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104 Photosynthesis Essay Topic Ideas & Examples

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Photosynthesis is a fundamental process in all living organisms that convert light energy into chemical energy. It is the process by which plants, algae, and some bacteria use sunlight to produce glucose, the main source of energy for all living organisms. Given its importance in sustaining life on Earth, it is a popular topic for essays and research papers in biology and related fields.

If you are struggling to come up with a topic for your photosynthesis essay, worry no more! We have compiled a list of 104 photosynthesis essay topic ideas and examples to help you get started.

The process of photosynthesis: a step-by-step explanation
The role of chlorophyll in photosynthesis
The importance of photosynthesis in the ecosystem
The evolution of photosynthesis in plants
Photosynthesis in different types of plants: C3, C4, and CAM plants
The relationship between photosynthesis and cellular respiration
Photosynthesis and global warming
The impact of climate change on photosynthesis
The effects of pollution on photosynthesis
The role of photosynthesis in agriculture
Photosynthesis and food production
The history of photosynthesis research
The discovery of photosynthesis
Photosynthesis and the oxygen revolution
The benefits of indoor plants on photosynthesis
Photosynthesis in aquatic plants
The role of photosynthesis in the carbon cycle
Photosynthesis and biotechnology
The future of photosynthesis research
Photosynthesis and biofuels
Photosynthesis and alternative energy sources
Photosynthesis and the development of sustainable agriculture
The photosynthetic efficiency of different plant species
The impact of light intensity on photosynthesis
Photosynthesis and plant adaptations to different environments
The role of photosynthesis in plant growth and development
Photosynthesis and nitrogen fixation
The effects of temperature on photosynthesis
Photosynthesis and plant stress responses
Photosynthesis and the production of secondary metabolites
The role of photosynthesis in plant defense mechanisms
Photosynthesis and the regulation of gene expression
The impact of genetic engineering on photosynthesis
Photosynthesis and plant biotechnology
Photosynthesis and the production of pharmaceuticals
The role of photosynthesis in biofuel production
Photosynthesis and the development of sustainable bioenergy sources
The effects of climate change on photosynthesis in polar regions
Photosynthesis and the adaptation of plants to changing environmental conditions
The role of photosynthesis in the biodiversity of plant species
Photosynthesis and the evolution of plant diversity
The impact of photosynthesis on ecosystem services
Photosynthesis and the conservation of biodiversity
The effects of deforestation on photosynthesis
Photosynthesis and the restoration of degraded ecosystems
The role of photosynthesis in ecosystem resilience
Photosynthesis and the restoration of wetlands
The impact of invasive species on photosynthesis
Photosynthesis and the management of invasive species
The role of photosynthesis in the restoration of native plant communities
Photosynthesis and the development of green infrastructure
The effects of urbanization on photosynthesis
Photosynthesis and the design of sustainable cities
The role of photosynthesis in urban agriculture
Photosynthesis and the development of green roofs
The impact of climate change on photosynthesis in urban environments
Photosynthesis and the development of green spaces in cities
The role of photosynthesis in urban planning
Photosynthesis and the development of sustainable transportation systems
The effects of air pollution on photosynthesis in urban areas
Photosynthesis and the health benefits of green spaces
The role of photosynthesis in the reduction of urban heat islands
Photosynthesis and the development of sustainable architecture
The impact of photosynthesis on the design of sustainable buildings
Photosynthesis and the use of natural light in buildings
The role of photosynthesis in the development of green building materials
Photosynthesis and the design of energy-efficient buildings
The effects of climate change on photosynthesis in buildings
Photosynthesis and the development of green infrastructure in buildings
The role of photosynthesis in the reduction of energy consumption in buildings
Photosynthesis and the development of sustainable water management systems
The impact of photosynthesis on the design of sustainable landscapes
Photosynthesis and the development of green roofs and walls
The role of photosynthesis in the development of sustainable agriculture
Photosynthesis and the production of food in urban environments
The effects of climate change on photosynthesis in agriculture
Photosynthesis and the development of sustainable farming practices
The role of photosynthesis in the production of organic food
Photosynthesis and the development of sustainable food systems
The impact of photosynthesis on food security
Photosynthesis and the development of sustainable food production systems
The role of photosynthesis in the reduction of food waste
Photosynthesis and the development of sustainable food distribution systems
The effects of climate change on photosynthesis in food production
Photosynthesis and the development of sustainable food packaging
The role of photosynthesis in the reduction of food miles
Photosynthesis and the development of sustainable food processing
The impact of photosynthesis on the design of sustainable food products
Photosynthesis and the development of plant-based alternatives to animal products
The role of photosynthesis in the production of plant-based proteins
Photosynthesis and the development of plant-based dairy alternatives
The effects of climate change on photosynthesis in the production of plant-based products
Photosynthesis and the development of plant-based meat alternatives
The role of photosynthesis in the production of plant-based seafood alternatives
Photosynthesis and the development of plant-based egg alternatives
The impact of photosynthesis on the design of plant-based food products
Photosynthesis and the development of sustainable food products
The impact of photosynthesis on the design of sustainable food systems

With these 104 photosynthesis essay topic ideas and examples, you are sure to find the perfect topic for your essay. Whether you are interested in the science of photosynthesis, its role in the environment, or its applications in biotechnology, there is a topic for everyone. So, get started on your photosynthesis essay today and explore the fascinating world of plant biology and energy production.

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Critical Thinking Questions

ATP and NADPH are forms of chemical energy produced from the light dependent reactions to be used in the light independent reactions that produce sugars.
ATP and NADPH are forms of chemical energy produced from the light independent reactions, to be used in the light dependent reactions that produce sugars.
ATP and NADPH are forms of chemical energy produced from the light dependent reactions to be used in the light independent reactions that produce proteins.
ATP and NADPH are forms of chemical energy produced from the light dependent reactions to be used in the light independent reactions that use sugars as reactants.
NADPH and ATP molecules are produced during the light-independent reactions and are used to power the light-dependent reactions.
Sugar and ATP are produced during the light-dependent reactions and are used to power the light-independent reactions.
Carbon dioxide and NADPH are produced during the light-independent reactions and are used to power the light-dependent reactions.
NADPH and ATP molecules are produced during the light-dependent reactions and are used to power the light-independent reactions.

Examine the illustration of the photosynthesis equation. How does the equation relate to both photosynthesis and cellular respiration, and what is the connection between the two processes?

Photosynthesis utilizes energy to build carbohydrates, while cellular respiration metabolizes carbohydrates.
Photosynthesis utilizes energy to metabolize carbohydrates, while cellular respiration builds carbohydrates.
Photosynthesis and cellular respiration both utilize carbon dioxide and water to produce carbohydrates.
Photosynthesis and cellular respiration both metabolize carbohydrates to produce carbon dioxide and water.
When photons strike photosystem (PS) I, pigments pass the light energy to chlorophyll, molecules that excite electrons, which are then passed to the electron transport chain. The cytochrome complex then transfers protons across the thylakoid membrane and transfers electrons from PS-II to PS-I. The products of the light-dependent reaction are used to power the Calvin cycle to produce glucose.
When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll a molecules that in turn excite electrons, which are then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS-I to PS-II. The products of the light-dependent reaction are used to power the Calvin cycle to produce glucose.
When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll a molecules that excite electrons, which are then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS-II to PS-I. The products of the light-dependent reaction are used to power the Calvin cycle to produce glucose.
When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll a molecules that excite electrons, which are then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS II to PS I. The products of the light-independent reaction are used to power the Calvin cycle to produce glucose.
Because UV rays and X-rays are high-energy waves, they penetrate the tissues and thus damage cells.
Because UV rays and X-rays are long-wavelength waves, they penetrate the tissues and thus damage cells.
Because UV rays and X-rays are low-energy waves, they cannot penetrate tissues and thus damage cells.
Because UV rays and X-rays are low-frequency waves, they can penetrate tissues and thus damage cells.
Photosynthesis is not possible.
Photosynthesis is possible.
Photosynthesis is possible only with blue light.
Photosynthesis is possible only with green light.
After splitting water in PS-I, high-energy electrons are delivered through the chloroplast electron transport chain to PS-II.
After the photosynthesis reaction, released products like glucose help in the transfer of electrons from PS-II to PS-I.
After splitting water in PS-II, high-energy electrons are delivered through the chloroplast electron transport chain to PS-I.
After the completion of the light-dependent reactions, the electrons are transferred from PS-II to PS-I.
This event will have no effect on the rate of photosynthesis in the leaf.
Photosynthesis in the leaf will slow down or possibly stop.
Photosynthesis in the leaf will increase exponentially.
Photosynthesis in the leaf will first decrease and then increase.
The product of the Calvin cycle is glyceraldehyde-3 phosphate and RuBP is regenerated.
The product of the Calvin cycle is glyceraldehyde-3 phosphate and RuBisCO is regenerated.
The product of the Calvin cycle is a 3-PGA molecule and glyceraldehyde-3 phosphate is regenerated.
The product of the Calvin cycle is glyceraldehyde-3 phosphate and oxygen is regenerated.
by using CAM photosynthesis and by closing stomatal pores during the night
by using CAM photosynthesis and by opening stomatal pores during the night
by using CAM photosynthesis and by keeping stomatal pores closed at all times
by bypassing CAM photosynthesis and by keeping stomatal pores closed at night
The prey of lions are generally herbivores, which depend on heterotrophs.
The prey of lions are generally smaller carnivorous animals, which depend on non-photosynthetic organisms.
The prey of lions are generally herbivores, which depend on autotrophs.
The prey of lions are generally autotrophs, which depend onother autotrophs.
It takes three turns to fix enough oxygen to export one G3P molecule.
It takes three turns to produce RuBisCO as an end product.
It takes three turns to produce ATP and NADPH for fixation of G3P.
It takes three turns to fix enough carbon to export one G3P molecule.

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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

Photosynthesis is a process used by plants, algae, and some bacteria to turn sunlight, water, and carbon dioxide into food and oxygen. Think of it like a recipe that plants use to make their own food. This happens in the leaves of plants, which have a green substance called chlorophyll.

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

To make their food, plants need three main things: sunlight, water, and carbon dioxide. Sunlight is the energy plants use to create their food. They get water from the ground through their roots. Carbon dioxide, a gas found in the air, is taken in through tiny holes in the leaves called stomata.

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 also helps the environment. Plants take in carbon dioxide, which is a gas that can make the Earth warmer when there is too much of it in the air. By using carbon dioxide to make food, plants help keep the air clean and the Earth’s temperature just right.

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.

In conclusion, photosynthesis is a vital process that allows plants to make food and oxygen using sunlight, water, and carbon dioxide. It is the foundation of the food chain and has a big impact on the environment and our lives. Understanding photosynthesis helps us appreciate how important plants are and why we need to take care of them and the environment they live in.

That’s it! I hope the essay helped you.

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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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"Photosynthesis and Cellular Respiration." IvyPanda , 21 Feb. 2022, ivypanda.com/essays/photosynthesis-and-cellular-respiration/.

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1. IvyPanda . "Photosynthesis and Cellular Respiration." February 21, 2022. https://ivypanda.com/essays/photosynthesis-and-cellular-respiration/.

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IvyPanda . "Photosynthesis and Cellular Respiration." February 21, 2022. https://ivypanda.com/essays/photosynthesis-and-cellular-respiration/.

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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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Home — Essay Samples — Science — Light — Photosynthesis Process

Photosynthesis Process

Categories: Light Photosynthesis

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Words: 423 |

Published: Feb 12, 2019

Words: 423 | Page: 1 | 3 min read

Works Cited

Campbell, N. A., & Reece, J. B. (2008). Photosynthesis and cellular respiration. In Biology (8th ed., pp. 190-220). Benjamin-Cummings Publishing Company.
Taiz, L., & Zeiger, E. (2010). Photosynthesis: Carbon reactions. In Plant physiology (5th ed., pp. 174-207). Sinauer Associates.
Raven, P. H., Evert, R. F., & Eichhorn, S. E. (2016). Photosynthesis and respiration. In Biology of Plants (8th ed., pp. 186-229). W. H. Freeman and Company.
Niyogi, K. K. (1999). Photoprotection revisited: Genetic and molecular approaches. Annual Review of Plant Physiology and Plant Molecular Biology, 50, 333-359. doi:10.1146/annurev.arplant.50.1.333
Siedow, J. N., & Day, D. A. (2000). Respiration and photorespiration. In Plant physiology (3rd ed., pp. 500-548). Academic Press.
Allen, J. F. (2002). Photosynthesis and cellular respiration considered as coupled redox cycles: A chemiosmotic bridge linking two epochs. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 357(1426), 707-717. doi:10.1098/rstb.2001.0993
Geigenberger, P. (2003). Response of plant metabolism to too little oxygen. Current Opinion in Plant Biology, 6(3), 247-256. doi:10.1016/S1369-5266(03)00038-8
Foyer, C. H., & Noctor, G. (2005). Redox homeostasis and antioxidant signaling: A metabolic interface between stress perception and physiological responses. The Plant Cell, 17(7), 1866-1875. doi:10.1105/tpc.105.033589
Sharkey, T. D. (2005). Effects of moderate heat stress on photosynthesis: Importance of thylakoid reactions, rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprene. Plant, Cell & Environment, 28(3), 269-277. doi:10.1111/j.1365-3040.2005.01324.x
Sweetlove, L. J., & Fernie, A. R. (2018). The impact of oxidative stress on metabolism: A compartmental analysis. Frontiers in Plant Science, 9, 1647. doi:10.3389/fpls.2018.01647

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Important Questions on Plant Photosynthesis

In recent patterns of SSC, Banking, Railways, and others, we find important questions from the General Science section. In this section, Biology topics are considered important. In this article, we will provide a Practice set from one of the most important topics of Biology i.e Plant Photosynthesis. This practice will help students in revising the topic thoroughly.

List of Important Questions on Plant Photosynthesis :

Que 1. What happens when the process of photosynthesis takes place? (a) Taking oxygen and releasing carbon dioxide (b) Taking nitrogen and releasing oxygen (c) Taking oxygen and releasing nitrogen (d) Taking carbon dioxide and releasing oxygen

Ans. (d) Taking carbon dioxide and releasing oxygen Explanation: The process of photosynthesis involves taking carbon dioxide and releasing oxygen.

Que 2. What gas played a major role in the process of photosynthesis? (a) Ammonia (b) Sulphur (c) Chlorine (d) Carbon dioxide

Ans. (d) Carbon dioxide Explanation: Carbon dioxide gas played a major role in the process of photosynthesis.

Que 3. Which gas is released during the process of photosynthesis? (a) Carbon dioxide (b) Oxygen (c) Nitrogen (d) Hydrogen

Ans. (b) Oxygen Explanation: Oxygen gas is released during the process of photosynthesis.

Que 4. Oxygen gas which is released during photosynthesis, comes from? (a) Water (b) Carbon dioxide (c) Ammonia (d) Chlorophyll

Ans. (a) Water Explanation: Oxygen gas which is released during photosynthesis, comes from water.

Que 5. Which of the given lights are strongly absorbed by plants? (a) Indigo and Yellow (b) Yellow and Violet (c) Blue and Red (d) Orange and Violet

Ans. (c) Blue and Red Explanation: Blue and Red lights are strongly absorbed by plants because photosynthesis occurs more in these lights.

Que 6. Which of the given options produces oxygen in a higher percentage? (a) Algae (b) Rice husks (c) Grasses (d) Trees

Ans. (a) Algae Explanation: Aquatic plants and Algae produces around 90% of oxygen.

Que 7. Name the process by which plants synthesize their food. (a) Respiration (b) Photosynthesis (c) Osmosis (d) Reverse Osmosis

Ans. (b) Photosynthesis Explanation: Photosynthesis is the process by which plants synthesize their food.

Que 8. The most effective light in the process of photosynthesis? (a) Violet (b) Green (c) Red (d) Yellow

Ans. (c) Red Explanation: The best photosynthesis takes place in the presence of the Red light spectrum.

Que 9. During the process of photosynthesis, which of the following conversion takes place? (a) Chemical energy converted to electrical energy (b) Wind energy converted to chemical energy (c) Chemical energy converted to light energy (d) Light energy converted to chemical energy

Ans. (d) Light energy converted to chemical energy Explanation: During the process of photosynthesis, Light energy is converted to chemical energy.

Que 10. Which of the following has the maximum efficiency in converting light energy to chemical energy? (a) Earthworm (b) Chlorella (c) Tiger (d) Snake

Ans. (b) Chlorella Explanation: Chlorella has the maximum efficiency in converting light energy to chemical energy.

Que 11. Which of the following performs photosynthesis? (a) Green leaves of a plant (b) Roots of a plant (c) Seed of a plant (d) Stem of a plant

Ans. (a) Green leaves of a plant Explanation: Green leaves of a plant generally performs photosynthesis.

Que 12. What function does Phloem perform? (a) Transportation of water (b)Transportation of food (c)Transportation of oxygen (d)Transportation of amino acid

Ans. (d) Transportation of food Explanation: Transportation of food is the main function of phloem.

Que 13. What are the effects of deforestation? (a) Carbon dioxide levels will increase (b) Oxygen level will increase (c) Carbon dioxide levels will decrease (d) Pollution will decrease

Ans. (a) Carbon dioxide level will increase Explanation: Carbon dioxide levels will increase due to deforestation, which is very harmful to the life of organisms living on the earth.

Que 14. Which of the given options has water-soluble photosynthetic pigments? (a) chlorophyll a (b) xanthophyll (c) Anthocyanin (d) chlorophyll b

Ans. (c) Anthocyanin Explanation: Anthocyanin has water-soluble photosynthetic pigments.

Que 15. The minerals involved in the decomposition reactions during photosynthesis are? (a)manganese and chlorine (b)potassium and manganese (c)potassium and chlorine (d)magnesium and chlorine

Ans. (a) manganese and chlorine Explanation: Manganese and chlorine are the minerals involved in the decomposition reactions during photosynthesis.

Que 16. Where does the photoreaction occur? (a) Endoplasmic reticulum (b) Cytoplasm (c) Stroma (d) Grana

Ans. (d) Grana Explanation: Photoreaction occurs in grana. Grana is the stacks of thylakoids embedded in the stroma of a chloroplast.

Que 17. Kranz’s anatomy can be seen in the leaves of? (a) wheat (b) sugarcane (c) potatoes (d) mustard

Ans. (b) sugarcane Explanation: Kranz’s anatomy can be seen in sugar cane leaves.

Que 18. The most suitable temperature for photosynthesis is? (a) 35-40℃ (b) 25-35℃ (c)20-25℃ (d)10-15℃

Ans. (b) 25-35℃ Explanation: The most suitable temperature for photosynthesis is 25-35℃

Que 19. The process of photosynthesis takes place in? (a) Endoplasmic reticulum (b) Nucleus (c) Golgi body (d) Chloroplast

Ans. (d) Chloroplast Explanation: The process of photosynthesis takes place in the Chloroplast.

Que 20. Which among the following is the full form of ATP? (a) Adenosine Triphosphate (b) Amine Triphosphate (c) Ammonium Triphosphate (d) Amine Try Poly Phosphate

Ans. (a) Adenosine Triphosphate Explanation: The full form of ATP is Adenosine Triphosphate.

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Essay Quiz On Photosynthesis (Light & Dark Reactions)

Multiple choice quiz + 1 brief essay on Photosynthesis (light & dark reactions)

Which best describes the activity of autotrophs?

They use sunlight to break down large complex, energy-rich organic molecules

They convert carbon dioxide and water into complex, energy-rich organic molecules

They use the energy harvested by other photosynthesizers

The use the energy harvest by heterotrophs

Rate this question:

Which is released as a byproduct of photosynthesis?

Carbon dioxide

Which best describes light-independent reactions?

They are the first stage of photosynthesis.

They utilize the energy stored in ATP and NADPH.

They use carbon dioxide to synthesize proteins.

They create energy-rich ATP and NADPH.

Why do we perceive chlorophyll as being green?

Because it is green.

Because it absorbs green light.

Because it reflects green light.

Because it absorbs yellow light.

Which would you expect to increase the rate of photosynthesis?

Incresing the carbon dioxide concentration

Decreasing the intensity of exposure to red light

Increasing the oxygen concentration

Decreasing the duration of exposure to red light

Which could be used to monitor the rate of photosynthesis in a plant?

Carbon dioxide production

Water production

Oxygen production

Hydrogen production

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104 Photosynthesis Essay Topic Ideas & Examples
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Photosynthesis is the process by which light energy is converted into chemical energy, while cellular respiration is the process by which the energy stored in glucose is converted into ATP. The Calvin cycle is the light-independent reaction of photosynthesis, in which carbon dioxide is used to create glucose molecules and other organic compounds.
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Ch. 8 Critical Thinking Questions
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In chemical terms, photosynthesis is a light-energized oxidation-reduction process. (Oxidation refers to the removal of electrons from a molecule; reduction refers to the gain of electrons by a molecule.) In plant photosynthesis, the energy of light is used to drive the oxidation of water (H 2 O), producing oxygen gas (O 2 ), hydrogen ions (H ...
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Cellular respiration happens in the dark. In the process of photosynthesis, the plants need sunlight for energy. Photosynthesis is the process that plants use light energy from the sun to make their own food. Cellular respiration is a chemical process plants use to release the stored chemical energy from glucose as usable chemical energy so it ...
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BIOLOGY ESSAY QUESTIONS WITH MARK SCHEMES 1. Explain the various ways in which a typical cell is adapted to its functions ... Required in the dark stage of photosynthesis; it combines with the hydrogen ion from the light stage; to form glucose, proteins and lipids; low concentrations reduces the rate of ...
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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.
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Unit test. Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere.
Important Questions on Plant Photosynthesis
This practice will help students in revising the topic thoroughly. List of Important Questions on Plant Photosynthesis : Que 1. What happens when the process of photosynthesis takes place? (a) Taking oxygen and releasing carbon dioxide. (b) Taking nitrogen and releasing oxygen. (c) Taking oxygen and releasing nitrogen.
Essay Quiz On Photosynthesis (Light & Dark Reactions)
C. They use carbon dioxide to synthesize proteins. D. They create energy-rich ATP and NADPH. Correct Answer. B. They utilize the energy stored in ATP and NADPH. Explanation. Light-independent reactions, also known as the Calvin cycle, occur in the second stage of photosynthesis.
Ap Biology Essay Questions On Photosynthesis
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Finished Papers. Essay Questions About Photosynthesis, Cover Letter Examples For Higher Education Administration, Weekly Spiral Homework, Phd Thesis Unemployment, Research Proposal Staff Turnover, Tourism Management Personal Statement Examples, Writing Prompt Third Grade Worksheet.

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Photosynthesis and Cellular Respiration Essay

Cover Image

An overview of photosynthesis

Introduction

The global carbon cycle

The site of photosynthesis in plants

Location of the photosynthetic machinery

Division of labour within the chloroplast

Photosynthetic electron and proton transfer chain

The photosynthetic electron and proton transfer chain

Z-scheme of photosynthetic electron transfer

Light absorption by pigments

Major photosynthetic pigments in plants

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

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

Basic mechanism of excitation energy transfer between chlorophyll molecules

Light-harvesting complexes

Reaction centres

Basic structure of a photosystem

Basic structure of the PSII–LHCII supercomplex from spinach

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

Basic structure of the PSI–LHCI supercomplex from pea

Plastoquinone/plastoquinol

Cytochrome b 6 f complex

Plastocyanin

Ferredoxin–NADP + reductase

ATP synthase

Membrane stacking and the regulation of photosynthesis

Lateral heterogeneity in thylakoid membrane organization

‘Dark’ reactions: the Calvin–Benson cycle

The Calvin–Benson cycle

CO 2 -concentrating mechanisms

Diagram of a C 4 plant leaf showing Kranz anatomy

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

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Important Questions on Plant Photosynthesis

List of Important Questions on Plant Photosynthesis :

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Essay Quiz On Photosynthesis (Light & Dark Reactions)

Which best describes the activity of autotrophs?

Which is released as a byproduct of photosynthesis?

Which best describes light-independent reactions?

Why do we perceive chlorophyll as being green?

Which would you expect to increase the rate of photosynthesis?

Which could be used to monitor the rate of photosynthesis in a plant?

Who will write my essay?

What is a good essay writing service?

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