic structure of a photosystem

Light energy is captured by the antenna pigments and transferred to the special pair of RC chlorophylls which undergo a redox reaction leading to reduction of an acceptor molecule. The oxidized special pair is regenerated by an electron donor.

It is worth asking why photosynthetic organisms bother to have a large antenna of pigments serving an RC rather than more numerous RCs. The answer lies in the fact that the special pair of chlorophylls alone have a rather small spatial and spectral cross-section, meaning that there is a limit to the amount of light they can efficiently absorb. The amount of light they can practically absorb is around two orders of magnitude smaller than their maximum possible turnover rate, Thus LHCs act to increase the spatial (hundreds of pigments) and spectral (several types of pigments with different light absorption characteristics) cross-section of the RC special pair ensuring that its turnover rate runs much closer to capacity.

Photosystem II

PSII is a light-driven water–plastoquinone oxidoreductase and is the only enzyme in Nature that is capable of performing the difficult chemistry of splitting water into protons, electrons and oxygen ( Figure 11 ). In principle, water is an extremely poor electron donor since the redox potential of the water–oxygen couple is +820 mV. PSII uses light energy to excite a special pair of chlorophylls, known as P680 due to their 680 nm absorption peak in the red part of the spectrum. P680* undergoes charge separation that results in the formation of an extremely oxidizing species P680 + which has a redox potential of +1200 mV, sufficient to oxidize water. Nonetheless, since water splitting involves four electron chemistry and charge separation only involves transfer of one electron, four separate charge separations (turnovers of PSII) are required to drive formation of one molecule of O 2 from two molecules of water. The initial electron donation to generate the P680 from P680 + is therefore provided by a cluster of manganese ions within the oxygen-evolving complex (OEC), which is attached to the lumen side of PSII ( Figure 12 ). Manganese is a transition metal that can exist in a range of oxidation states from +1 to +5 and thus accumulates the positive charges derived from each light-driven turnover of P680. Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P680 within PSII by light and is known as the S-state cycle ( Figure 12 ). After the fourth turnover of P680, sufficient positive charge is built up in the manganese cluster to permit the splitting of water into electrons, which regenerate the original state of the manganese cluster, protons, which are released into the lumen and contribute to the proton gradient used for ATP synthesis, and the by-product O 2 . Thus charge separation at P680 provides the thermodynamic driving force, whereas the manganese cluster acts as a catalyst for the water-splitting reaction.

Basic structure of the PSII–LHCII supercomplex from spinach

The organization of PSII and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Drawn from PDB code 3JCU

S-state cycle of water oxidation by the manganese cluster (shown as circles with roman numerals representing the manganese ion oxidation states) within the PSII oxygen-evolving complex

Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P680 within PSII by light. Each of the electrons given up by the cluster is eventually repaid at the S 4 to S 0 transition when molecular oxygen (O 2 ) is formed. The protons extracted from water during the process are deposited into the lumen and contribute to the protonmotive force.

The electrons yielded by P680* following charge separation are not passed directly to plastoquinone, but rather via another acceptor called pheophytin, a porphyrin molecule lacking the central magnesium ion as in chlorophyll. Plastoquinone reduction to plastoquinol requires two electrons and thus two molecules of plastoquinol are formed per O 2 molecule evolved by PSII. Two protons are also taken up upon formation of plastoquinol and these are derived from the stroma. PSII is found within the thylakoid membrane of plants as a dimeric RC complex surrounded by a peripheral antenna of six minor monomeric antenna LHC complexes and two to eight trimeric LHC complexes, which together form a PSII–LHCII supercomplex ( Figure 11 ).

Photosystem I

PSI is a light-driven plastocyanin–ferredoxin oxidoreductase ( Figure 13 ). In PSI, the special pair of chlorophylls are known as P700 due to their 700 nm absorption peak in the red part of the spectrum. P700* is an extremely strong reductant that is able to reduce ferredoxin which has a redox potential of −450 mV (and is thus is, in principle, a poor electron acceptor). Reduced ferredoxin is then used to generate NADPH for the Calvin–Benson cycle at a separate complex known as FNR. The electron from P700* is donated via another chlorophyll molecule and a bound quinone to a series of iron–sulfur clusters at the stromal side of the complex, whereupon the electron is donated to ferredoxin. The P700 species is regenerated form P700 + via donation of an electron from the soluble electron carrier protein plastocyanin.

Basic structure of the PSI–LHCI supercomplex from pea

The organization of PSI and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Drawn from PDB code 4XK8.

PSI is found within the thylakoid membrane as a monomeric RC surrounded on one side by four LHC complexes known as LHCI. The PSI–LHCI supercomplex is found mainly in the unstacked regions of the thylakoid membrane ( Figure 13 ).

Plastoquinone/plastoquinol

Plastoquinone is a small lipophilic electron carrier molecule that resides within the thylakoid membrane and carries two electrons and two protons from PSII to the cyt b 6 f complex. It has a very similar structure to that of the molecule ubiquinone (coenzyme Q 10 ) in the mitochondrial inner membrane.

Cytochrome b 6 f complex

The cyt b 6 f complex is a plastoquinol–plastocyanin oxidoreductase and possess a similar structure to that of the cytochrome bc 1 complex (complex III) in mitochondria ( Figure 14 A). As with Complex III, cyt b 6 f exists as a dimer in the membrane and carries out both the oxidation and reduction of quinones via the so-called Q-cycle. The Q-cycle ( Figure 14 B) involves oxidation of one plastoquinol molecule at the Qp site of the complex, both protons from this molecule are deposited in the lumen and contribute to the proton gradient for ATP synthesis. The two electrons, however, have different fates. The first is transferred via an iron–sulfur cluster and a haem cofactor to the soluble electron carrier plastocyanin (see below). The second electron derived from plastoquinol is passed via two separate haem cofactors to another molecule of plastoquinone bound to a separate site (Qn) on the complex, thus reducing it to a semiquinone. When a second plastoquinol molecule is oxidized at Qp, a second molecule of plastocyanin is reduced and two further protons are deposited in the lumen. The second electron reduces the semiquinone at the Qn site which, concomitant with uptake of two protons from the stroma, causes its reduction to plastoquinol. Thus for each pair of plastoquinol molecules oxidized by the complex, one is regenerated, yet all four protons are deposited into the lumen. The Q-cycle thus doubles the number of protons transferred from the stroma to the lumen per plastoquinol molecule oxidized.

( A ) Structure drawn from PDB code 1Q90. ( B ) The protonmotive Q-cycle showing how electrons from plastoquinol are passed to both plastocyanin and plastoquinone, doubling the protons deposited in the lumen for every plastoquinol molecule oxidized by the complex.

Plastocyanin

Plastocyanin is a small soluble electron carrier protein that resides in the thylakoid lumen. The active site of the plastocyanin protein binds a copper ion, which cycles between the Cu 2+ and Cu + oxidation states following its oxidation by PSI and reduction by cyt b 6 f respectively.

Ferredoxin is a small soluble electron carrier protein that resides in the chloroplast stroma. The active site of the ferredoxin protein binds an iron–sulfur cluster, which cycles between the Fe 2+ and Fe 3+ oxidation states following its reduction by PSI and oxidation by the FNR complex respectively.

Ferredoxin–NADP + reductase

The FNR complex is found in both soluble and thylakoid membrane-bound forms. The complex binds a flavin–adenine dinucleotide (FAD) cofactor at its active site, which accepts two electrons from two molecules of ferredoxin before using them reduce NADP + to NADPH.

ATP synthase

The ATP synthase enzyme is responsible for making ATP from ADP and P i ; this endergonic reaction is powered by the energy contained within the protonmotive force. According to the structure, 4.67 H + are required for every ATP molecule synthesized by the chloroplast ATP synthase. The enzyme is a rotary motor which contains two domains: the membrane-spanning F O portion which conducts protons from the lumen to the stroma, and the F 1 catalytic domain that couples this exergonic proton movement to ATP synthesis.

Membrane stacking and the regulation of photosynthesis

Within the thylakoid membrane, PSII–LHCII supercomplexes are packed together into domains known as the grana, which associate with one another to form grana stacks. PSI and ATP synthase are excluded from these stacked PSII–LHCII regions by steric constraints and thus PSII and PSI are segregated in the thylakoid membrane between the stacked and unstacked regions ( Figure 15 ). The cyt b 6 f complex, in contrast, is evenly distributed throughout the grana and stromal lamellae. The evolutionary advantage of membrane stacking is believed to be a higher efficiency of electron transport by preventing the fast energy trap PSI from ‘stealing’ excitation energy from the slower trap PSII, a phenomenon known as spillover. Another possible advantage of membrane stacking in thylakoids may be the segregation of the linear and cyclic electron transfer pathways, which might otherwise compete to reduce plastoquinone. In this view, PSII, cyt b 6 f and a sub-fraction of PSI closest to the grana is involved in linear flow, whereas PSI and cyt b 6 f in the stromal lamellae participates in cyclic flow. The cyclic electron transfer pathway recycles electrons from ferredoxin back to plastoquinone and thus allows protonmotive force generation (and ATP synthesis) without net NADPH production. Cyclic electron transfer thereby provides the additional ATP required for the Calvin–Benson cycle (see below).

Lateral heterogeneity in thylakoid membrane organization

( A ) Electron micrograph of the thylakoid membrane showing stacked grana and unstacked stromal lamellae regions. ( B ) Model showing the distribution of the major complexes of photosynthetic electron and proton transfer between the stacked grana and unstacked stromal lamellae regions.

‘Dark’ reactions: the Calvin–Benson cycle

CO 2 is fixed into carbohydrate via the Calvin–Benson cycle in plants, which consumes the ATP and NADPH produced during the light reactions and thus in turn regenerates ADP, P i and NADP + . In the first step of the Calvin–Benson cycle ( Figure 16 ), CO 2 is combined with a 5-carbon (5C) sugar, ribulose 1,5-bisphosphate in a reaction catalysed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The reaction forms an unstable 6C intermediate that immediately splits into two molecules of 3-phosphoglycerate. 3-Phosphoglycerate is first phosphorylated by 3-phosphoglycerate kinase using ATP to form 1,3-bisphosphoglycerate. 1,3-Bisphosphoglycerate is then reduced by glyceraldehyde 3-phosphate dehydrogenase using NADPH to form glyceraldehyde 3-phosphate (GAP, a triose or 3C sugar) in reactions, which are the reverse of glycolysis. For every three CO 2 molecules initially combined with ribulose 1,5-bisphopshate, six molecules of GAP are produced by the subsequent steps. However only one of these six molecules can be considered as a product of the Calvin–Benson cycle since the remaining five are required to regenerate ribulose 1,5-bisphosphate in a complex series of reactions that also require ATP. The one molecule of GAP that is produced for each turn of the cycle can be quickly converted by a range of metabolic pathways into amino acids, lipids or sugars such as glucose. Glucose in turn may be stored as the polymer starch as large granules within chloroplasts.

The Calvin–Benson cycle

Overview of the biochemical pathway for the fixation of CO 2 into carbohydrate in plants.

A complex biochemical ‘dance’ ( Figure 16 ) is then involved in the regeneration of three ribulose 1,5-bisphosphate (5C) from the remaining five GAP (3C) molecules. The regeneration begins with the conversion of two molecules of GAP into dihydroxyacetone phosphate (DHAP) by triose phosphate isomerase; one of the DHAP molecules is the combined with another GAP molecule to make fructose 1,6-bisphosphate (6C) by aldolase. The fructose 1,6-bisphosphate is then dephosphorylated by fructose-1,6-bisphosphatase to yield fructose 6-phosphate (6C) and releasing P i . Two carbons are then removed from fructose 6-phosphate by transketolase, generating erythrose 4-phosphate (4C); the two carbons are transferred to another molecule of GAP generating xylulose 5-phosphate (5C). Another DHAP molecule, formed from GAP by triose phosphate isomerase is then combined with the erythrose 4-phosphate by aldolase to form sedoheptulose 1,7-bisphosphate (7C). Sedoheptulose 1,7-bisphosphate is then dephosphorylated to sedoheptulose 7-phosphate (7C) by sedoheptulose-1,7-bisphosphatase releasing P i . Sedoheptulose 7-phosphate has two carbons removed by transketolase to produce ribose 5-phosphate (5C) and the two carbons are transferred to another GAP molecule producing another xylulose 5-phosphate (5C). Ribose 5-phosphate and the two molecules of xylulose 5-phosphate (5C) are then converted by phosphopentose isomerase to three molecules of ribulose 5-phosphate (5C). The three ribulose 5-phosphate molecules are then phosphorylated using three ATP by phosphoribulokinase to regenerate three ribulose 1,5-bisphosphate (5C).

Overall the synthesis of 1 mol of GAP requires 9 mol of ATP and 6 mol of NADPH, a required ratio of 1.5 ATP/NADPH. Linear electron transfer is generally thought to supply ATP/NADPH in a ratio of 1.28 (assuming an H + /ATP ratio of 4.67) with the shortfall of ATP believed to be provided by cyclic electron transfer reactions. Since the product of the Calvin cycle is GAP (a 3C sugar) the pathway is often referred to as C 3 photosynthesis and plants that utilize it are called C 3 plants and include many of the world's major crops such as rice, wheat and potato.

Many of the enzymes involved in the Calvin–Benson cycle (e.g. transketolase, glyceraldehyde-3-phosphate dehydrogenase and aldolase) are also involved in the glycolysis pathway of carbohydrate degradation and their activity must therefore be carefully regulated to avoid futile cycling when light is present, i.e. the unwanted degradation of carbohydrate. The regulation of the Calvin–Benson cycle enzymes is achieved by the activity of the light reactions, which modify the environment of the dark reactions (i.e. the stroma). Proton gradient formation across the thylakoid membrane during the light reactions increases the pH and also increases the Mg 2+ concentration in the stroma (as Mg 2+ flows out of the lumen as H + flows in to compensate for the influx of positive charges). In addition, by reducing ferredoxin and NADP + , PSI changes the redox state of the stroma, which is sensed by the regulatory protein thioredoxin. Thioredoxin, pH and Mg 2+ concentration play a key role in regulating the activity of the Calvin–Benson cycle enzymes, ensuring the activity of the light and dark reactions is closely co-ordinated.

It is noteworthy that, despite the complexity of the dark reactions outlined above, the carbon fixation step itself (i.e. the incorporation of CO 2 into carbohydrate) is carried out by a single enzyme, Rubisco. Rubisco is a large multisubunit soluble protein complex found in the chloroplast stroma. The complex consists of eight large (56 kDa) subunits, which contain both catalytic and regulatory domains, and eight small subunits (14 kDa), which enhance the catalytic function of the L subunits ( Figure 17 A). The carboxylation reaction carried out by Rubisco is highly exergonic (Δ G °=−51.9 kJ·mol- 1 ), yet kinetically very slow (just 3 s −1 ) and begins with the protonation of ribulose 1,5-bisphosphate to form an enediolate intermediate which can be combined with CO 2 to form an unstable 6C intermediate that is quickly hydrolysed to yield two 3C 3-phosphoglycerate molecules. The active site in the Rubisco enzyme contains a key lysine residue, which reacts with another (non-substrate) molecule of CO 2 to form a carbamate anion that is then able to bind Mg 2+ . The Mg 2+ in the active site is essential for the catalytic function of Rubisco, playing a key role in binding ribulose 1,5-bisphosphate and activating it such that it readily reacts with CO 2.. Rubisco activity is co-ordinated with that of the light reactions since carbamate formation requires both high Mg 2+ concentration and alkaline conditions, which are provided by the light-driven changes in the stromal environment discussed above ( Figure 17 B).

( A ) Structure of the Rubisco enzyme (the large subunits are shown in blue and the small subunits in green); four of each type of subunit are visible in the image. Drawn from PDB code 1RXO. ( B ) Activation of the lysine residue within the active site of Rubisco occurs via elevated stromal pH and Mg 2+ concentration as a result of the activity of the light reactions.

In addition to carboxylation, Rubisco also catalyses a competitive oxygenation reaction, known as photorespiration, that results in the combination of ribulose 1,5-bisphosphate with O 2 rather than CO 2 . In the oxygenation reaction, one rather than two molecules of 3-phosphoglycerate and one molecule of a 2C sugar known as phosphoglycolate are produced by Rubisco. The phosphoglycolate must be converted in a series of reactions that regenerate one molecule of 3-phosphoglycerate and one molecule of CO 2 . These reactions consume additional ATP and thus result in an energy loss to the plant. Although the oxygenation reaction of Rubisco is much less favourable than the carboxylation reaction, the relatively high concentration of O 2 in the leaf (250 μM) compared with CO 2 (10 μM) means that a significant amount of photorespiration is always occurring. Under normal conditions, the ratio of carboxylation to oxygenation is between 3:1 and 4:1. However, this ratio can be decreased with increasing temperature due to decreased CO 2 concentration in the leaf, a decrease in the affinity of Rubisco for CO 2 compared with O 2 and an increase in the maximum rate of the oxygenation reaction compared with the carboxylation reaction. The inefficiencies of the Rubisco enzyme mean that plants must produce it in very large amounts (∼30–50% of total soluble protein in a spinach leaf) to achieve the maximal photosynthetic rate.

CO 2 -concentrating mechanisms

To counter photorespiration, plants, algae and cyanobacteria have evolved different CO 2 -concentrating mechanisms CCMs that aim to increase the concentration of CO 2 relative to O 2 in the vicinity of Rubisco. One such CCM is C 4 photosynthesis that is found in plants such as maize, sugar cane and savanna grasses. C 4 plants show a specialized leaf anatomy: Kranz anatomy ( Figure 18 ). Kranz, German for wreath, refers to a bundle sheath of cells that surrounds the central vein within the leaf, which in turn are surrounded by the mesophyll cells. The mesophyll cells in such leaves are rich in the enzyme phosphoenolpyruvate (PEP) carboxylase, which fixes CO 2 into a 4C carboxylic acid: oxaloaceatate. The oxaloacetate formed by the mesophyll cells is reduced using NADPH to malate, another 4C acid: malate. The malate is then exported from the mesophyll cells to the bundle sheath cells, where it is decarboxylated to pyruvate thus regenerating NADPH and CO 2 . The CO 2 is then utilized by Rubisco in the Calvin cycle. The pyruvate is in turn returned to the mesophyll cells where it is phosphorylated using ATP to reform PEP ( Figure 19 ). The advantage of C 4 photosynthesis is that CO 2 accumulates at a very high concentration in the bundle sheath cells that is then sufficient to allow Rubisco to operate efficiently.

Diagram of a C 4 plant leaf showing Kranz anatomy

The C 4 pathway (NADP + –malic enzyme type) for fixation of CO 2

Plants growing in hot, bright and dry conditions inevitably have to have their stomata closed for large parts of the day to avoid excessive water loss and wilting. The net result is that the internal CO 2 concentration in the leaf is very low, meaning that C 3 photosynthesis is not possible. To counter this limitation, another CCM is found in succulent plants such as cacti. The Crassulaceae fix CO 2 into malate during the day via PEP carboxylase, store it within the vacuole of the plant cell at night and then release it within their tissues by day to be fixed via normal C 3 photosynthesis. This is termed crassulacean acid metabolism (CAM).

This article is a reviewed, revised and updated version of the following ‘Biochemistry Across the School Curriculum’ (BASC) booklet: Weaire, P.J. (1994) Photosynthesis . For further information and to provide feedback on this or any other Biochemical Society education resource, please contact [email protected]. For further information on other Biochemical Society publications, please visit www.biochemistry.org/publications .

adenosine diphosphate

adenosine triphosphate

carbohydrate

cytochrome b 6 f

dihydroxyacetone phosphate

excitation energy transfer

ferredoxin–NADP + reductase

glyceraldehyde 3-phosphate

light-harvesting complex

nicotinomide–adenine dinucleotide phosphate

phosphoenolpyruvate

inorganic phosphate

reaction centre

ribulose-1,5-bisphosphate carboxylase/oxygenase

I thank Professor Colin Osborne (University of Sheffield, Sheffield, U.K.) for useful discussions on the article, Dr Dan Canniffe (Penn State University, Pennsylvania, PA, U.S.A.) for providing pure pigment spectra and Dr P.J. Weaire (Kingston University, Kingston-upon-Thames, U.K.) for his original Photosynthesis BASC article (1994) on which this essay is partly based.

The Author declares that there are no competing interests associated with this article.

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Essay on Photosynthesis in Plants

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In this essay we will discuss about Photosynthesis in Plants. After reading this essay you will learn about: 1. Meaning of Photosynthesis 2. Significance of Photosynthesis to Mankind 3. History 4. Photosynthetic Apparatus 5. Pigments 6. Quantum Requirement and Quantum Yield 7. Mechanism 8. Evidences for Existence of Light and Dark Reactions 9. Source of Oxygen 10. Factors Affecting.

• Essay on the Factors Affecting Photosynthesis

Essay # 1. Meaning of Photosynthesis:

Although literary meaning of photosynthesis is ‘synthesis with the help of light’ but this term is usually applied to a very important vital process by which the green plants synthesize organic matter in presence of light. Photosynthesis is sometimes called as carbon assimila­tion and is represented by the following traditional equation.

Chlorophylls and other photosynthetic pigments are found in the form of protein pigment complexes mainly in thylakoid membranes of grana. The latter are sites of primary photochemi­cal reaction. Some of the protein-pigment complexes are also found in stroma lamellae.

Dark reaction of photosynthesis occurs in stroma. Besides necessary enzymes, some ribosomes and DNA have also been found in chloroplasts which give them (chloroplasts) a partial genetic autonomy.

Essay # 5. Photosynthesis Pigments:

Photosynthetic pigments are of three types:

(1) Chlorophylls,

(2) Carotenoids, and

(3) Phycobillins.

i. Chlorophylls and carotenoids are insoluble in water and can be extracted only with organic solvents.

ii. Phycobillins are soluble in water.

iii. Carotenoids include carotenes and xanthophylls. The latter are also called as carotenols.

iv. Different pigments absorb light of different wavelengths and characteristic absorption peak in vivo and in vitro.

v. They show property of fluoresces.

Distribution of Photosynthetic Pigments in Plant Kingdom :

The distribution of the different types of photosynthetic pigments in plant kingdom is shown in table 11.1.

A new form of chlorophyll has been discovered recently by Chen et al (2010) from stromatolites of Shark Bay in Western Australia which they have called as chlorophyll f. This pigment is believed to absorb light upto 706 nm in vitro, with a fluorescence of 722 nm. (stro­matolites are structures formed from layers of cyanobacteria (blue-green algae), and other mi­croorganisms, calcium carbonate and sediments).

Structure of Photosynthetic Pigments :

(1) Chlorophylls:

They are magnesium porphyrin compounds. The porphyrin ring consists of four pyrrol rings joined together by CH bridges. A long chain of C atoms called as phytol chain is attached to porphyrin ring at iv pyrrol ring.

I. Chemical structures of chlorophyll-a and chlorophyll-b are well established.

v. (In modern scientific literature, some plant physiologists equate PAR with visible part of spec­trum of radiant energy which is erroneous. This is because such scientists working on photobiology use commercially available instruments that are limited to that portion of spectrum between 400-700 nm only, thus excluding visible light in the 700-760 and 390-400 nm range.)

vi. Only about 1% of the total solar energy received by the earth is absorbed by the pigments and is utilised in photosynthesis.

vii. There is very weak absorption by pigments in green part of the spectrum and hence, the chloroplasts appear green in green plants.

Absorption Spectra of Chlorophylls:

They chiefly absorb in the violet-blue and red parts of the spectrum. The absorption band shown by the chlorophylls in violet-blue region is also called as soret band. Characteristic absorption peaks shown by different chlorophylls both in vivo (i.e., intact cell) and in vitro (i.e., in solvents) are given in Table 11.2.

Absorption Spectra of Carotenoids:

These pigments absorb light energy in blue, blue- green and green parts of the spectrum.

Absorption Spectra of Phycobillins:

This can be explained further by a schematic model for the photo-oxidation of water given by Bessel Kok et al (1970) which is widely accepted and is called as S state mecha­nism or sometimes as water oxidizing clock. It consists of a series of 5 states called as S 0 , S 1 , S 2 , S 3 and S 4 which represent successively more oxidised forms of the water oxidizing system or oxygen evolving complex (OEC) S 0 is uncharged state.

Each short flash of light (photon or hv) converts S 0 to S 1 , S 1 to S 2 , S 2 to S 3 and S 3 to S 4 . After the S 4 state has ac­quired four positive charges, it gets four electrons back in one step oxidation of two molecules of H 2 O and returns back to S 0 with four fewer charges than S 4 (fig. 11.14).

However, the chemical nature of S state in this ‘clock’ is yet unknown. Once it was believed that P680 becomes oxidised by loss of one electron after a brief flash of light to P680 + but P680 cannot be S because it can lose only one electron and can accumulate only one positive charge.

Later studies have shown that various S states probably represent oxi­dation states of manganese including Mn 2+ , Mn 3+ and Mn 4+ . This hypothesis has received strong support from a variety of experiments, especially X-ray absorption and ESR studies which detect the manganese directly (Yano at al, 2006).

It is now known that the immediate electron donor to PSII is a tyrosine (an amino acid) residue which is often designated as Z or Y z in subunit D 1 of PSII reaction centre. (Y is code letter for tyrosine; hence Z is now called as Y z ). It is believed that tyrosine radical regains its electron by oxidizing a cluster of 4 Mn ions in OEC.

With each single electron transfer, the Mn cluster becomes more oxidized. Four single electron transfers (each correspond­ing with one photon (hv) of light) produce four positive charges on Mn cluster. In this state, Mn complex can take four electrons (4e-) from a pair of water molecules. The exact mechanism of photo-oxidation of H 2 O 2 however, remains elusive.

(The OEC is a 33kD complex situated on lumenal side of thylakoid. The 4H + released by photoly­sis of 2H 2 O molecules are released into lumen of thylakoid where they add to the proton gradient nec­essary for photophosphorylation. Apart from Mn 2+ and Cr ions, Ca 2+ ions are also believed to be essen­tial for photolysis of water.)

(v) Electron Transport and the Production of Assimilatory Power (i.e., NADPH + H + + ATP):

It has already been said that when chlorophyll-a molecule receives a photon of light it becomes excited and expels the extra energy along with an electron in both the pigment systems. This electron after travelling through a number of electron carriers is either cycled back or is consumed in reducing NADP + (Nicotinamide Adenine Dinucleotide Phosphate) to NADPH + H + .

The extra light energy carried by the electron is utilised in the formation of ATP molecules at certain places during its transport. This process of the formation of ATP from ADP and inorganic phosphate (Pi) in photosynthesis is called as photosynthetic phosphorylation or photophosphorylation. Arnon has contributed a lot in our understanding of the electron transport and photo­phosphorylation in chloroplasts.

These are of two types:

(a) Non-cyclic Electron Transport and Non-cyclic Photophosphorylation (Z-Scheme):

This process of electron transport involves both PSI and PSII which act in tandem or series and is initiated by the absorption of a photon (quantum) of light by P700 form of chlorophyll- a molecule in pigment system I which gets excited. An electron is ejected from it so that an electron deficiency or a ‘hole’ is left in the P700 molecule (or in other words a positive charge comes on chlorophyll-a-molecule).

This ejected electron is trapped by FRS (Ferredoxin reduc­ing substance) which is an unknown oxidation-reduction system with a redox potential (E 0 ‘) of -0.6 volts and may be a pteridene. The electron is now transferred to a non-heme iron protein called ferredoxin (Fd) with E’ 0 of-0.432 V. From ferredoxin the electron is transferred to NADP (E 0 ‘ = -0.32 V) via intermediate protein electron carrier ferredoxin-NADP reductase (FNR) so that NADP is reduced to NADPH + H + .

Most recent researches have shown that FRS is in-fact a series of electron carriers which in their reduced form are very unstable and difficult to be identified and are designated as A 0 A 1 Fe-S 1 ,Fe-S A & Fe-S B . A 0 is probably a chlorophyll molecule that receives electron from P700.

A 1 is be­lieved to be phylloquinone (vit. K 1 ). Fe-S x , Fe-S A and Fe-S B are iron-sulphur centres situated on proteins in core complex I (CCI) and act as additional electron carriers. From Fe-S centres, the elec­tron is transferred to ferredoxin (Fd) which is a small, water soluble iron-sulphur protein situated on stroma side of thylakoid membrane (Fig. 11.16).

Now, when a photon (quantum) of light is absorbed by P680 form of chlorophyll-a mol­ecule in pigment system II, it gets excited and an electron is ejected from it so that an electron deficiency or a ‘hole’ is left behind in the P680 molecule. The ejected electron is trapped by a compound of unknown identity usually designated Y (Compound Y is some­times called as Q because it also causes quenching of the characteristic fluorescence of chlorophyll-a in pigment system II).

This unknown compound forms oxidation-reduction sys­tem with a redox-potential (E 0 ‘) value more negative than 0.0 V. From Q the electron passes downhill along a series of compounds or intermediate electron carriers and is ultimately received by pigment system I where it ‘fills the hole.’ Redox potential of P700 in pigment system is + 0.43 V.

The series of compounds consists of (i) cytochrome b-559 (E 0 ‘ = + 0. 055 V), (ii) plastoquinone (PQ) whose chemical structure shows similarity with vitamins of K Series. It has a redox potential (E 0 ‘) of + 0.113 V, (iii) cytochrome ƒ (E 0 ‘ = + 0.36 V) and (iv) plastocyanin (PC) which is copper containing protein (E 0 ‘ = + 0.39 V).

At one place during the electron transport i.e., between plastoquinone and cytochrome ƒ there is enough change in free energy which allows phosphorylation of one molecule of ADP to form one ATP molecule (photophosphorylation).

Most recent researches have shown that from p680, the electron is transferred to unknown compound ‘Q’ via pheophytin. The latter is special form of chlorophyll-a which lacks magnesium atom (Fig. 11.2B). The unknown compound Q exists in two forms Q A & Q B .

It is now known that Q A and Q B are infact specialized plastoquinones (PQ) which receive elec­tron from pheophytin and transfer it to Cyt. b 6 f complex. Q A is attached strongly to D 2 protein, while Q B is attached loosely to D 1 protein in core complex II (CC II). After the Q B has received two electrons from Q A (one by one in two turns), it also takes two protons (2H + ) from stroma and is fully reduced to uncharged plastoquinol or plastohydroquinone (PQH 2 or PQ B H 2 ).

The PQH 2 is now re­leased from the reaction centre and is replaced by another molecule of PQ which now occupies the Q B site (11.16). From PQH 2 , electrons are transferred to cytochrome b 6 f complex and its two protons (2H + ) are expelled into the lumen of thylakoid. Finally, the electrons from Cyt b 6 f complex reach to PSI via plastocyanin (PC).

(It is important to note that Q A is one electron acceptor, while Q B is two electrons acceptor).

i. Cytochrome ƒ is a typical c type of cytochrome, ‘ ƒ ’ is abbreviated from ‘frons’ which in Latin means leaf).

The ‘hole’ in pigment system I has been filled by the electron coming from pigment sys­tem II. But the ‘hole’ or an electron deficiency is still there in pigment system II. This is ful­filled by the electron coming from photolysis of water. Water here acts as electron donor. It has redox-potential (E’ 0 ) of +0.82 V. This transfer of electron from water probably involves a strong oxidant which is yet unknown and is designated as Z or Yz.

In the above scheme of electron transport the electron ejected from pigment system II did not return to its place of origin, instead it was taken by pigment system I. Similarly, the elec­tron ejected from pigment system I did not cycle back and was consumed in reducing NADP + . Therefore, this electron transport has been called as non-cycle electron transport and the accompanying photophosphorylation as non-cyclic photophosphorylation.

ii. Arrangement of PSI and PSII and various components of non-cyclic electron transport chain when depicted on paper according to their redox-potential values, takes a zig-zag shape like the letter ‘Z’ (Fig. 11.15) hence, non-cyclic electron transport is also called by the name Z-scheme.

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Biology: Photosynthesis and Respiration Essay

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Introduction

Photosynthesis is the process by which plants assemble carbon-based compounds which are the building blocks and energy stores of life. Plants first entrap sunlight energy and convert it to a chemical energy in ATP molecules which are in form of bonds. ATP brings energy to reactions where glucose is formed from water and carbon dioxide. To finish, glucose molecules are combined to form starch and other molecules. Oxygen is also produced during photosynthesis which is released in to the atmosphere (Koning, 1994, p. 1). The process of photosynthesis is summarized in the equation below;

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

Aerobic respiration is a procedure of cellular respiration that utilizes oxygen to split molecules to release electrons and form energy (Gregory, 2006, p. 2). In this process adenosine triphosphate (ATP) is produced which is liable for storing up and transporting most energy to other body cells. Aerobic respiration has two by-products which are water and carbon dioxide. It usually involves three main stages of reactions glycolysis which include the Kreb’s cycle and electron transport phosphorylation. The equation below is a summary of aerobic respiration;

C 6 H 12 O 6 +6O 2 →→6CO 2 +6H 2 O

How the two processes are linked between plants and animals based on the reactants and products of both pathways

The two processes are the life blood of plants and animals. These processes link in the way that the by-products of one process are used as the raw materials of the other. Photosynthesis uses carbon dioxide and water from aerobic respiration to produce oxygen, food (glucose) and water. Whereas aerobic respiration in animals will require glucose and oxygen from photosynthesis to produce energy (ATP molecules) as well as carbon dioxide and water used again in photosynthesis.

A description of how energy is transferred from sunlight to ATP, from ATP to sugars, and from sugars to your cells

Sunlight is trapped by organelles called chloroplasts in the form of chlorophyll (a red and blue light) to start the process of photosynthesis. In this process molecules of carbon dioxide gas and water are combined in the presence of the solar energy and chemical energy is formed. Calvin cycle then takes place to convert ATP to sugars through carbon fixation where 6 molecules of carbon dioxide are combined with Ribulose Biphosphate to form Phosphoglycerate (PGA) (Bergman, 1999, p. 1). It is then converted into G3P (Glyceraldehyde-3-phosphate) which is a sugar. The sugars are then consumed by human beings in the form of starch.

The role of fermentation in allowing an organism to generate energy for its cell(s) in the absence of oxygen

In the deficiency of oxygen, pyruvic acid can be converted into compounds such as lactic acid through the combination of glycolysis and other additional pathways in the process of fermentation. This is important during exercise especially because breathing cannot provide the body with all the oxygen needed for aerobic respiration and the cells turn to lactic acid fermentation, therefore providing the muscles with the energy required in exercise.

How the energy from the sun ends up as chemical energy for the anaerobic organism or cell

Before fermentation occurs, one glucose molecule is split into two pyruvate molecules through glycolysis summarized as;

C 6 H 12 O 6 +2 ADP i +2 P+2NAD + →2CH 3 COCOO – + 2ATP +2NADH + 2H 2 O +2H +

Thereafter, fermentation can take place where sugars are converted into cellular energy producing carbon dioxide and ethanol because of the absence of oxygen as shown below (Paustian,2000, p.2);

C 12 H 22 O 11 +H 2 O+Invertase → 2C 6 H 12 O 6

C 6 H 12 0 6 +Zymase→2C 2 H 5 OH+2CO 2

How an enzyme catalyzes a reaction

During a reaction a substrate that requires processing is carried towards the enzymes. Enzymes accelerate reactions via lowering the free energy of activation barrier, which is the Ea barrier (Kornberg, 1989, p.198). The enzymes are substrate definite and therefore can just speed up the creation of one form of a substrate. Usually, weak hydrogen or ionic bonds join the substrate to the enzyme. Then the enzyme lessens the Ea Barrier of a reaction by appropriately adjusting the substrates, damaging substrate bonds, giving a good microenvironment for the reaction to occur in the optimum PH. temperature and I.E and participating thoroughly in the reaction.

There are three main steps of the cycle of enzyme-substrate interactions

• Enzyme + substrate
• Enzyme-substrate complex
• Enzyme + product

How enzyme activity regulated by the cell

Cells regulate enzyme activity through end-product inhibition. The enzyme catalyzing one of the stages in the metabolic pathway is inhibited by the end-product.

Subsequently, if the quantity of product swells, the pathway is hindered and less is formed. If the quantity reduces, the inhibition is condensed and more is manufactured.

Additionally, the gene that produces the enzyme is possibly switched on or off by courier molecules for instance hormones.

Reference list

Bergman, J. (1999). ATP: The perfect energy currency for the cell; creation research society quarterly. Web.

Gregory, M. (2006). Cellular respiration. The biology web . Web.

Kornberg, A. (1989). For the love of enzymes . Harvard University Press. Cambridge, MA.

Koning, R. E. (1994). Respiration. Plant Physiology Information Website . Web.

Paustian, T. (2000). University of Wisconsin-Madison. Web.

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Photosynthesis

Affiliation.

• 1 Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, U.K. [email protected].
• PMID: 27784776
• PMCID: PMC5264509
• DOI: 10.1042/EBC20160016
• Correction: Photosynthesis. Johnson MP. Johnson MP. Essays Biochem. 2017 Oct 31;61(4):429. doi: 10.1042/EBC20160016_COR. Print 2017 Oct 31. Essays Biochem. 2017. PMID: 29089380 Free PMC article. No abstract available.

Photosynthesis sustains virtually all life on planet Earth providing the oxygen we breathe and the food we eat; it forms the basis of global food chains and meets the majority of humankind's current energy needs through fossilized photosynthetic fuels. The process of photosynthesis in plants is based on two reactions that are carried out by separate parts of the chloroplast. The light reactions occur in the chloroplast thylakoid membrane and involve the splitting of water into oxygen, protons and electrons. The protons and electrons are then transferred through the thylakoid membrane to create the energy storage molecules adenosine triphosphate (ATP) and nicotinomide-adenine dinucleotide phosphate (NADPH). The ATP and NADPH are then utilized by the enzymes of the Calvin-Benson cycle (the dark reactions), which converts CO 2 into carbohydrate in the chloroplast stroma. The basic principles of solar energy capture, energy, electron and proton transfer and the biochemical basis of carbon fixation are explained and their significance is discussed.

Keywords: membrane; photosynthesis; thylakoid.

© 2016 The Author(s).

PubMed Disclaimer

Figure 1. The global carbon cycle

The relationship between respiration, photosynthesis and global CO 2…

Figure 2. Location of the photosynthetic machinery

( A ) The model plant Arabidopsis thaliana…

Figure 3. Division of labour within the…

Figure 3. Division of labour within the chloroplast

The light reactions of photosynthesis take place…

Figure 4. The photosynthetic electron and proton…

Figure 4. The photosynthetic electron and proton transfer chain

The linear electron transfer pathway from…

Figure 5. Z-scheme of photosynthetic electron transfer

The main components of the linear electron transfer…

Figure 6. Major photosynthetic pigments in plants

The chemical structures of the chlorophyll and carotenoid…

Figure 7. Basic absorption spectra of the…

Figure 7. Basic absorption spectra of the major chlorophyll and carotenoid pigments found in plants

Figure 8. Jablonski diagram of chlorophyll showing…

Figure 8. Jablonski diagram of chlorophyll showing the possible fates of the S 1 and…

Figure 9. Basic mechanism of excitation energy…

Figure 9. Basic mechanism of excitation energy transfer between chlorophyll molecules

Two chlorophyll molecules with…

Figure 10. Basic structure of a photosystem

Light energy is captured by the antenna pigments…

Figure 11. Basic structure of the PSII–LHCII…

Figure 11. Basic structure of the PSII–LHCII supercomplex from spinach

The organization of PSII and…

Figure 12. S-state cycle of water oxidation…

Figure 12. S-state cycle of water oxidation by the manganese cluster (shown as circles with…

Figure 13. Basic structure of the PSI–LHCI…

Figure 13. Basic structure of the PSI–LHCI supercomplex from pea

The organization of PSI and…

Figure 14. Cytochrome b 6 f complex

( A ) Structure drawn from PDB code 1Q90. (…

Figure 15. Lateral heterogeneity in thylakoid membrane…

Figure 15. Lateral heterogeneity in thylakoid membrane organization

( A ) Electron micrograph of the…

Figure 16. The Calvin–Benson cycle

Overview of…

Overview of the biochemical pathway for the fixation of CO…

Figure 17. Rubisco

( A ) Structure…

( A ) Structure of the Rubisco enzyme (the large subunits are…

Figure 18. Diagram of a C 4…

Figure 18. Diagram of a C 4 plant leaf showing Kranz anatomy

Figure 19. The C 4 pathway (NADP…

Figure 19. The C 4 pathway (NADP + –malic enzyme type) for fixation of CO…

• Editorial Note: Photosynthesis. [No authors listed] [No authors listed] Essays Biochem. 2021 Jul 26;65(2):405. doi: 10.1042/EBC-2016-0016C_EDN. Epub 2021 Jul 16. Essays Biochem. 2021. PMID: 34309653 Free PMC article. No abstract available.

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Photosynthesis & Respiration ( OCR A Level Biology )

Revision note.

Biology Lead

Photosynthesis & Respiration

• Photosynthesis occurs in autotrophic organisms such as plants, algae and cyanobacteria
• In the process of photosynthesis, light energy is trapped and used to convert simple inorganic compounds into complex organic compounds. Energy is stored within these organic compounds
• Respiration occurs in all living organisms
• Respiration is the process by which energy is released from organic molecules in living cells. The process can be aerobic (using oxygen) or anaerobic (without using oxygen)
• There are several similarities and differences between photosynthesis and the two types of respiration
• For example, the coenzyme NADP is used in photosynthesis whereas the coenzyme NAD is used in both aerobic and anaerobic respiration

Remember, energy is never created or destroyed; it is converted from one form to another!

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Solar-Powered Life: How Plants And Other Organisms Produce Their Own Food

Some organisms can produce their own food through a process called photosynthesis. These organisms transform light energy, carbon dioxide, and water into sugars, which allow them to grow their bodies, reproduce, and be a source of energy for other organisms. Studying photosynthesis in nature and in the laboratory has given scientists important insights into the effects of climate change on plants and other photosynthetic organisms. For example, such studies help scientists understand whether there will continue to be enough food for humans to eat as the climate changes. In this article, we discuss the importance of photosynthetic organisms; how light energy, carbon dioxide, and water are transformed into sugar during photosynthesis; the challenges that today’s land plants face; and how and why scientists measure photosynthesis in plants.

Sunlight And Sugar-Making

Sugars give all living organisms the energy they need to move, grow, and reproduce. Some organisms (including humans) get the sugars they need from eating food. Other organisms, called primary producers , do not have to eat because they can make their own sugars. Most primary producers use sunlight to combine carbon dioxide and other compounds into sugars, through a process called photosynthesis. Photosynthesis is essential for all living creatures since it takes carbon dioxide (an important greenhouse gas) out of the air, puts oxygen into the air, and makes the foods that other organisms eat.

Plants are the most famous primary producers, but did you know that plants are not the only organisms that can do photosynthesis? There are lots of other types of primary producers that are photosynthetic. The Earth formed over 4.6 billion years ago, and land plants have been around for just the last 500 million years or so. Some bacteria, called cyanobacteria, have been living in the ocean, doing photosynthesis and releasing oxygen, for 3 billion years longer than plants [ 1 ]. Other non-plant organisms, including algae, are also primary producers and do photosynthesis in lakes and oceans. All the non-plant organisms that do photosynthesis actually produce most of the oxygen that we breathe.

Where Does Photosynthesis Occur?

The production of sugars by primary producers is a complex chemical process that uses sunlight, water, and carbon dioxide ( Figure 1 ). Plants and bacteria use chloroplasts (small organs inside their cells and leaves) to do photosynthesis. These tiny organs have green chlorophyll pigments used to capture energy from sunlight and make sugars.

• Figure 1 - The process of photosynthesis in a plant leaf.
• The key ingredients are sunlight, water, and carbon dioxide. The light-dependent reactions that occur within the chloroplasts require light and result in the production of ATP and NADPH. The light-independent reactions, or Calvin cycle, occur in the inner space of the chloroplasts and result in the production of sugar (figure credit: Alejandra Castillo).

Photosynthesis consists of two main types of reactions: those that are dependent on light and those that are not. Light-dependent reactions are the first step in producing sugars. During this step, two pairs of chlorophyll molecules absorb light energy and transform it into chemical energy. As a result, plants generate two important molecules: ATP and NAPDH .

The light-independent reactions are also called the Calvin cycle. In these reactions, plants use the ATP and NADPH molecules created in the light-dependent reactions. ATP and NAPDH help plants turn carbon dioxide, which they take up from the air through their stomata , into sugars ( Figure 2 ). The plants can then use these sugars to keep growing their roots, stems, and leaves, as well as to make flowers, fruits, and seeds. Animals and fungi also use those sugars as food when they eat the plants. So, the next time you see a plant, remember that it uses solar power to produce its own food—and to make all the food that we animals eat. Thank you, plants!

• Figure 2 - Flowers, leaves, and stomata of three plant species that grow on big tropical rock outcrops in Colombia: Spruce’s acanthella (left) , lance-leaved rocktrumpet (middle) , and orinoco tabebuia (right) .
• Big leaves tend to have fewer stomata. This means that small leaves like the ones of Spruce’s acanthella have a lot of small stomata, while big leaves like the ones of orinoco tabebuia have fewer stomata (figure credit: Alejandra Castillo).

Today’s Land Plants Face Challenges

When we use fossil fuels (e.g., coal, natural gas, and oil), we increase the amount of carbon dioxide in Earth’s atmosphere. You may think that more carbon dioxide would be good for plants and allow them to produce more sugar and more oxygen. Unfortunately, more carbon dioxide does not always translate into more photosynthesis. This is because plants also need lots of water to do photosynthesis. Plants get water from the soil, through their roots. This water gives hydrogen to chlorophyll, to keep the light-dependent reactions working, and it is also the source of the oxygen that plants put into the air. In addition, when plants open their stomata to take up carbon dioxide from the air, they lose a lot of water through evapotranspiration . In fact, on average, plants lose about 400 molecules of water for every one molecule of carbon dioxide that they get.

Unfortunately, increasing amounts of carbon dioxide in the air are causing climate change, which is making it hotter and causing lots of places to have less rain or longer dry seasons. Less rain and hotter temperatures mean that many plants have less water available. So, when we use fossil fuels and put more carbon dioxide into the air, we may actually be making it harder for plants to do photosynthesis. Scientists have tested this idea by growing plants in air with extra carbon dioxide. The scientists found that plants could, in fact, do more photosynthesis and grow faster for a while because of the extra carbon dioxide—but this boost did not last for long. Soon, the plants started growing slower or even dying because there was not enough water or nutrients in the soil to keep them alive [ 2 ].

How Do We Measure Photosynthesis In Nature?

Scientists who study plants use very sophisticated machines called infrared gas analyzers (IRGAs) to measure how fast plants do photosynthesis and turn carbon dioxide into sugars ( Figure 3 ). IRGAs detect the infrared light that is absorbed by various gases in the air. To use the IRGA, the scientists put a leaf or even a small plant inside a special airtight chamber. Then, they fill the chamber with air that has a known amount of carbon dioxide. Next, they keep measuring the amount of carbon dioxide in the chamber. If the plant is doing photosynthesis, it will take carbon dioxide out of the air, and the concentration of carbon dioxide in the chamber will decrease. The faster the plant does photosynthesis, the faster the carbon dioxide is removed from the chamber.

• Figure 3 - A scientist measuring the rate of photosynthesis in the leaf of a plant using an Infra-Red Gas Analyzer (IRGA; figure credit: Alejandra Castillo).

Using IRGA, scientists can also measure the concentration of water in the air inside the chamber. Remember that plants lose lots of water through evapotranspiration as they take up carbon dioxide—so the more water that gets added to the air, the faster the plant is losing water as it does photosynthesis. Some types of plants (e.g., cacti) can do lots of photosynthesis without losing much water. These plants may have special tricks or adaptations for using less water, so they are especially good at living in deserts or other dry places. Other types of plants lose lots of water when they do photosynthesis. These thirsty plants would have a hard time living in dry places, and they may have a tough time surviving if climate change continues to make our world hotter and drier.

Another thing that scientists can test with IRGA is how much light plants need to do photosynthesis [ 3 ]. They can also test how fast plants do photosynthesis with different amounts of carbon dioxide in the air, or at different temperatures [ 4 ]. These types of measurements can be slow. For example, it takes about 45 min to measure how much light a leaf needs for photosynthesis, since the scientist must expose the leaf to lots of different light levels, and they must give the plant time to adjust and relax between each treatment. Forty-five minutes might not seem like a lot, but keep in mind that some scientists need to measure photosynthesis in the middle of a wet jungle or a hot desert. Keeping the IRGA machine working for that long can be challenging, since these machines are very fragile and use lots of battery power. Scientists also do not just measure one leaf! To do a good study, they may try to measure photosynthesis on hundreds of leaves from lots of plants. This is a lot of work, but it is all worthwhile if it helps scientists understand what certain types of plants need to do photosynthesis and if these plants are in danger from climate change.

Why Do We Need This Information?

Scientists measure photosynthesis for lots of reasons. One reason is to study the effects of climate change on how many vegetables and fruits our plants can grow [ 5 ]. For example, scientists can grow the plants people like to eat, like beans, tomatoes, carrots, or avocados, in different temperatures and with varying amounts of water. To change the temperatures, scientists can use special greenhouses to make the plants hotter. They can also give the plants all the water they need, or they can block out the rain and force plants to live with less water. Scientists can even change what time of year the plants get water. Through these clever experiments, scientists can monitor the health and photosynthesis of plants grown under differing conditions, to see if the plants will be able to keep producing our food when the climate changes. Given how valuable plants and primary producers are for our planet, this is very exciting and important research.

In a Nutshell, Photosynthesis Is Amazing!

Plants and other photosynthetic organisms use solar power to make their own food and, in the process, they provide us with food and oxygen, remove carbon dioxide from the air, and help protect the planet from climate change. Scientists measure photosynthesis to study how plants work and how photosynthesis may be affected by climate change. Scientists use their creativity and IRGAs to measure photosynthesis in different kinds of plants and under varying conditions. This important information will help scientists understand how plants will perform in a hotter and drier world, and if plants will be able to keep doing so many great things for humans and for all life on Earth. If you were a scientist, what plant experiments would you do?

Primary Producers : ↑ Organisms capable of doing their own food by transforming sunlight, water, minerals, and carbon dioxide into organic carbon (sugar).

Chloroplasts : ↑ Small organ-like structures (organelle) found withing the plant’s cell in which photosynthesis occurs.

Chlorophyll : ↑ Pigment found in the chloroplast of the plant’s cells in charge of absorbing blue and red light used toward sugar production.

ATP : ↑ Adenosine triphosphate, the “energy currency” of the cell. ATP is used to perform cellular reactions that require energy.

NAPDH : ↑ Nicotinamide adenine dinucleotide phosphate hydrogen, an energy-carrying molecule that provides energy for the Calvin cycle, in the form of hydrogen atoms.

Stomata : ↑ Cell structures in leaves, composed of an opening surrounded by two guard cells, that control the exchange of gases and water with the environment.

Evapotranspiration : ↑ Movement of water from Earth’s surface into the atmosphere via both evaporation and transpiration (loss of water through plant leaves).

Infrared Gas Analyzer : ↑ Detectives of the infrared light that is absorbed by gases in the air. These detectives use special light sensors that measure the amount of carbon dioxide.

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.

[1] ↑ Sánchez-Baracaldo, P., and Cardona, T. 2020. On the origin of oxygenic photosynthesis and Cyanobacteria. New Phytol. 225:1440–6. doi: 10.1111/nph.16249

[2] ↑ Li, F., Guo, D., Gao, X., and Zhao, X. 2021. Water deficit modulates the CO 2 fertilization effect on plant gas exchange and leaf-level water use efficiency: a meta-analysis. Front. Plant Sci. 12:775477. doi: 10.3389/fpls.2021.775477

[3] ↑ Aragón, L., Messier, J., Atuesta-Escobar, N., and Lasso, E. 2023. Tropical shrubs living in an extreme environment show convergent ecological strategies but divergent ecophysiological strategies. Ann. Bot. 2023:mcad002. doi: 10.1093/aob/mcad002

[4] ↑ Taylor, T. C., Smith, M. N., Slot, M., and Feeley, K. J. 2019. The capacity to emit isoprene differentiates the photosynthetic temperature responses of tropical plant species. Plant Cell Environ. 42:2448–57. doi: 10.1111/pce.13564

[5] ↑ Tito, R., Vasconcelos, H. L., and Feeley, K. J. 2018. Global climate change increases risk of crop yield losses and food insecurity in the tropical Andes. Glob. Change Biol. 24:e592–602. doi: 10.1111/gcb.13959

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The microalgae chlamydomonas for bioremediation and bioproduct production.

Graphical Abstract

1. Introduction: Why Microalgae and Why Chlamydomonas ?

2. wastewater and advantages of using microalgae for its bioremediation, 3. microalgae cultivation methods, 4. chlamydomonas phycoremediation, 5. chlamydomonas bioproduct generation, 5.1. biomass, 5.2. biochar, 5.3. biofertilizers, 5.4. bioplastic, 5.5. biofuels, 5.5.1. biodiesel, 5.5.2. bioethanol, 5.5.3. biogas, 5.5.4. hydrogen, 5.6. high-value bioproducts.

Microalgae	Bioproduct	Experimental Condition	Productivity/Characteristic	References
Chlamydomonas reinhardtii CC-2937	Biomass	Erlenmeyer flasks containing 50 mL of Tris-acetate-phosphate media on a shaker under constant light of 75 µmol photons m s	23 g/L	[ ]
Chlamydomonas sp.	Biochar	Bioreactor, Tris-acetate-phosphate with nitrate at 28 °C, light intensity of 150 µmol photons m s , and bubbled with 3% CO	94% w/w dry biomass	[ ]
Chlamydomonas sp. JSC4	Biochar	Bioreactor, Tris–acetate-phosphate at 25 °C, light intensity of 70 µmol photons m s , and bubbled air-CO (v/v, 97/3)	93.9% w/w dry biomass	[ ]
Chlamydomonas sp. Tai-03	Biochar	Photoautotrophic mode using BG-11 medium at 26 °C, continuous aeration of 2.5% CO , and light intensity of µmol photons m s	95.4% w/w dry biomass	[ ]
Chlamydomonas applanata M9V	Biofertilizer	Allen Arnon medium with Imipenem at 100 µg mL and incubated for a week at 25.5 °C after shaking at 200 rpm for 24 h	Increased soil organic matter by 1.77–23.10%, total carbon by 7.14–14.46%, and C:N ratio by 2.99–11.73%	[ ]
Chlamydomonas reinhardtii	Biofertilizer	250 mL Erlenmeyer flasks containing minimal media at 25 °C, 140 rpm, and 135 µmol photons m s continuous white light	Maximum uptake of nitrogen, phosphorus, and potassium increased by 185.17%, 119.36% and 78.04%, respectively	[ ]
Chlamydomonas reinhardtii cc124	Biofertilizer	Bioreactor, Tris-acetate-phosphate, 25 °C, 16/8 h light/dark regime, white light, and shaker set at 180 rpm	Increased the plants’ shoot length, leaf size, fresh weight, number of flowers, and pigment content	[ ]
Chlamydomonas reinhardtii	Biofertilizer	1 L flasks in a climatic chamber at a 16 h light/8 h dark regime at 22 °C/18 °C and light intensity µmol photons m s using Tris-acetate-phosphate	Increased the number of secondary roots, improved micro-nutrient accumulation in roots and shoots	[ ]
Chlamydomonas sp.	Biofertilizer	Batch cultures incubated at 25 °C, in a 12:12 h light-and-dark cycle, and 130 µmol photons m s	Increased growth, cell division, elongation, reproduction and respiration	[ ]
Chlamydomonas sajao	Biofertilizer	Minimal medium, tubes incubated for 1 week at 25 °C at 5000-lx cool white light on a 16/8 h (light/dark) photo regime	Increased soil wet aggregate stability (33–77%)	[ ]
Chlamydomonas reinhardtii cc-849	Bioplastic (PHB)	Tris-acetate-phosphate medium, continuous light of 90 µmol photons m s at 22 °C	126 nmol ·min ·mg prot	[ ]
Chlamydomonas reinhardtii UVM4	Bioplastic (PHB)	Tris-acetate-phosphate medium, continuous light of 80 µmol photons m s 25 °C, and 120 rpm shaking	21.6 mg/g	[ ]
Chlamydomonas reinhardtii C-9	Bioplastic (Cell-plastic)	80 L Photobioreactor, 25 °C, 150 µmol photons m s , and 15,000 ppm CO in BG-11 medium	60% wt protein 6.6% wt carbohydrates 5.0% wt lipids	[ ]
Chlamydomonas sp. JSC4	Biodiesel	Bioreactor, Tris-acetate-phosphate at 25 °C, and light intensity of 70 µmol photons m s	96.2% oil recovery	[ ]
Chlamydomonas reinhardtii UTEX 90	Bioethanol	Photo-bioreactor, Tris-acetate-phosphate medium, 96 h at 23 °C, and 130 rpm in a 2.5 L	235 mg/g algal biomass	[ ]
Chlamydomonas reinhardtii UTEX 90	Bioethanol	Photobioreactor, 23 °C, Tris-acetate-phosphate medium, andcontinuous illumination at 450 µmol photons m s	29.2% from algal biomass	[ ]
Chlamydomonas reinhardtii UTEX 90	Bioethanol	Tris-acetate-phosphate medium, 25 °C, 100 µmol photons m s , and 100 rpm	90–94% from algal biomass	[ ]
Chlamydomonas sp. QWY37	Bioethanol	BG-11 medium, 27–30 °C, continuous supply of 2.5% CO , and continuous illumination of 250 µmol photons m s	61 g/L	[ ]
Chlamydomonas reinhardtii cc124	Biogas	Tris-acetate-phosphate medium, 25 °C, and white light at 400 µmol photons m s	587 mL of biogas per gram	[ ]
Chlamydomonas reinhardtii CC-1690	Biogas	Photoautotrophically, glass bottles (max. capacity 3.5 L), and continuous white light at 300 µmol photons m s	750 mL of biogas per gram	[ ]
Chlamydomonas reinhardtii 6145	Biogas	Tris-acetate-phosphate medium, 12:8 light–dark cycles, 25 °C, and illumination of 36 µmol photons m s	542 mL of biogas per gram	[ ]
Chlamydomonas reinhardtii C137	Hydrogen	Anaerobic conditions involved using sulfur-starved culture under continuous illumination for up to 150 h	140 mL/L	[ ]
Chlamydomonas reinhardtii 704	Hydrogen	Tris-acetate-phosphate medium, 25 °C, and white light at 12 µmol photons m s with acetic acid	65 mL/L	[ ]
Chlamydomonas reinhardtii pgr5	Hydrogen	Tris-acetate-phosphate medium, 25 °C, white light at 90 µmol photons m s , and constant agitation	65 mL/L	[ ]
Chlamydomonas reinhardtii cc124	Hydrogen	Tris-acetate-phosphate medium, 25 °C, white light at 180 µmol photons m s , and Argon atmosphere	3.26 mmol/L	[ ]
Chlamydomonas reinhardtii HCR 89	Glycolate	Minimal-salts medium, 25 °C, 100 µmol photons m s , 125 rpm, and 0.035% CO	130 µmol/mg	[ ]
Chlamydomonas reinhardtii Cia5	Glycolate	125 mL flasks of liquid Tris-acetate-phosphate medium on a shaker platform set at 100 rpm. Continuously illuminated at 65 µmol photons m s , 25 °C, and no additional CO provided	0.3 g/L	[ ]
Chlamydomonas reinhardtii AG 11–32b	Glycolate	Batch preculture at 20 °C, at a light intensity of 100 µmol photons m s , Tris-phosphate minimal medium with Tris buffer (39.95 mM), and the addition of 3.08 µM FeSO ·7H O plus 2.3 µM Na2-EDTA	41 mM	[ ]
Chlamydomonas reinhardtii hpr1	Glycolate	Tris-acetate-phosphate at 25 °C under 80 µmol photons m s continuous light. Tris-minimal medium with aeration of 3% CO	350 × 10 nmol/cell	[ ]
Chlamydomonas reinhardtii UPN22	Bioisoprenoid	Tris-acetate-phosphate plus nitrate at 22 °C under 150 µmol photons m s continuous light and 120 rpm	152 mg/L	[ ]
Chlamydomonas reinhardtii 137c	Hydroxyalkanoy- loxyalkanoate	Minimal high-salt medium with Spectinomycin at 25 °C under 50 µmol photons m s continuous light and 125 rpm	0.20 mg/L intracellular 0.16 mg/L extracellular	[ ]
Chlamydomonas reinhardtii fap	7-heptadecene	Minimal high salt and Tris-acetate-phosphate in 24 deep well plates of 25 mL culture under 100 µmol photons m s at 25 °C. For day–night cycle experiment, autotrophically in 1L-photobiorectors in turbidostat mode	1.5% of total fatty acid methyl esters	[ ]
Chlamydomonas sp. KR025878	ε-Polylysine	BG11 medium, under continuous illumination at 50 µmol photons m s at 27 °C with 100 rpm shaking. FeCl at 100 mg/L as flocculant and supplementation with lysine, aspartate, and 4 mM citric acid	2.24 g/L	[ ]
Chlamydomonas reinhardtii UVM4	Polyamine (Cadaverine)	Mixotrophically in liquid or in solid Tris-acetate-phosphate medium and 250 µmol photons m s at 22 °C. Phototrophic in minimal medium supplied with 3–5% (v/v) CO enriched air	0.24 g/L after 9 days and maximal productivity of 0.1 g/L/d	[ ]
Chlamydomonas reinhardtii ODC1	Polyamine (Putrescine)	Mixotrophic growth conditions on solid Tris-acetate phosphate, 350 µmol photons m s at 22 °C. For high-cell-density cultivations, 6x medium supplied with up to 10% (v/v) CO -enriched air in 6-well plates	Maximum yield of 200 mg/L	[ ]
Chlamydomonas reinhardtii TAI114	Protoporphyrin IX	Minimal-salts medium, 25 °C, 150 µmol photons m s , 100 rpm, and 3–5% CO	3–8% w/w of the dried biomass	[ ]
Chlamydomonas agloeformis ChA	Antioxidants (flavonol)	Minimal-salts medium nitrate, 26 °C with 24:0 light–dark photoperiod, and a light intensity of 100 µmol photons m s	203.80 ± 97.02 mg/100 g dried weight
Chlamydomonas reinhardtii BKT	Antioxidants (Astaxanthin)	Tris-acetate-phosphate and 100–150 µmol photons m s at 25 °C. High-salt minimal media were used for photoautotrophic conditions. Growth was conducted using shaking flasks or stirring flasks	4.3 mg/L/day	[ ]
Chlamydomonas reinhardtii bkt5	Antioxidants (Astaxanthin)	Tris-acetate-phosphate, 100 µmol photons m s at 25 °C. Growth in Multi-Cultivator MC-1000 (Photon Systems Instruments, Drásov, Czech Republic)	Up to 2.5 mg/g dry weight	[ ]
Chlamydomonas reinhardtii ATG1-ATG8	Antioxidants (β-Carotene)	Tris-acetate-phosphate with Paromomycin 25 µg/m under continuous illumination of 100 µmol photons m s at 25 °C and shaken at 90 rpm	23.75 mg/g dry cell weight	[ ]
Chlamydomonas reinhardtii VTC2	Antioxidants (vitamin C)	Mixotrophically in Tris-acetate-phosphate medium with arginine in 25–250 mL Erlenmeyer flasks on a rotatory shaker at 22 °C and 80 µmol photons m s	Up to 1.3 mM	[ ]
Chlamydomonas reinhardtii	Omega-3 fatty acids	Tris-acetate-phosphate medium, 100 rpm with ambient CO level, 23 °C, and 16:8 h alternating light–dark cycle with a photon irradiance of 100 µmol photons m s	0.2–1.6 mg/g	[ ]
Chlamydomonas reinhardtii CC-124	Sulphated polysaccharide	Tris-acetate-phosphate medium pH 7 and continuous illumination at 300 µmol photons m s	130 mg/g	[ ]
Chlamydomonas reinhardtii CR25	Therapeutic protein (ICAM)	Bioreactor, Tris-acetate-phosphate medium pH 7 with 15 μg/mL of Zeocin, and continuous illumination at 125 µmol photons m s	46.6 mg/L	[ ]
Chlamydomonas reinhardtii SRTA	Therapeutic protein (SARS-CoV-2)	Tris-acetate-phosphate medium pH 7 with 100 µg/mL spectinomycin and continuous illumination at 125 µmol photons m s	11.2 ± 1.8 µg/L	[ ]

6. Conclusions and Future Perspective

Author contributions, data availability statement, acknowledgments, conflicts of interest.

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Click here to enlarge figure

Microalgae	Wastewater Type	Cultivation/Growth Conditions	Bioremediation/Biomass Productivity	References
Chlamydomonas reinhardtii (NIES-2235)	Municipal Swine	Photobioreactor/28 ± 1 °C. Fluorescent lamps 80 μmol photons m s and 16 h light/8 h dark for 1 week	Biomass: 187 mg dry weight/L	[ ]
Chlamydomonas debaryana IITRIND3	Domestic Sewage Dairy	Photobioreactor/pH 7.4 at 27 °C and 140 rpm with white light illumination (200 mmol m s )	COD (105 mg L )/Biomass: 193 mg L /day	[ ]
Chlamydomonas debaryana AT24	Swine wastewater	Photobioreactor/20–30 °C illuminated with white light (300–900 μmol photons m s ). Air bubble (100 mL/min). 15 days cultivation	COD (29.8–46.0 mg L )	[ ]
Chlamydomonas reinhardtii	Industrial	Photobioreactor/25 ± 1 °C. 120 μmol photons m s	N removal (55.8 mg L ); P removal (17.4 mg L )/Biomass: 820 mg L /day	[ ]
Chlamydomonas mexicana	Piggery wastewater	Batch/27 ± 1 °C and 150 rpm under continuous illumination for 20 days	N removal (23 mg L ); P removal (5.1 mg L ); Inorganic carbon (189 mg L ); Calcium removal (17 mg L )/Biomass: 1.3 g L	[ ]
Chlamydomonas reinhardtii sp.ck	Municipal	Photobioreactor/400 mL algae culture + Modified Provasoli-based minimal medium/100%–10% wastewater	Volatile solids (3.2–1.2 g L )/Biomass: 277 mg dry wight/L	[ ]

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Bellido-Pedraza, C.M.; Torres, M.J.; Llamas, A. The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production. Cells 2024 , 13 , 1137. https://doi.org/10.3390/cells13131137

Bellido-Pedraza CM, Torres MJ, Llamas A. The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production. Cells . 2024; 13(13):1137. https://doi.org/10.3390/cells13131137

Bellido-Pedraza, Carmen M., Maria J. Torres, and Angel Llamas. 2024. "The Microalgae Chlamydomonas for Bioremediation and Bioproduct Production" Cells 13, no. 13: 1137. https://doi.org/10.3390/cells13131137

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