best essay on photosynthesis

100 Best Photosynthesis Essay Topics and Ideas

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Photosynthesis is one of the many reasons for the existence of a variety of organisms on Earth. It is the complex procedure through which plants absorb light, carbon dioxide, and minerals from their surroundings ding and transform them into oxygen and chemical energy. Photosynthesis also occurs in a few other living organisms but they use it mainly to grow and transmit light. If you want to choose the right essay topic that will help you to share your knowledge of photosynthesis, then you must read this blog. Here, we have shared some unique photosynthesis essay topics. Choose the one you like best.

What is Photosynthesis?

Photosynthesis is the method through which green plants, algae, and a specific group of bacteria called cyanobacteria change the energy from light into chemical energy. In green plants, the energy from light gets captured during photosynthesis and transforms water, carbon dioxide, and minerals into oxygen and other energy-providing organic compounds.

What is the Significance of Photosynthesis?

Photosynthesis is crucial for the existence of the vast majority of life on Earth. It is because of the following reasons:

Offers a source of food to all types of organisms

It is the process through which all energy in the biosphere is accessible to living things. Photosynthetic organisms are primary producers of food. Therefore, they lie at the base of the food web and are used directly or indirectly by all higher forms of life.

Provides oxygen to the atmosphere

Almost all the oxygen in the atmosphere is available because of photosynthesis. If photosynthesis does not occur on Earth, no food or other organic matter will be available on Earth, most organisms would cease to exist, and Earth’s atmosphere would ultimately become nearly empty of gaseous oxygen. Only the chemosynthetic bacteria would exist on earth in this condition since it uses the chemical energy of specific inorganic compounds for existence and is not dependent on the conversion of light energy.

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Tips for Choosing the Right Photosynthesis Essay Topic

If you are a student of bioscience you may get assigned to write a paper on photosynthesis. When you attempt to choose your topic, keep the following points in mind:

Select a topic that is appropriate to the length of your paper

All types of academic assignments are not of the same length, for example, an essay may be 1000 words but research papers can length up to 3000 words. Choose a topic that allows you to state your facts and wrap up your content within the specific limit. If you write a research paper on photosynthesis, you can detail every step of photosynthesis. However, for essay writing, you can add more details and state the facts more clearly if you focus on one of the several steps of photosynthesis.

Pick a topic that you find interesting

If you do not have an interest in the subject you are writing about, then you can never persuade your reader to believe in your arguments. However, if you like a subject, you will care for it, have more to say on the topic, and your writing will be better. Moreover, you can pass on your enthusiasm for the subject through your writing and your readers will like the subject and find interest in it too.

Ensure you have sufficient material

Not all topics that you find interesting will have a lot of information to share with your reader. Therefore, before you settle down on a topic ensure that you can put in sufficient information to justify the word limit. It is essential to remember that adding fluffs will never help you gain a high score.

Change the topic if it does not work out

Sometimes the topic that seems quite easy to write at the beginning gets tougher to work on as you progress with it. If you come across a topic that you cannot expand beyond a specific word limit or you struggle to develop a compelling paper on it, leave it right away and write on another topic of your choice. Never grind out pages of an essay on a subject you know is a poor choice.

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List of Remarkable Photosynthesis Essay Topics

Are you in search of some notable ideas for writing an essay on photosynthesis? If so, then take a look at these photosynthesis essay topics .

Best Photosynthesis Essay Topics

Explore the fascinating world of photosynthesis with the best essay topics suggested below. The recommended topics will be ideal for students and researchers seeking to delve into the science, importance, and applications of this vital biological process.

• What is the concept of photosynthesis?
• Why is the rate of photosynthesis in waterweed more than that of other plants?
• Resemblance and dissimilarity of photosynthesis and cellular respiration.
• What is the relationship of Photosynthesis with Cellular Respiration?
• Determine the rate of photosynthesis from the oxygen gas evolution.
• What did Devens Gust discuss about photosynthesis in his book “Why Study Photosynthesis?”
• Process of photosynthesis and fermentation in plant cellular metabolism.
• Discuss Photosynthesis and Respiration in plant biology.
• Examine the natural procedure of Photosynthesis.
• Share your ideas on the intricacy of photosynthesis and respiration
• What are the latest developments in artificial photosynthesis?
• The rate of photosynthetic in terrestrial plants.

Informative Photosynthesis Essay Topics

If you want to create an informative science essay, then focus on any topics related to photosynthesis. Listed below are some ideas based on photosynthesis that might be helpful for you in crafting an essay with educational value.

• Draw a comparison between photosynthesis and cellular respiration.
• How does the intensity of light intensity impact the rate of photosynthesis?
• What is the connection between carbon dioxide and the rate of photosynthesis?
• What is the future of artificial photosynthesis?
• How would you differentiate light-dependent and light-independent reactions of photosynthesis?
• Dissimilarity and relationship between respiration and photosynthesis
• How does the color of the ray of light impact on the photosynthesis?
• Examine the features that affect the rate of photosynthesis in leaves.
• How would you analyze the photosynthetic pigments in the chlorophyll extracts?
• What is the impact of runaway photosynthesis on the life in the universe
• Shed light on the recent research and experiments on the molecules in food and photosynthesis.
• What do you understand by the Photosynthetic Organism Aesculus Hippocastanum?

Captivating Essay Topics on Photosynthesis

The photosynthesis essay that you prepare should grab the attention of your readers. So, keep in mind to choose an engaging and relevant topic on photosynthesis. The following are some photosynthesis topics you may consider for composing a captivating essay.

• Share some common facts about Hydrilla and photosynthesis.
• What are the reactants and products of photosynthesis?
• How to improve the Light Reactions of Photosynthesis?
• Shed light on the development of oxygen-releasing photosynthesis.
• How does photosynthesis impact our human health?
• Analyze the relationship between circadian clocks and photosynthesis.
• Explore the restrictive factors of photosynthesis.
• Analyze the chemical equation for photosynthesis.
• What is the use of adenosine triphosphate and energy in photosynthesis?
• Explore photosynthesis and its impacts on the environment.
• What are the components of usual photosynthetic tools in solar cells?
• Analyze how photosynthesis converts light energy organisms into chemical energy.
• What are the basic requirements for photosynthesis?
• Analyze the impacts and restrictions of drought on photosynthesis.
• Which experiment caused the discovery of photosynthesis?

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Simple Photosynthesis Essay Topics for Students

If you are a school student, then for your science assignment, you may take into account any simple topic that is related to photosynthesis. Find here a collection of simple essay topics, tailored particularly for school students.

• Examine the floating leaf disk to explore photosynthesis.
• The phase and special circumstances required for photosynthesis.
• Examine the chemical yield of photosynthesis.
• The experimentation of the color of light and its impact on photosynthesis
• What is the function of molecular oxygen play in photosynthesis?
• Examine how light impacts on photosynthesis.
• Find the feature that can augment the rate of photosynthesis.
• Give an in-depth impression of the methods of photosynthesis in plants.
• Investigate the rate of photosynthesis in chloroplasts.
• The characteristics of the method of photosynthesis.
• Find the relationship between the positive charges of electrons and intercellular fluid in photosynthesis.
• The identification of the rates of photosynthesis through chloroplasts.
• Write about the unsure chemical reaction in photosynthesis.

Interesting Photosynthesis Essay Topics

When it comes to writing a photosynthesis essay, always choose a topic that aligns with your interest and study goals so that you may conveniently complete your assignment. These are some photosynthesis essay ideas that might be exciting for you to discuss and write about.

• Analyze the effect of different kinds of environments on photosynthesis.
• The method of marine photosynthesis and factors that impact it.
• Distinctiveness of photosynthesis of sugar plants
• Theory, equation, and steps of photosynthesis.
• How Does Photosynthesis Produce Sugar?
• What is the significance of Photosynthesis?
• Which scientist discovered photosynthesis and how?
• Why is photosynthesis essential for plants?
• What factors impact photosynthesis?
• Why is it impracticable to create photosynthesis in an artificial setting?
• Where does photosynthesis take place?
• What are the various levels of photosynthesis?
• Analyze the impact of Carbon Dioxide and light variations on the C3 Photosynthesis of Eruca Sativa.

Unique Photosynthesis Research Topics

To make your photosynthesis research paper stand top in your class, then work on any original topics that were not explored earlier. Writing about unique topics will help you to identify novel solutions and enhance your subject knowledge. In your science thesis, you may discuss any of these unique photosynthesis research ideas.

• Explore the rate of photosynthesis in plants
• How is the method and rate of photosynthesis in animal-eating plants different from that of plants that are dependent on sunlight and water for food?
• Where does the energy for photosynthesis originate from?
• Which organelle is accountable for photosynthesis?
• What are the primary steps of Photosynthesis?
• How carbon dioxide can impact the rate of photosynthesis?
• Why can photosynthesis not take place at very high temperatures?
• What would happen if photosynthesis could not take place on Earth?
• What is the highest temperature at which photosynthesis can take place?
• Which components augment photosynthesis in plants?
• Analyze how light affects photosynthesis.

Popular Photosynthesis Essay Ideas

You may also prepare your photosynthesis essay on any of the below-listed popular topics. But when you write about a frequently chosen topic, examine it from various perspectives and present insights that are new to your readers.

• Which plants perform photosynthesis at night?
• What gas is given away during photosynthesis?
• Explore the reactants and products of photosynthesis.
• How can the light reactions of photosynthesis be enhanced in plants?
• Can photosynthesis take place without light?
• Why is it impossible for humans to photosynthesize?
• What is the main objective of photosynthesis?
• Find out the products of photosynthesis and examine their role in the process of photosynthesis.
• What are the levels of photosynthesis?
• Explain how the process of photosynthesis is linked to photovoltaic technologies.
• Prepare a comparative essay on photosynthesis vs. solar cell production.

Read more: Newspaper Essay Topics and Ideas

Top Photosynthesis Essay Prompts

In case, you wish to prepare a photosynthesis essay deserving of an A+ grade, then make sure to select a high-quality topic. The following are some top-rated photosynthesis essay questions that might be helpful for you in drafting a top-score-fetching science academic paper.

• Discuss the practical applications of photosynthesis.
• Explain the impact of climate change on photosynthesis.
• Analyze the impact of photosynthesis on agricultural productivity.
• What is the importance of water in photosynthesis?
• Discuss how desert plants adapt to undergo photosynthesis.
• Study the possibilities of photosynthesis in space exploration.
• Explore the applications of photosynthesis in medical research.
• Discuss the importance of minerals in photosynthesis.
• Examine the relationship between photosynthesis and biodiversity.
• Describe the role of soil nutrients in supporting photosynthesis.
• Explore the applications of photosynthesis in biotechnology and genetic engineering.
• Describe the connection between photosynthesis and transpiration.

Final Words

Photosynthesis is an interesting subject that has several essay themes to investigate. In your science essay, you can very well discuss photosynthesis and its relevance, applications, or environmental consequences. Especially, when you write about photosynthesis, you will get a chance to enhance your writing skills and also understand more about this important biological phenomenon. All the essay ideas we have recommended in the list above will be useful for you in researching and learning the value of photosynthesis in daily life. So, for creating a photosynthesis essay, without any hesitation, from the list, choose any relevant topic of your choice or approach us for essay help. From essay topic selection to proofreading, skilled essay writers from our team with strong knowledge of photosynthesis will guide you in completing your academic paper according to your guidelines.

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Development of the idea

Overall reaction of photosynthesis.

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

Why is photosynthesis important?

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

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• 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.

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.

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Photosynthesis: Essay on Photosynthesis (2098 Words)

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Here is your essay on Photosynthesis!

[I] Photosynthesis:

Photosynthesis is one of the most fundamental biological reactions.

The chlorophyll bearing plants trap the free energy of sunlight as photons and transform and store it as chemical potential energy by combining CO 2 and water.

The end products of photosynthesis are carbohydrates with loss of oxygen. These directly or indirectly serve as the source of energy for all living beings, except chemosynthetic bacteria.

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[II] Food storage :

Some unpigmented plastids like leucoplasts store the essential food materials like protein, oil and starch. Later on these are used during germination of seeds and development.

[III] Hereditary carrier :

Recent studies show that these plastids, like chromosomes, are transmitted directly to the daughter cells during cell division. Cytoplasmic inheritance of plastids in Mirabilis is the well-known example. They produce phenotypic effects in Oenothera and other plants.

[IV] Chloroplasts as semiautonomous organoid :

The chloroplast matrix contains dissolved salts and enzymes of photo­synthesis. Besides these, like mitochondria, it contains RNA, DNA and ribosomes, and is capable of carrying on protein synthesis.

The chloroplast ribosomes are of the same size as ribosomes in prokaryotes. Chloroplasts are also semi-autonomous like mitochondria. They can grow and divide, and their DNA contains a portion of the genetic information needed for the synthesis of chloroplast proteins.

[V] Inheritance of chloroplasts :

Cells have the capacity to outgrow their chloroplasts and the rate of multiplication of chloroplasts is partly independent of the rate of multiplication of entire cells. Brawerman and Chargaff (1960) discovered it in Euglena gracilis after a temperature shock.

Cells which were permitted to multiply rapidly became irreversibly bleached, whereas cells prevented from dividing regained their normal ability to produce chloroplasts. They concluded that Euglena contains an autonomously replicating factor which is necessary for chloroplast formation.

[VI] DNA in chloroplasts :

Chloroplasts contain both DNA and the necessary mechanism for synthesizing specific RNA’s and proteins from a DNA template. DNA is found in chloroplasts (Stocking and Gifford, 1959). Ris and Plaut (1962) have also found DNA in the chloroplasts of alga Chlamydomonas. It has now been generally accepted that characteristic chloroplast DNA’s or chloroplast chromosomes occur in the photosynthetic organelles of algae and higher plants.

According to Brawerman (1966) this DNA differs from nuclear DNA in GC (guanosine and cytosine) content. Chloroplasts also contain a DNA-dependent RNA polymerase; it appears that specific RNA’s are synthesized from chloroplast DNA as a template (Kirk 1966). Chloroplasts DNA are capable of self-duplication.

[VII] Chloroplast ribosomes :

Lyttleton (1962) isolated chloroplast ribosomes, which are estimated to make up 3 to 7% of the chloroplast dry mass. Chloroplast ribosomes are smaller than cytoplasmic ribosomes. These are 60-66S. Chloroplast ribosomes also dissociate reversibly into 50S and 35S subunits, in a way that is found in E. coli ribosomes (Boardman et al., 1966). Chloroplasts have three types of RNA required for protein synthesis: ribosomal, transfer and messenger. Chloroplast ribosomes associate to form polysomes for synthesis of proteins (Gunning and Steer, 1975).

[VIII] Protein synthesis :

Protein synthesis in mitochondria and chloroplasts is similar to that of prokaryotes. For example, the size of chloroplast ribosomes is the same as ribosomes of blue-green algae, and ribosomes of chloroplasts and mitochondria more closely resemble prokaryotic ribosomes in antibiotic sensitivity than they do eukaryotic ribosomes.

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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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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

Essay on Photosynthesis

Students are often asked to write an essay on Photosynthesis in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Photosynthesis

What is photosynthesis.

Photosynthesis is how plants make their own food using sunlight. It happens in the leaves of plants. Tiny parts inside the leaves, called chloroplasts, use sunlight to turn water and carbon dioxide from the air into sugar and oxygen. The sugar is food for the plant.

The Ingredients

The main things needed for photosynthesis are sunlight, water, and carbon dioxide. Roots soak up water from the soil. Leaves take in carbon dioxide from the air. Then, using sunlight, plants create food and release oxygen.

The Process

In the chloroplasts, sunlight energy is changed into chemical energy. This energy turns water and carbon dioxide into glucose, a type of sugar. Oxygen is made too, which goes into the air for us to breathe.

Why It’s Important

Photosynthesis is vital for life on Earth. It gives us food and oxygen. Without it, there would be no plants, and without plants, animals and people would not survive. It also helps take in carbon dioxide, which is good for the Earth.

250 Words Essay on Photosynthesis

Why is photosynthesis important.

This process is very important because it is the main way plants make food for themselves and for us, too. Without photosynthesis, plants could not grow, and without plants, animals and humans would not have oxygen to breathe or food to eat.

How Photosynthesis Works

Photosynthesis happens in two main stages. In the first stage, the plant captures sunlight with its leaves. The sunlight gives the plant energy to split water inside its leaves into hydrogen and oxygen. The oxygen is released into the air, and the hydrogen is used in the next stage.

In the second stage, the plant mixes the hydrogen with carbon dioxide from the air to make glucose, which is a type of sugar that plants use for energy. This energy helps the plant to grow, make flowers, and produce seeds.

The Cycle of Life

Photosynthesis is a key part of the cycle of life on Earth. By making food and oxygen, plants support life for all creatures. When animals eat plants, they get the energy from the plants, and when animals breathe, they use the oxygen that plants release. It’s a beautiful cycle that keeps the planet alive.

500 Words Essay on Photosynthesis

Photosynthesis is a process used by plants, algae, and some bacteria to turn sunlight, water, and carbon dioxide into food and oxygen. This happens in the green parts of plants, mainly the leaves. The green color comes from chlorophyll, a special substance in the leaves that captures sunlight.

The Ingredients of Photosynthesis

The photosynthesis recipe.

When sunlight hits the leaves, the chlorophyll captures it and starts the food-making process. The energy from the sunlight turns water and carbon dioxide into glucose, a type of sugar that plants use for energy, and oxygen, which is released into the air. This process is like a recipe that plants follow to make their own food.

The Importance of Photosynthesis

Photosynthesis is very important for life on Earth. It gives us oxygen, which we need to breathe. Plants use the glucose they make for growth and to build other important substances like cellulose, which they use to make their cell walls. Without photosynthesis, there would be no food for animals or people, and no oxygen to breathe.

The Benefits to the Environment

Photosynthesis and the food chain.

All living things need energy to survive, and this energy usually comes from food. Plants are at the bottom of the food chain because they can make their own food using photosynthesis. Animals that eat plants get energy from the glucose in the plants. Then, animals that eat other animals get this energy too. So, photosynthesis is the start of the food chain that feeds almost every living thing on Earth.

Photosynthesis in Our Lives

Photosynthesis affects our lives in many ways. It gives us fruits, vegetables, and grains to eat. Trees and plants also give us wood, paper, and other materials. Plus, they provide shade and help make the air fresh and clean.

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Home — Essay Samples — Science — Biology — Photosynthesis

Essays on Photosynthesis

Photosynthesis is a crucial process that occurs in plants, algae, and some bacteria, allowing them to convert light energy into chemical energy in the form of glucose. This process is essential for the survival of nearly all living organisms on Earth, as it provides the primary source of energy for the food chain and produces the oxygen we breathe. As such, photosynthesis is a fascinating and important topic for study and research. In this essay, we will explore a wide range of photosynthesis essay topics, providing a comprehensive resource for students and researchers interested in this vital biological process.

The Importance of the Topic

Understanding photosynthesis is crucial for numerous reasons. Firstly, it allows us to appreciate the incredible complexity and efficiency of the natural world. Photosynthesis is a fundamental process that underpins the entire ecosystem, and studying it can provide valuable insights into the interconnectedness of life on Earth. Furthermore, photosynthesis has significant implications for agriculture and food production, as well as for addressing environmental challenges such as climate change and air pollution. By delving into the various aspects of photosynthesis, researchers can uncover new ways to improve crop yields, develop sustainable energy sources, and mitigate the impacts of human activity on the environment.

Advice on Choosing a Topic

When selecting a photosynthesis essay topic, it is important to consider your specific interests and goals. There are countless facets of photosynthesis to explore, from the biochemical mechanisms of light capture and carbon fixation to the ecological and evolutionary implications of this process. If you are interested in biochemistry and molecular biology, you might choose a topic related to the enzymes and molecular structures involved in photosynthesis. Alternatively, if you are more intrigued by environmental science and ecology, you could explore the role of photosynthesis in ecosystems and its interactions with other biogeochemical cycles. Ultimately, the best topic for you will be one that aligns with your passions and allows you to make a meaningful contribution to the field of photosynthesis research.

Photosynthesis is a vast and multifaceted topic that offers numerous opportunities for study and exploration. Whether you are a student, a researcher, or simply someone with a curious mind, there is no shortage of intriguing photosynthesis essay topics to delve into. By delving into the various aspects of photosynthesis, researchers can uncover new ways to improve crop yields, develop sustainable energy sources, and mitigate the impacts of human activity on the environment.

Why Photosynthesis is Essential to Life on Earth

The negative impact of abiotic pressures on photosynthesis, made-to-order essay as fast as you need it.

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Photosynthesis Process

Study of the process of the photosynthesis and its features, an analysis of the photosynthesis process from the light dependent and light independent reactions, the effect of light intensity on the rate of photosynthesis in elodea (pondweed), let us write you an essay from scratch.

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The Three Experiments on Photosynthesis, Chromatography, and The Wavelength of Light

The experiment that led to the discovery of photosynthesis, the process of photosynthesis within a carbon cycle, review of types and features of cyanobacteria, get a personalized essay in under 3 hours.

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Review of The Abiotic and Biotic Factors and Chemosynthesis Process

Review of the effects of nitrogen used as fertilizer, photosynthesis and cellular respiration, discussion on the fossil fuels and creation of algae into biofuels, the williamson ether synthesis: a cornerstone of organic chemistry, cyclohexanone lab report: analyzing the synthesis and properties, relevant topics.

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Photosynthesis and Cellular Respiration Essay

Photosynthesis is one of the primary sources of energy for living organisms. The fossilized photosynthetic fuels account for almost 90% of the energy in the world (Johnson, 2016). Cellular respiration is a process that takes place in the living organism and converts nutrients into energy. This essay will examine photosynthesis and cellular respiration separately and identify similarities, differences, and interconnectedness between two processes. Two processes are similar in that they both deals with energy, but they are different because one process involves catabolic reactions and another anabolic one.

The purpose of photosynthesis is to convert atmospheric carbon dioxide into carbohydrates using light energy. The light splits one of the reactants, water in the mesophyll of the leaf into oxygen, electrons, and protons during the light-dependent phase (Johnson, 2016). Then carbon dioxide enters the mesophyll of the leaf through openings, stomata, during the light-independent phase. These two reactions differ in light utilization and molecules production. The first reaction products are oxygen, adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH) that are used as energy storages, while by the end of the second reaction, the carbohydrate is obtained, and molecules mentioned above are used (Flügge et al., 2016). Photosynthesis occurs in the chloroplast with the light-dependent reaction taking place in the thylakoid membrane, and light-independent reaction in the stroma. The energy produced in the light reaction is used to fix carbon dioxide and produce carbohydrates while oxygen is released outside. According to the following equation of the photosynthesis, C → O2 + 2H20 + photons (CH2O)n + electrons + O2 carbon monoxide and water are transferred into carbohydrates under the light with the release of atmospheric oxygen.

The purpose of cellular respiration is to convert nutrients into energy. The reactants of the respiration are glucose circulating in the blood and oxygen obtained from breathing, while the product is ATP. Cellular respiration starts from glycolysis in the mitochondria’s stroma, where the glucose is broken down into pyruvate (Bentley & Connaughton, 2017). Then it continues with the citric acid cycle that generates ATP, NADH, and FADH2. In the final stage, the electron transport chain uses these molecules to generate more ATP. The energy produced is then used for metabolic processes in the organism, while carbon dioxide is released with breathing (BBC Bitesize, n.d.). According to the following equation of the cellular respiration, C → 6H12O6 + 6O2 6CO2 + 6H2O the glucose is broken down into carbon dioxide and water with the presence of oxygen.

There are two main differences between photosynthesis and cellular respiration. The first one is the anabolic process, during which complex compounds are synthesized, while the second one is catabolic, which involves breaking down the compounds (Panawala, 2017). The second crucial difference is that photosynthesis is found only in chloroplasts, while cellular respiration is found in any living cell, making it a universal process. There are also two main similarities between photosynthesis and respiration. The first similarity is that both processes involve the production of ATP (Stauffer et al., 2018). The second similarity is that both processes utilize ATP but for different purposes.

Photosynthesis and cellular respiration are connected in such a way that they allow to perform metabolic functions normally. Moreover, these processes help to regulate the concentration of oxygen and carbon dioxide in the atmosphere. If photosynthesis stopped occurring, the level of oxygen would drop dramatically This would lead to deaths of all living organisms whose lives depend on this molecule. Whereas if cellular respiration stopped happening, living creatures would not be able to generate energy and sustain life.

To conclude, photosynthesis plays a crucial role in maintaining life on Earth. Photosynthesis uses light energy to produce oxygen, while cellular respiration uses oxygen to break down complex molecules and provide energy. These processes are different in their metabolic nature, but similar in terms of energy storage. If photosynthesis did not exist, the life for oxygen-dependent creatures would become extinct. Similarly, in the case of cellular respiration disappearing, living organisms would not be able to produce energy.

BBC Bitesize . (n.d.). Respiration. 2020. Web.

Bentley, M., & Connaughton, V, P. (2017). A simple way for students to visualize cellular respiration: Adapting the board game MousetrapTM to model complexity . CourseSource. 4, 1-6. Web.

Flügge, W., Westhoff, P., & Leister, D. (2016). Recent advances in understanding photosynthesis. F1000 Research, 5, 1-10.

Johnson, M. P. (2016). Photosynthesis. Essays Biochemistry , 60 (3), 255-273.

Panawala, L. (2017). Difference between photosynthesis and respiration. IE PEDIAA. Web.

Stauffer S., Gardner A., Ungu D.A.K., López-Córdoba A., & Heim M. (2018). Cellular respiration. In Labster virtual lab experiments: Basic biology (pp. 43-55). Springer.

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