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Photosynthesis Process

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Published: Feb 12, 2019

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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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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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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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Explainer: how photosynthesis works.

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

green leaves lit up from behind with sunlight

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

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Let the light shine in

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

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

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

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

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

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

Give me some sugar

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

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

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

The Calvin cycle has four major steps:

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

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

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

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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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8.1: Overview of Photosynthesis - The Purpose and Process of Photosynthesis

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Learning Objectives

Describe the process of photosynthesis

The Importance of Photosynthesis

The processes of all organisms—from bacteria to humans—require energy. To get this energy, many organisms access stored energy by eating food. Carnivores eat other animals and herbivores eat plants. But where does the stored energy in food originate? All of this energy can be traced back to the process of photosynthesis and light energy from the sun.

Photosynthesis is essential to all life on earth. It is the only biological process that captures energy from outer space (sunlight) and converts it into chemical energy in the form of G3P ( Glyceraldehyde 3-phosphate) which in turn can be made into sugars and other molecular compounds. Plants use these compounds in all of their metabolic processes; plants do not need to consume other organisms for food because they build all the molecules they need. Unlike plants, animals need to consume other organisms to consume the molecules they need for their metabolic processes.

The Process of Photosynthesis

During photosynthesis, molecules in leaves capture sunlight and energize electrons, which are then stored in the covalent bonds of carbohydrate molecules. That energy within those covalent bonds will be released when they are broken during cell respiration. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago.

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

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

Photosynthesis evolved as a way to store the energy in solar radiation as high-energy electrons in carbohydrate molecules.
Plants, algae, and cyanobacteria, known as photoautotrophs, are the only organisms capable of performing photosynthesis.
Heterotrophs, unable to produce their own food, rely on the carbohydrates produced by photosynthetic organisms for their energy needs.
photosynthesis : the process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts
photoautotroph : an organism that can synthesize its own food by using light as a source of energy
chemoautotroph : a simple organism, such as a protozoan, that derives its energy from chemical processes rather than photosynthesis

Biology Article

Photosynthesis

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

Table of Contents

What is Photosynthesis?
Site of photosynthesis

Photosynthesis definition states that the process exclusively takes place in the chloroplasts through photosynthetic pigments such as chlorophyll a, chlorophyll b, carotene and xanthophyll. All green plants and a few other autotrophic organisms utilize photosynthesis to synthesize nutrients by using carbon dioxide, water and sunlight. The by-product of the photosynthesis process is oxygen.Let us have a detailed look at the process, reaction and importance of photosynthesis.

What Is Photosynthesis in Biology?

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

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

A visual representation of the photosynthesis reaction

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

Where Does This Process Occur?

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

Also Read: Photosynthesis Early Experiments

Photosynthesis Equation

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

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

Structure Of Chlorophyll

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

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

Process Of Photosynthesis

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

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

Structure of Chloroplast. Note the presence of the thylakoid

“Photosynthesis Steps:”

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

The process of photosynthesis occurs in two stages:

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

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

Light Reaction of Photosynthesis (or) Light-dependent Reaction

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

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

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

Dark Reaction of Photosynthesis (or) Light-independent Reaction

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

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

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

* G3P – glyceraldehyde-3-phosphate

Calvin photosynthesis Cycle (Dark Reaction)

Also Read: Cyclic And Non-Cyclic Photophosphorylation

Importance of Photosynthesis

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

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The Process of Photosynthesis

Introduction.

Photosynthesis is fundamental to the energy flow process in living organisms. “Plants are the primary producers and they make use of sunlight to produce sugars for energy production.” (Govindjee, 1997, p. 45) Excess nutrients are stored and the plants are eaten up by herbivores and omnivores which rely on the energy stored in the plant cells to keep alive. The herbivores are subsequently consumed by the omnivores and carnivores; this process continues from one living organism to another creating a food chain that sustains life on earth.

Problem statement

The energy flow process is fundamental to the sustenance of life on earth. For survival, each organism requires nutrition; the nutrition is sourced from another organism as food substrates except for the green plants which manufacture their food using sunlight and minerals. Many types of research have been conducted and reveal that the process of photosynthesis is the main process that sustains life. This experimental study is designed to ascertain how the process of photosynthesis leads to energy production and how it is affected by variation in light intensity and wavelength.

Relevance of the question

This study is essential for a proper understanding of the role of plants as primary producers in the food chain process.

Literature review

The photosynthesis process occurs primarily in the leaves with little taking place in the stem for some plants. The main parts of the leaf in which are involved in the process include; “the upper and lower epidermis, the mesophyll, the vascular bundles and the stomates.” (Photosynthesis, 2000) The epidermis lacks chlorophyll and therefore photosynthesis does not occur there, the epidermis only acts to protect the internal part of the leaf. “The stomates are tiny holes in the epidermis through which gaseous exchange takes place.” (Photosynthesis, 2000, para.2)

Through the stomates, Co2 enters, and O2 leaves. “The vascular bundles constitute the plants transport system through which water and other nutrients are moved around the plant.” (Photosynthesis, 2000, para.2)

Cross-sectional view of a leaf revealing the various parts.

Chlorophyll is composed of the outer and inner membranes, “there is also an inter membrane space stroma and thylakoids which are stacked in grana. The chlorophyll is normally built in the membrane of the thylakoids.” (Photosynthesis, 2000, para.3)

Thus, photosynthesis is the main process in which the “radiant light energy is absorbed by the chloroplast’s pigment, chlorophyll and converted into chemical energy in the molecular form of ATP and NADH.” (Photosynthesis, 2000) The energy is then utilized in driving the Calvin cycle in the production of three-carbon sugars. The three-carbon sugars are subsequently converted to various types of carbohydrates.

According to Govindjee, it is currently possible to carry out photolysis in the laboratory. The reaction was first performed by Robert Hill in 1937 and thus became known as the Hill Reaction. The Hill Reaction requires only intact isolated chloroplasts rather than the entire intact plant cell. However, since isolated chloroplasts do not reduce carbon dioxide directly, it is essential to provide a hydrogen acceptor for the reduction process and permit electron transport to take place. The hydrogen acceptor that was chosen for this experiment is the synthetic compound 2,6-dichlorophenol-indophenol or DPIP , which is blue in the oxidized form and colorless when reduced. As indicated above, the source of hydrogen for the reduction in water. This hydrogen can reduce DPIP and turn it from a blue substance to a colorless one.

Oxygen is also formed, but it is not monitored in this experiment. The change in color of the DPIP solution, which is directly proportional to the number of hydrogen produced in the Hill Reaction, can be assayed using colorimetric spectrophotometry. The more hydrogen produced, the more DPIP that is reduced and the more colorless the solution containing DPIP becomes. Therefore, the course of this reaction can be assessed by the bleaching of the artificial hydrogen acceptor. (1997, p. 144))

Light + CO2 + 2H2O → n (CH2O) + H2O + O2

The experiment entailed the use of both boiled and fresh chloroplast suspension. The boiled chloroplast was used as a control and photosynthetic activities were monitored in the fresh chloroplast. The results were measured using a spectrophotometer and tabulated. The same procedure was repeated with variations in the light intensity and wavelength.

Spectrophotometer
Spectrophotometer cuvettes
Phosphate buffer (pH6.5)
Chloroplast suspension
Sodium hydrosulfite
Distilled water
Wrapping foil
Digital timer
DPIP solution

The source of hydrogen for the reduction in water. This hydrogen can reduce DPIP and turn it from a blue substance to a colorless one. Oxygen is also formed, but it is not monitored in this experiment. The change in color of the DPIP solution, which is directly proportional to the number of hydrogen produced in the Hill Reaction, can be assayed using colorimetric spectrophotometry. The more hydrogen produced, the more DPIP that is reduced and the more colorless the solution containing DPIP becomes. Therefore, the course of this reaction can be assessed by the bleaching of the artificial hydrogen acceptor. The experimental procedure was selected because it has a high specificity and therefore results will have a low error margin. The chemicals used are readily available were purchased in various forms and reconstituted in the laboratory before the practice according to the manufacture’s guidelines.

Obtain fresh and boiled chloroplast suspension prepared before the practical time. The boiling of the suspension will render the chloroplasts non-functional and will serve as one of the experimental controls. Allow the solution to return to room temperature before use. The chloroplast is boiled preferably at 100 degrees for 5 to 7 minutes.
Obtain four clean spectrophotometer cuvettes. Label them 1-4 and prepare them as follows: Cuvette 1: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of boiled chloroplast suspension, Cuvette 2: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of chloroplast suspension, Cuvette 3: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of chloroplast suspension, Cuvette 4: 3.0 ml of phosphate buffer (pH 6.5) and 1.0 ml of chloroplast suspension
Add 50 μl of the 0.1% DPIP solution to cuvette 4 only. Shake it to mix the solution well. Then add a few (very few) crystals of sodium hydrosulfite (Na2S2O4) to it. Sodium hydrosulfite reduces the DPIP and removes the blue color.
Be sure that the spectrophotometer is set to 600nm. Use the ‘reduced’ cuvette 4 as a blank to zero the spectrophotometer.
Add the same amount of DPIP that you added to cuvette 4 to cuvette 1. Mix the solution well and take a reading as quickly as possible using the spectrophotometer, because cuvettes with functional chloroplasts should immediately start producing hydrogen that reduces the DPIP. Record this value as ‘time zero’. Start the digital timer.
Repeat step 5 for cuvettes 2 and 3. Record the ‘time zero’ values and their starting time points.
Immediately after taking the first readings, wrap cuvette 3 completely in aluminum foil and place cuvettes 1-3 in a beaker of water at room temperature. Set the light source 10 inches from the cuvettes and turn it on to the highest setting. Note that the water in the beaker serves as a thermal buffer to prevent any experimental artifact due to warming by the source light.
Every two minutes record the optical density of cuvettes 1 and 2. Leave cuvette 3 in the dark until the end of the procedure.
Continue taking readings until the optical density reading of cuvette 2 does not change.
Remove cuvette 3 from its foil wrapping. Quickly read the optical density of this cuvette. Take two-minute readings until the optical density reading does not change for two or three-time points. Make a graphical plot with time as the independent variable and optical density as the dependent variable.

Note: Different colors of light indicate the difference in wavelength thus the use of various colors of cellophane to measure wavelength.

Dependent, independent, and controlled variables

The independent variable is the time of exposure whereby the duration of the reaction is changed to determine the effect of time on the DPIP reduction (visualized as the clearing of the blue color). The dependent variable is the optical density which varies depending on the duration the cuvette was left to react. The controlled variables include the use of the same amount of constituent chemical for each tube, temperature of the reactions where all the tubes are reacted at room temperature.

Threat reduction to internal validity was done by:

Taking measurements immediately after the timed duration had expired; was done to prevent errors in the measurement of the optical density. Providing the same conditions for all measurements taken i.e. the spectrophotometer was blanked by the same solution for all the readings which was done at 600nm. All the necessary conditions for the materials were provided prior to and during the experiment.

Plants form an important part of the food chain. “Green plants manufacture their on food using the photosynthesis process.” (Photosynthesis, 2000, para.1) Sunlight is an important aspect of this process; the practical examines the process of photosynthesis, particularly the role played by light. Therefore:

HA: Light is a fundamental factor for the photosynthetic process where it’s used to reduce carbon dioxide and break the water molecule. In this experiment, the hydrogen from the water molecule reacts with DCIP to reduce its blue color. The spectrophotometer is used to measure the intensity of the reduced color.

Ho: Light is not a fundamental factor in the process of photosynthesis and does not reduce carbon dioxide or break the water molecule to produce hydrogen used in the experiment to reduce DCIP’s blue color that is measured using a spectrophotometer.

Use of appropriate methods, tools, and technologies to collect quantitative data

In this experiment quantitative data was collected as optical density readings. The readings were taken at 600 nm after the spectrophotometer was blanked using cuvette 4 which had 3.0 ml of phosphate buffer (pH 6.5) 1.0 ml of chloroplast suspension and 50 μl of the 0.1% DPIP solution added and the reaction left to complete( color clears). The optical density reading for each cuvette was recorded against the time duration in the lab notebook.

Data Collection

Data collection is an important aspect for the success of any experiment, for this particular experiment, the data was collected by the use of a spectrophotometer. Every reaction was timed according to the manual and on expiry of the time duration, the optical density readings were taken using the spectrophotometer and recorded in the laboratory notebook for use in the tabulation and plotting of graphs for analysis.

Experiment: the hill reaction.

Results of the Experiment

The results for the experimental were plotted as below

This experiment was designed to investigate the photosynthesis process in which light is utilized for energy production. In the hill reaction, the rate of photosynthesis remained the same for cuvettes 1 and 3, there was no evidence of photosynthetic activity in the fourth cuvette. In cuvette 5, the photosynthetic activity decreased with time. From the experiments, it is deduced that photosynthesis occurs in two stages, the light-dependent, and the light-independent stage. Generally, “in the first stage energy is captured and stored in the form of ATP and NADPH.” (Photosynthesis, 2000)

In the second stage light-independent stage the energy stored is used to process sugars using carbon. The results of this experiment agree with the hypothesis that light is a fundamental factor for the photosynthetic process where it’s used to reduce carbon dioxide and break the water molecule. In this experiment, the hydrogen from the water molecule reacts with DCIP to reduce its blue color. The spectrophotometer is used to measure the intensity of the reduced color

The experiment was carried out successfully and the results indicate that indeed plants manufacture their own food using sunlight as a source of energy. Therefore sunlight is an important factor in this process. However, the success of the practical was due to the experimental design which provided all the necessary material and conditions for the laboratory model of photosynthesis. A good experimental design gives results with minimal errors and thus can be used to conclude. “Validity refers how closeness the values are for repeated measures.”(Govindjee, 1997, p. 67) Replication of the above experiment gives results from which comparison can be drawn to give a test of validity for this experimental design.

The experiment can be replicated by another person provided he/she formulates a suitable experimental design that will include the identification, manipulation, and measurement of all the parameters in the investigation. The experimental design must be reliable and carefully selected to reduce the error margin to minimum values as this is an important aspect of this experiment.

Reference list

Govindjee, G. (1997). Experiments in plant biology. Berlin: Springer.

Photosynthesis. (2000). Web.

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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.

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Article Contents

Photosynthetic research in plant science.

Article contents
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Supplementary Data

Ayumi Tanaka, Amane Makino, Photosynthetic Research in Plant Science, Plant and Cell Physiology , Volume 50, Issue 4, April 2009, Pages 681–683, https://doi.org/10.1093/pcp/pcp040

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Photosynthesis is a highly regulated, multistep process. It encompasses the harvest of solar energy, transfer of excitation energy, energy conversion, electron transfer from water to NADP + , ATP generation and a series of enzymatic reactions that assimilate carbon dioxide and synthesize carbohydrate.

Photosynthesis has a unique place in the history of plant science, as its central concepts were established by the middle of the last century, and the detailed mechanisms have since been elucidated. For example, measurements of photosynthetic efficiency (quantum yield) at different wavelengths of light (Emerson and Lews 1943 ) led to the insight that two distinct forms of Chl must be excited in oxygenic photosynthesis. These results suggested the concept of two photochemical systems. The reaction center pigments of PSII and PSI (P680 and P700, respectively) were found by studying changes in light absorbance in the red region (Kok 1959 , Döring et al. 1969 ). Chls with absorbance maxima corresponding to these specific wavelengths were proposed as the final light sink. These Chls were shown to drive electron transfer by charge separation. The linkage of electron transfer and CO 2 assimilation was suggested by studies on Hill oxidant (Hill 1937 ). A linear electron transport system with two light-driven reactions (Z scheme) was proposed based upon observations of the redox state of cytochromes (Hill and Bendall 1960 , Duysens et al. 1961 ), and photophosphorylation was found to be associated with thylakoid fragments (Arnon et al. 1954 ). The metabolic pathway that assimilates carbon by fixation of CO 2 was discovered by Calvin's group who used 14 CO 2 radioactive tracers in the 1950s (Bassham and Calvin 1957 ). This was the first significant discovery in biochemistry made using radioactive tracers. The primary reaction of CO 2 fixation is catalyzed by Rubisco (Weissbach et al. 1956 ), initially called Fraction 1 protein (Wildman and Bonner 1947 ). Rubisco is the most abundant protein in the world, largely because it is also the most inefficient with the lowest catalytic turnover rate (1–3 s –1 ). Another CO 2 fixation pathway was then found in sugarcane (Kortschak et al. 1964, Hatch and Slack 1965) and named C 4 photosynthesis.

Although photosynthesis plays the central role in the energy metabolism of plants, historically there have not been strong interactions between photosynthesis research and other fields of plant science. Many techniques and tools developed for photosynthesis research have not been widely used in other fields because they were developed to examine phenomena unique to photosynthesis. For example, excitation energy transfer and charge separation are fundamental but unique processes of photosynthesis. Another reason for the historic isolation of photosynthesis research within plant science is that it was long believed that CO 2 fixation and carbohydrate production are the sole function of photosynthesis, with carbohydrates representing the only link between photosynthesis and other biological phenomena.

However, this situation has begun to change. Recent research has revealed that photosynthesis is closely related to a variety of other physiological processes. It is a major system for controlling the redox state of cells, playing an important role in regulating enzyme activity and many other cellular processes (Buchanan and Balmer 2005 , Hisabori et al. 2007 ). Photosynthesis also generates reactive oxygen species, which are now appreciated as being regulatory factors for many biological processes rather than inevitable by-products of photosynthesis (Wagner et al. 2004 , Beck 2005 ). Precursor molecules of Chl, which are a major component of photosynthesis, act as a chloroplast-derived signal, and are involved in regulating the cell cycle (Kobayashi et al. 2009 ). In light of this new information, it seems important to re-evaluate the function(s), both potential and demonstrated, of photosynthesis from a variety of view points. Photosynthesis research now employs the methods and tools of molecular biology and genetics, which are central methods for plant science in general. Meanwhile, Chl fluorescence and gas exchange measurements, developed especially for photosynthesis research, are now widely used in stress biology and ecology.

Photosynthesis research also contributes to our understanding of ecological phenomena and even the global environments (Farquhar et al. 1980 , de Pury and Farquhar 1997 , Monsi and Saeki 2005 ). Indeed, photosynthesis is now an integral component of simulation models used to predict the future of our planet. Improving the efficiency of photosynthesis by artificial modification of photosynthetic proteins and pathways has long been considered impossible or at best problematic, because, over evolutionary time, photosynthesis has become complex and tightly regulated. However, recent advances have made it possible to manipulate photosynthesis using molecular genetic technology (Andrews and Whiney 2003 , Raines 2006 ). These advances may have positive influences on crop productivity (Parry et al. 2007 ) as photosynthetic rates have frequently been correlated with biomass accretion (Kruger and Volin 2006 ). Thus, we can expect many more novel concepts to be added to this history of photosynthetic research.

As photosynthesis research tackles new challenges, we should also continue to re-evaluate past research. Oxygen evolution, energy dissipation and cyclic electron transport are crucial processes during photosynthesis, yet their mechanisms still remain to be clarified. We have very limited knowledge of the formation and degradation of photosynthetic apparatus. Also, although photosynthesis plays a central role in C and N metabolism in plants, we do not yet understand how potential photosynthesis is related to crop productivity.

Plant and Cell Physiology would like to contribute to the development of novel concepts, pioneering new fields and solving the unanswered questions of photosynthesis. This special issue covers a wide range of topics in photosynthesis research. Terashima et al. (pp. 684–697) readdress the enigmatic question of why leaves are green. They show that the light profile through a leaf is steeper than that of photosynthesis, and that the green wavelengths in white light are more effective in driving photosynthesis than red light. Evans (pp. 698–706) proposes a new model using Chl fluorescence to explore modifications in quantum yield with leaf depth. This new multilayered model can be applied to study variations in light absorption profiles, photosynthetic capacity and calculation of chloroplastic CO 2 concentration at different depths through the leaf.

Singlet oxygen, 1 O 2 , is produced by the photosystem and Chl pigments. 1 O 2 not only causes physiological damage but also activates stress response programs. The flu mutant of Arabidopsis thaliana overaccumulates protochlorophyllide that upon illumination generates singlet oxygen, causing growth cessation and cell death. Coll et al. (pp. 707–718) have isolated suppressor mutants, dubbed ‘singlet oxygen-linked death activator’ (soldat), that specifically abrogate 1 O 2 -mediated stress responses in young flu seedlings, and they discuss the processes of acclimation to stresses. Phephorbide a is a degradation product of Chl and one of the most powerful photosensitzing molecules. Mutants defective in pheophorbide a oxygenase, which converts phephorbide a to open tetrapyrrole, accumulate pheophorbide a and display cell death in a light-dependent manner. Hirashima et al. (pp. 719–729) report that pheophorbide a is involved in this light-independent cell death.

Plants regulate the redox level of the plastoquinone pool in response to the light environment. In acclimation to high-light conditions, the redox level is kept in an oxidized state by the plastoquinone oxidation system (POS). Miyake et al. (pp. 730–743) investigated the mechanism of POS using the Chl fluorescence parameter, qL.

Nagai and Makino (pp. 744–755) examine in detail the differences between rice and wheat, the two most commercially important crops, in the temperature responses of CO 2 assimilation and plant growth. They find that the difference in biomass production between the two species at the level of the whole plant depends on the difference in N-use efficiency in leaf photosynthesis and growth rate. Sage and Sage (pp. 756–772) examine chlorenchyma structure in rice and related Oryza species in relation to photosynthetic function. They find that rice chlorenchyma architecture includes adaptations to maximize the scavenging of photorespired CO 2 and to enhance the diffusive conductance of CO 2 . In addition, they consider that the introduction of Kranz anatomy does not require radical anatomical alterations in engineering C 4 rice.

Bioinformatics has become a powerful tool, especially in photosynthetic research, because photosynthetic organisms have a wide taxonomic distribution among prokaryotes and eukaryotes. Ishikawa et al. (pp. 773–788) present the results of a pilot study of functional orthogenomics, combining bioinformatic and experimental analyses to identify nuclear-encoded chloroplast proteins of endosymbiontic origin (CRENDOs). They conclude that phylogenetic profiling is useful in finding CPRENDOs, although the physiological functions of orthologous genes may be different in chloroplasts and cyanobacteria.

We hope you enjoy this special issue, and would like to invite you to submit more excellent papers to Plant and Cell Physiology in the field of photosynthesis.

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Photosynthesis

Matthew p. johnson.

Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, U.K.

An overview of photosynthesis

Introduction.

Light reactions:

Dark reactions:

Standard free energy change

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

The site of photosynthesis in plants

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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

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Relationship between redox potentials and standard free energy changes

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How the photosystems work

Light absorption by pigments.

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

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 .

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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).

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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

Reaction centres

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Photosystem II

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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

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Photosystem I

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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.

Other electron transfer chain components

Plastoquinone/plastoquinol.

Cytochrome b 6 f complex

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Plastocyanin

Ferredoxin–NADP + reductase

ATP synthase

Membrane stacking and the regulation of photosynthesis

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‘Dark’ reactions: the Calvin–Benson cycle

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

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CO 2 -concentrating mechanisms

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Acknowledgments

Abbreviations

Competing Interests

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

Photosynthesis

Affiliation.

1 Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, U.K. [email protected].
PMID: 27784776
PMCID: PMC5264509
DOI: 10.1042/EBC20160016

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

Keywords: membrane; photosynthesis; thylakoid.

Publication types

Electron Transport*
Photosynthesis*
Photosynthetic Reaction Center Complex Proteins / chemistry
Photosynthetic Reaction Center Complex Proteins / genetics
Photosynthetic Reaction Center Complex Proteins / metabolism*
Plants / metabolism*
Photosynthetic Reaction Center Complex Proteins

Regenerating worms have genetic control over their algal partners

Many organisms are far more complex than just a single species. Humans, for example, are full of a variety of microbes. Some creatures have even more special connections, though. Acoels, unique marine worms that regenerate their bodies after injury, can form symbiotic relationships with photosynthetic algae that live inside them. These collections of symbiotic organisms are called a holobiont, and the ways that they “talk” to each other are something scientists are trying to understand – especially when the species in question are an animal and a solar-powered microbe.

Bo Wang , assistant professor of bioengineering in the Schools of Engineering and of Medicine at Stanford, has started to find some answers. His lab, in conjunction with the University of San Francisco, studies Convolutriloba longifissura , a species of acoel that hosts the symbiotic algae Tetraselmis. According to new research, published in Nature Communications , the researchers found that, when C. longifissura regenerates, a genetic factor that takes part in the acoel regeneration also controls how the algae inside of them reacts.

“We don’t know yet how these species talk to each other or what the messengers are. But this shows their gene networks are connected,” said Wang, who is a senior author of the paper.

Splitting worms

Because holobiont is a relatively new concept, scientists still aren’t sure what the nature of some relationships are. The odd name “acoel” is Greek for “no cavity,” as the worms have no separation between their inner and outer organs (called a coelom). In these animals, all their organs share the same space. Some acoels also have symbiotic algae sharing the space of the animals' organs, and photosynthesizing inside the animals' body. This relationship provides a safe zone for the algae and extra energy from photosynthesis to the acoel.

“There was no guarantee that there was communication because the algae are not within the acoel’s cells, they’re floating around them,” says James Sikes, a researcher at the University of San Francisco and co-senior author of the paper. Sikes has been working with acoels for about 20 years, and their symbiotic relationship differentiates them from other animals that regenerate, like planarian flatworms and axolotls.

When these acoels reproduce asexually, they first bisect themselves. The head region grows a tail and becomes a new acoel. The tail, however, acts like the mythological Hydra and grows two new heads, which, then, split into two separate animals.

Animal regeneration requires communication across many different cell types, but in this case, it may also involve another organism entirely. Researchers were curious about how the algal colonies inside reacted to this process – in particular, whether they continued to photosynthesize as normal, and if not, what was controlling that? This was especially puzzling as the team found that photosynthesis wasn’t required for acoels to regenerate – they could do it in the dark. But there has to be conversation between the species for their long-term survival.

“Testing if photosynthesis was affected was an adventure. None of us knew what we were doing,” says Dania Nanes Sarfati, lead author of the paper, who was a doctoral student in Wang’s lab and Stanford Bio-X Bowes Fellow . “One of the most exciting things was that we could actually measure algal photosynthesis happening inside the animal.”

In addition, through sequencing, the team was able to differentiate the genes of the two species and figure out which pathways were responding to injury. These measurements helped them realize that the algae inside were undergoing a major reconstruction of their photosynthetic machinery during the regeneration – but the process by which it was being controlled was shocking.

The role of runt

When the results came back, Wang said the unpredictable happened. During regeneration, both the acoel’s regrowth and the algal photosynthesis appeared to be controlled by a common transcription factor in acoels called runt.

In the early stage, right after injury, runt is activated, kicking off the regeneration process . Meanwhile, algal photosynthesis drops off, but there is an upregulation in algal genes associated with photosynthesis – likely to compensate for the loss in photosynthesis due to the split. However, when the researchers knocked down runt , it halted both regeneration and the algal responses.

What’s special about runt is that it’s highly conserved, meaning the same factor is responsible for regeneration in many different organisms, including non-symbiotic acoels. But now it’s clear that instead of just controlling the acoel’s regenerative process, it also controls the communication with another species.

How holobionts communicate

Understanding how partners in symbiotic relationships communicate at the molecular level opens up many new questions for this field of research. “Are there rules of symbiosis? Do they exist?” said Nanes Sarfati. “This research sparks these kinds of questions, which we can link to other organisms.”

Wang believes it introduces more ways of investigating how symbiotic species interact and couple with each other to form holobionts. Some of these interactions could be potentially driven by chemicals, proteins, or environmental factors. But more concerningly, these interactions are now becoming vulnerable points under the challenge of climate change, causing symbiotic partners to separate. Sikes highlighted that he, Wang, and Nanes Sarfati all began from the animal side of the symbiotic relationship but realized that algae respond to host injury as well, potentially sparking similar questions in other systems.

“We often assume we know a lot, but we’re humbled when we look at different species,” Wang said. “They can do things in completely unexpected ways, which highlights the need to study more organisms and is becoming possible with technology.”

Additional Stanford co-authors include former graduate student Yuan Xue, PhD ’21; PhD student Eun Sun Song; Stephen Quake , the Lee Otterson Professor of Bioengineering in the Schools of Engineering and Medicine and professor of applied physics in the School of Humanities and Sciences; and Adrien Burlacot , assistant professor (by courtesy) of biology in the School of Humanities and Sciences.

Burlacot is also a principal investigator at the Carnegie Institution for Science. Quake is also former co-president of Chan Zuckerberg Biohub San Francisco and the current Head of Science at the Chan Zuckerberg Initiative, and a member of Stanford Bio-X , the Cardiovascular Institute , the Wu Tsai Human Performance Alliance , the Stanford Cancer Institute , and the Wu Tsai Neurosciences Institute . Wang is also a member of Bio-X and the Wu Tsai Neurosciences Institute.

This research was funded by a Bio-X Bowes Fellowship, a Stanford Interdisciplinary Graduate Fellowship, the Beckman Young Investigator Program, the Carnegie Institution for Science, and the National Institutes of Health.

Related : Bo Wang , assistant professor of bioengineering

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Photosynthesis Process

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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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8.1: Overview of Photosynthesis - The Purpose and Process of Photosynthesis

Learning Objectives

The Importance of Photosynthesis

The Process of Photosynthesis

Photosynthesis

What Is Photosynthesis in Biology?

Where Does This Process Occur?

Photosynthesis Equation

Structure Of Chlorophyll

Process Of Photosynthesis

Light Reaction of Photosynthesis (or) Light-dependent Reaction

Dark Reaction of Photosynthesis (or) Light-independent Reaction

Importance of Photosynthesis

Frequently Asked Questions