significance of photosynthesis essay

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

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• 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

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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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Middle school biology - NGSS

Course: middle school biology - ngss   >   unit 5, photosynthesis in ecosystems.

• Understand: photosynthesis in ecosystems

Key points:

• Energy enters an ecosystem when light energy from the sun is transformed into chemical energy. This happens during photosynthesis .
• Photosynthesis is carried out by photosynthetic organisms .
• Photosynthetic organisms take in and use carbon dioxide and water from the air and soil.
• Photosynthetic organisms release oxygen into the air. Organisms throughout the ecosystem use this oxygen to breathe.
• Photosynthetic organisms produce sugars, which become part of the organism’s biomass . When the photosynthetic organism is eaten, its biomass provides matter and energy to the organism that eats it.

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Essay on Photosynthesis in Plants

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

• Essay on the Factors Affecting Photosynthesis

Essay # 1. Meaning of Photosynthesis:

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

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

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

Essay # 5. Photosynthesis Pigments:

Photosynthetic pigments are of three types:

(1) Chlorophylls,

(2) Carotenoids, and

(3) Phycobillins.

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

ii. Phycobillins are soluble in water.

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

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

v. They show property of fluoresces.

Distribution of Photosynthetic Pigments in Plant Kingdom :

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

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

Structure of Photosynthetic Pigments :

(1) Chlorophylls:

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

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

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

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

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

Absorption Spectra of Chlorophylls:

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

Absorption Spectra of Carotenoids:

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

Absorption Spectra of Phycobillins:

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

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

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

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

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

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

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

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

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

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

These are of two types:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Why Is Photosynthesis Important for All Organisms?

How Does a Plant Convert Light Energy to Chemical Energy?

Photosynthesis is important to living organisms because it is the number one source of oxygen in the atmosphere. Without photosynthesis, the carbon cycle could not occur, oxygen-requiring life would not survive and plants would die. Green plants and trees use photosynthesis to make food from sunlight, carbon dioxide and water in the atmosphere: It is their primary source of energy. The importance of photosynthesis in our life is the oxygen it produces. Without photosynthesis there would be little to no oxygen on the planet.

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

Photosynthesis is important for all living organisms because it provides the oxygen needed by most living creatures for survival on the planet.

Reasons Why Photosynthesis Is Important

• It is the number one source of oxygen in the atmosphere.
• It contributes to the carbon cycle between the earth, the oceans, plants and animals.
• It contributes to the symbiotic relationship between plants, humans and animals.
• It directly or indirectly affects most life on Earth.
• It serves as the primary energy process for most trees and plants.

How Photosynthesis Works

Photosynthesis uses light energy from the sun and carbon dioxide and water in the atmosphere to make food for plants, trees, algae and even some bacteria. It releases oxygen as a byproduct. The chlorophyll in these living organisms, which also contributes to their green hues, absorbs the sunlight and combines it with carbon dioxide to convert these compounds into an organic chemical called adenosine triphosphate (ATP). ATP is crucial in the relationship between energy and living things, and is known as the "energy currency for all life."

Importance of Cellular Respiration to Photosynthesis

Cellular respiration allows all living cells to extract energy in the form of ATP from food and offer that energy for the vital processes of life. All living cells in plants, animals and humans take part in cellular respiration in one form or another. Cellular respiration is a three-step process. In step one, the cytoplasm of the cell breaks down glucose in a process called glycolysis, producing two pyruvate molecules from one glucose molecule and releasing a bit of ATP. In the second step, the cell transports the pyruvate molecules into the mitochondria, the energy center of the cells, without using oxygen, This is known as anaerobic respiration. The third step of cellular respiration involves oxygen and is called aerobic respiration, in which the food energy enters an electron transport chain where it produces ATP.

Cellular respiration in plants is essentially the opposite of photosynthesis. Living creatures breathe in oxygen and release carbon dioxide as a byproduct. A plant uses the carbon dioxide exhaled by animals and humans in combination with the sun's energy during cellular respiration to produce the food that it requires. Plants eventually release oxygen back into the atmosphere, resulting in a symbiotic relationship between plants, animals and humans.

Non-Photosynthetic Plants

While most plants use photosynthesis to produce energy, there are some that are non-photosynthetic. Plants that do not use photosynthesis to produce food are usually parasitic, which means they rely on a host for nutrient generation. Examples include Indian pipe ( Monotropa uniflora ) – also known as the ghost or corpse plant – and beechdrops ( Epifagus americana ), which steals nutrients found in beech tree roots. The Indian pipe plant is a ghostly white color because it contains no chlorophyll. Plants in the fungi kingdom – mushrooms, molds and yeasts – rely on their environment for food instead of photosynthesis.

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• University of California Santa Barbara: How Does Photosynthesis Affect Other Organisms?
• Columbia University: The Carbon Cycle and Earth's Climate
• State University of New York Cortland: Non-Photosynthetic Plants
• California State University, Sacramento: Kingdom Fungi

About the Author

As a journalist and editor for several years, Laurie Brenner has covered many topics in her writings, but science is one of her first loves. Her stint as Manager of the California State Mining and Mineral Museum in California's gold country served to deepen her interest in science which she now fulfills by writing for online science websites. Brenner is also a published sci-fi author. She graduated from San Diego's Coleman College in 1972.

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What is photosynthesis?

Photosynthesis is arguably the most important biological process on earth. By liberating oxygen and consuming carbon dioxide, it has transformed the world into the hospitable environment we know today. Directly or indirectly, photosynthesis fills all of our food requirements and many of our needs for fiber and building materials. The energy stored in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the energy in firewood, which is a major fuel in many parts of the world. This being the case, scientific research into photosynthesis is vitally important. If we can understand and control the intricacies of the photosynthetic process, we can learn how to increase crop yields of food, fiber, wood, and fuel, and how to better use our lands. The energy-harvesting secrets of plants can be adapted to man-made systems which provide new, efficient ways to collect and use solar energy. These same natural "technologies" can help point the way to the design of new, faster, and more compact computers, and even to new medical breakthroughs. Because photosynthesis helps control the makeup of our atmosphere, understanding photosynthesis is crucial to understanding how carbon dioxide and other "greenhouse gases" affect the global climate. In this document, we will briefly explore each of the areas mentioned above, and illustrate how photosynthesis research is critical to maintaining and improving our quality of life.

Photosynthesis and food. All of our biological energy needs are met by the plant kingdom, either directly or through herbivorous animals. Plants in turn obtain the energy to synthesize foodstuffs via photosynthesis. Although plants draw necessary materials from the soil and water and carbon dioxide from the air, the energy needs of the plant are filled by sunlight. Sunlight is pure energy. However, sunlight itself is not a very useful form of energy; it cannot be eaten, it cannot turn dynamos, and it cannot be stored. To be beneficial, the energy in sunlight must be converted to other forms. This is what photosynthesis is all about. It is the process by which plants change the energy in sunlight to kinds of energy that can be stored for later use. Plants carry out this process in photosynthetic reaction centers. These tiny units are found in leaves, and convert light energy to chemical energy, which is the form used by all living organisms. One of the major energy-harvesting processes in plants involves using the energy of sunlight to convert carbon dioxide from the air into sugars, starches, and other high-energy carbohydrates. Oxygen is released in the process. Later, when the plant needs food, it draws upon the energy stored in these carbohydrates. We do the same. When we eat a plate of spaghetti, our bodies oxidize or "burn" the starch by allowing it to combine with oxygen from the air. This produces carbon dioxide, which we exhale, and the energy we need to survive. Thus, if there is no photosynthesis, there is no food. Indeed, one widely accepted theory explaining the extinction of the dinosaurs suggests that a comet, meteor, or volcano ejected so much material into the atmosphere that the amount of sunlight reaching the earth was severely reduced. This in turn caused the death of many plants and the creatures that depended upon them for energy.

Photosynthesis and energy. One of the carbohydrates resulting from photosynthesis is cellulose, which makes up the bulk of dry wood and other plant material. When we burn wood, we convert the cellulose back to carbon dioxide and release the stored energy as heat. Burning fuel is basically the same oxidation process that occurs in our bodies; it liberates the energy of "stored sunlight" in a useful form, and returns carbon dioxide to the atmosphere. Energy from burning "biomass" is important in many parts of the world. In developing countries, firewood continues to be critical to survival. Ethanol (grain alcohol) produced from sugars and starches by fermentation is a major automobile fuel in Brazil, and is added to gasoline in some parts of the United States to help reduce emissions of harmful pollutants. Ethanol is also readily converted to ethylene, which serves as a feedstock to a large part of the petrochemical industry. It is possible to convert cellulose to sugar, and then into ethanol; various microorganisms carry out this process. It could be commercially important one day.

Our major sources of energy, of course, are coal, oil and natural gas. These materials are all derived from ancient plants and animals, and the energy stored within them is chemical energy that originally came from sunlight through photosynthesis. Thus, most of the energy we use today was originally solar energy!

Photosynthesis, fiber, and materials. Wood, of course, is not only burned, but is an important material for building and many other purposes. Paper, for example, is nearly pure photosynthetically produced cellulose, as is cotton and many other natural fibers. Even wool production depends on photosynthetically-derived energy. In fact, all plant and animal products including many medicines and drugs require energy to produce, and that energy comes ultimately from sunlight via photosynthesis. Many of our other materials needs are filled by plastics and synthetic fibers which are produced from petroleum, and are thus also photosynthetic in origin. Even much of our metal refining depends ultimately on coal or other photosynthetic products. Indeed, it is difficult to name an economically important material or substance whose existence and usefulness is not in some way tied to photosynthesis.

Photosynthesis and the environment. Currently, there is a lot of discussion concerning the possible effects of carbon dioxide and other "greenhouse gases" on the environment. As mentioned above, photosynthesis converts carbon dioxide from the air to carbohydrates and other kinds of "fixed" carbon and releases oxygen to the atmosphere. When we burn firewood, ethanol, or coal, oil and other fossil fuels, oxygen is consumed, and carbon dioxide is released back to the atmosphere. Thus, carbon dioxide which was removed from the atmosphere over millions of years is being replaced very quickly through our consumption of these fuels. The increase in carbon dioxide and related gases is bound to affect our atmosphere. Will this change be large or small, and will it be harmful or beneficial? These questions are being actively studied by many scientists today. The answers will depend strongly on the effect of photosynthesis carried out by land and sea organisms. As photosynthesis consumes carbon dioxide and releases oxygen, it helps counteract the effect of combustion of fossil fuels. The burning of fossil fuels releases not only carbon dioxide, but also hydrocarbons, nitrogen oxides, and other trace materials that pollute the atmosphere and contribute to long-term health and environmental problems. These problems are a consequence of the fact that nature has chosen to implement photosynthesis through conversion of carbon dioxide to energy-rich materials such as carbohydrates. Can the principles of photosynthetic solar energy harvesting be used in some way to produce non-polluting fuels or energy sources? The answer, as we shall see, is yes.

Why study photosynthesis?

Because our quality of life, and indeed our very existence, depends on photosynthesis, it is essential that we understand it. Through understanding, we can avoid adversely affecting the process and precipitating environmental or ecological disasters. Through understanding, we can also learn to control photosynthesis, and thus enhance production of food, fiber and energy. Understanding the natural process, which has been developed by plants over several billion years, will also allow us to use the basic chemistry and physics of photosynthesis for other purposes, such as solar energy conversion, the design of electronic circuits, and the development of medicines and drugs. Some examples follow.

Photosynthesis and agriculture. Although photosynthesis has interested mankind for eons, rapid progress in understanding the process has come in the last few years. One of the things we have learned is that overall, photosynthesis is relatively inefficient. For example, based on the amount of carbon fixed by a field of corn during a typical growing season, only about 1 - 2% of the solar energy falling on the field is recovered as new photosynthetic products. The efficiency of uncultivated plant life is only about 0.2%. In sugar cane, which is one of the most efficient plants, about 8% of the light absorbed by the plant is preserved as chemical energy. Many plants, especially those that originate in the temperate zones such as most of the United States, undergo a process called photorespiration. This is a kind of "short circuit" of photosynthesis that wastes much of the plants' photosynthetic energy. The phenomenon of photorespiration including its function, if any, is only one of many riddles facing the photosynthesis researcher.

If we can fully understand processes like photorespiration, we will have the ability to alter them. Thus, more efficient plants can be designed. Although new varieties of plants have been developed for centuries through selective breeding, the techniques of modern molecular biology have speeded up the process tremendously. Photosynthesis research can show us how to produce new crop strains that will make much better use of the sunlight they absorb. Research along these lines is critical, as recent studies show that agricultural production is leveling off at a time when demand for food and other agricultural products is increasing rapidly.

Because plants depend upon photosynthesis for their survival, interfering with photosynthesis can kill the plant. This is the basis of several important herbicides, which act by preventing certain important steps of photosynthesis. Understanding the details of photosynthesis can lead to the design of new, extremely selective herbicides and plant growth regulators that have the potential of being environmentally safe (especially to animal life, which does not carry out photosynthesis). Indeed, it is possible to develop new crop plants that are immune to specific herbicides, and to thus achieve weed control specific to one crop species.

Photosynthesis and energy production. As described above, most of our current energy needs are met by photosynthesis, ancient or modern. Increasing the efficiency of natural photosynthesis can also increase production of ethanol and other fuels derived from agriculture. However, knowledge gained from photosynthesis research can also be used to enhance energy production in a much more direct way. Although the overall photosynthesis process is relatively wasteful, the early steps in the conversion of sunlight to chemical energy are quite efficient. Why not learn to understand the basic chemistry and physics of photosynthesis, and use these same principles to build man-made solar energy harvesting devices? This has been a dream of chemists for years, but is now close to becoming a reality. In the laboratory, scientists can now synthesize artificial photosynthetic reaction centers which rival the natural ones in terms of the amount of sunlight stored as chemical or electrical energy. More research will lead to the development of new, efficient solar energy harvesting technologies based on the natural process.

The role of photosynthesis in control of the environment. How does photosynthesis in temperate and tropical forests and in the sea affect the quantity of greenhouse gases in the atmosphere? This is an important and controversial issue today. As mentioned above, photosynthesis by plants removes carbon dioxide from the atmosphere and replaces it with oxygen. Thus, it would tend to ameliorate the effects of carbon dioxide released by the burning of fossil fuels. However, the question is complicated by the fact that plants themselves react to the amount of carbon dioxide in the atmosphere. Some plants, appear to grow more rapidly in an atmosphere rich in carbon dioxide, but this may not be true of all species. Understanding the effect of greenhouse gases requires a much better knowledge of the interaction of the plant kingdom with carbon dioxide than we have today. Burning plants and plant products such as petroleum releases carbon dioxide and other byproducts such as hydrocarbons and nitrogen oxides. However, the pollution caused by such materials is not a necessary product of solar energy utilization. The artificial photosynthetic reaction centers discussed above produce energy without releasing any byproducts other than heat. They hold the promise of producing clean energy in the form of electricity or hydrogen fuel without pollution. Implementation of such solar energy harvesting devices would prevent pollution at the source, which is certainly the most efficient approach to control.

Photosynthesis and electronics. At first glance, photosynthesis would seem to have no association with the design of computers and other electronic devices. However, there is potentially a very strong connection. A goal of modern electronics research is to make transistors and other circuit components as small as possible. Small devices and short connections between them make computers faster and more compact. The smallest possible unit of a material is a molecule (made up of atoms of various types). Thus, the smallest conceivable transistor is a single molecule (or atom). Many researchers today are investigating the intriguing possibility of making electronic components from single molecules or small groups of molecules. Another very active area of research is computers that use light, rather than electrons, as the medium for carrying information. In principle, light-based computers have several advantages over traditional designs, and indeed many of our telephone transmission and switching networks already operate through fiber optics. What does this have to do with photosynthesis? It turns out that photosynthetic reaction centers are natural photochemical switches of molecular dimensions. Learning how plants absorb light, control the movement of the resulting energy to reaction centers, and convert the light energy to electrical, and finally chemical energy can help us understand how to make molecular-scale computers. In fact, several molecular electronic logic elements based on artificial photosynthetic reaction centers have already been reported in the scientific literature.

Photosynthesis and medicine. Light has a very high energy content, and when it is absorbed by a substance this energy is converted to other forms. When the energy ends up in the wrong place, it can cause serious damage to living organisms. Aging of the skin and skin cancer are only two of many deleterious effects of light on humans and animals. Because plants and other photosynthetic species have been dealing with light for eons, they have had to develop photoprotective mechanisms to limit light damage. Learning about the causes of light- induced tissue damage and the details of the natural photoprotective mechanisms can help us can find ways to adapt these processes for the benefit of humanity in areas far removed from photosynthesis itself. For example, the mechanism by which sunlight absorbed by photosynthetic chlorophyll causes tissue damage in plants has been harnessed for medical purposes. Substances related to chlorophyll localize naturally in cancerous tumor tissue. Illumination of the tumors with light then leads to photochemical damage which can kill the tumor while leaving surrounding tissue unharmed. Another medical application involves using similar chlorophyll relatives to localize in tumor tissue, and thus act as dyes which clearly delineate the boundary between cancerous and healthy tissue. This diagnostic aid does not cause photochemical damage to normal tissue because the principles of photosynthesis have been used to endow it with protective agents that harmlessly convert the absorbed light to heat.

Conclusions

The above examples illustrate the importance of photosynthesis as a natural process and the impact that it has on all of our lives. Research into the nature of photosynthesis is crucial because only by understanding photosynthesis can we control it, and harness its principles for the betterment of mankind. Science has only recently developed the basic tools and techniques needed to investigate the intricate details of photosynthesis. It is now time to apply these tools and techniques to the problem, and to begin to reap the benefits of this research.

Written by and Copyright ©1996 Devens Gust Professor of Chemistry and Biochemistry, Arizona State University

A  translation  of this article into Belorussian by Martha Ruszkowski is available

Learn more about Devens Gust

Photosynthesis and the environment

• Published: 14 December 2013
• Volume 119 , pages 1–2, ( 2014 )

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• Asaph B. Cousins 1 ,
• Matt Johnson 2 &
• Andrew D. B. Leakey 3 , 4  

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Nearly 240 years after Joseph Priestley’s influential experiments involving a mouse, a plant and a bell jar the need and desire to study photosynthesis and the environment has not diminished. In fact, it is well recognized that the relationship between photosynthesis and the environment is key to understanding the health of our planet, in addition to providing clean air, water and food security across the globe. Although there is a wealth of information, scaling across time (femtosecond to gigayear) and space (angstrom to globe), on the response of photosynthesis to changing environmental conditions there is still much to be learned about the interaction between photosynthetic processes and the environment in which it happens. In fact this has never been truer as our planet’s climate changes at unprecedented rates and the population of humankind continues to grow (both in number and girth).

With this in mind we present a special issue of Photosynthesis Research entitled, “Photosynthesis and the Environment”, which is a compilation of exciting new work on photosynthesis and the environment at a range of scales from the biophysical and molecular to the physiological and biogeochemical, including evolutionary and ecological perspectives. Integration across these scales and the merging of traditionally distinct approaches are key features of the work. The research ideas presented here come from some of the best early career researchers in photosynthesis whom have received their Ph.D. within 15 years of 2013. We solicited early career scientist for this review issue as they can potentially provide a unique perspective on the future of photosynthesis research. Additionally, these scientists are in the trenches of training the next generation(s) of interdisciplinary scientists as well as engaging the non-scientific community about the importance of both fundamental and applied photosynthetic research.

We’ve organized the structure of this special issue scaling from large to small. The first publications address issues relating to global modeling of photosynthesis (Dietze 2013 ), the use of biochemical parameters to constrain these models (Rogers 2013 ), and the influence of climate (Desai 2013 ) and seasonal changes (Stoy et al. 2013 ) to canopy level photosynthesis. At the physiological level, manuscripts discuss the use of leaf optical measurements (Ainsworth et al. 2013 ), the role of internal CO 2 diffusion (Buckley and Warren 2013 ), the thermal acclimation of photosynthesis (Way and Yamori 2013 ), and the thermal response of different photosynthetic functional types (Yamori et al. 2013 ). Following this, a set of manuscripts addresses integration of photosynthesis with other key processes including water use and respiration; specifically discussing genetic variation in water use efficiency (Easlon et al. 2013 ), the role of redox state on stomatal regulation (Busch 2013 ), and the interaction of mitochondrial metabolism and photosynthesis (Araújo et al. 2013 ). The special features of C 4 photosynthesis are then discussed both in terms of natural variation in C 4 Kranz (Covshoff et al. 2013 ), and single-cell C 4 photosynthesis (Sharpe and Offermann 2013 ). Ultimately, at the molecular and biochemical level, manuscripts address circadian regulation of photosynthesis (Dodd et al. 2013 ), Rubisco (Cavanagh and Kubien 2013 ), Rubisco activase (Mueller-Cajar et al. 2013 ), pigment regulation of light harvesting (Holleboom and Walla 2013 ), pigment biosynthesis (Sobotka 2013 ), thylakoid reactions (Johnson and Ruban 2013 ) and thylakoid organization (Sznee et al. 2013 ).

We are excited about the findings and opinions presented here and the discussion of future research directions collected in these manuscripts. As for many centuries, this is an exciting time to study photosynthesis, and it is clear that this area of research has a bright future that will assimilate much more valuable knowledge as this multidisciplinary field continues to move forward.

Ainsworth EA, Serbin SP, Skoneczka JA, Townsend PA (2013) Using leaf optical properties to detect ozone effects on foliar biochemistry. Photosynth Res. doi: 10.1007/s11120-013-9837-y

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Araújo WL, Nunes-Nesi A, Fernie AR (2013) On the role of plant mitochondrial metabolism and its impact on photosynthesis in both optimal and sub-optimal growth conditions. Photosynth Res. doi: 10.1007/s11120-013-9807-4

Buckley TN, Warren CR (2013) The role of mesophyll conductance in the economics of nitrogen and water use in photosynthesis. Photosynth Res. doi: 10.1007/s11120-013-9825-2

Busch FA (2013) Opinion: the red-light response of stomatal movement is sensed by the redox state of the photosynthetic electron transport chain. Photosynth Res. doi: 10.1007/s11120-013-9805-6

Cavanagh AP, Kubien DS (2013) Can phenotypic plasticity in Rubisco performance contribute to photosynthetic acclimation? Photosynth Res. doi: 10.1007/s11120-013-9816-3

Covshoff S, Burgess SJ, Kneřová J, Kümpers BMC (2013) Getting the most out of natural variation in C 4 photosynthesis. Photosynth Res. doi: 10.1007/s11120-013-9872-8

Desai AR (2013) Influence and predictive capacity of climate anomalies on daily to decadal extremes in canopy photosynthesis. Photosynth Res. doi: 10.1007/s11120-013-9925-z

Dietze MC (2013) Gaps in knowledge and data driving uncertainty in models of photosynthesis. Photosynth Res. doi: 10.1007/s11120-013-9836-z

Dodd AN, Kusakina J, Hall A, Gould PD, Hanaoka M (2013) The circadian regulation of photosynthesis. Photosynth Res. doi: 10.1007/s11120-013-9811-8

Easlon HM, Nemali KS, Richards JH, Hanson DT, Juenger TE, McKay JK (2013) The physiological basis for genetic variation in water use efficiency and carbon isotope composition in Arabidopsis thaliana . Photosynth Res. doi: 10.1007/s11120-013-9891-5

Holleboom C-P, Walla PJ (2013) The back and forth of energy transfer between carotenoids and chlorophylls and its role in the regulation of light harvesting. Photosynth Res. doi: 10.1007/s11120-013-9815-4

Johnson MP, Ruban AV (2013) Rethinking the existence of a steady-state Δ ψ component of the proton motive force across plant thylakoid membranes. Photosynth Res. doi: 10.1007/s11120-013-9817-2

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Mueller-Cajar O, Stotz M, Bracher M (2013) Maintaining photosynthetic CO 2 fixation via protein remodelling: the Rubisco activases. Photosynth Res. doi: 10.1007/s11120-013-9819-0

Rogers A (2013) The use and misuse of Vc, max in earth system models. Photosynth Res. doi: 10.1007/s11120-013-9818-1

Sharpe RM, Offermann S (2013) One decade after the discovery of single-cell C 4 species in terrestrial plants: what did we learn about the minimal requirements of C 4 photosynthesis? Photosynth Res. doi: 10.1007/s11120-013-9810-9

Sobotka R (2013) Making proteins green; biosynthesis of chlorophyll-binding proteins in cyanobacteria. Photosynth Res. doi: 10.1007/s11120-013-9797-2

Stoy PC, Trowbridge AM, Bauerle WL (2013) Controls on seasonal patterns of maximum ecosystem carbon uptake and canopy-scale photosynthetic light response: contributions from both temperature and photoperiod. Photosynth Res. doi: 10.1007/s11120-013-9799-0

Sznee K, Crouch LI, Jones MR, Dekker JP, Frese RN (2013) Variation in supramolecular organisation of the photosynthetic membrane of Rhodobacter sphaeroides induced by alteration of PufX. Photosynth Res. doi: 10.1007/s11120-013-9949-4

Way DA, Yamori W (2013) Thermal acclimation of photosynthesis: on the importance of adjusting our definitions and accounting for thermal acclimation of respiration. Photosynth Res. doi: 10.1007/s11120-013-9873-7

Yamori W, Hikosaka K, Way DA (2013) Temperature response of photosynthesis in C 3 , C 4 , and CAM plants: temperature acclimation and temperature adaptation. Photosynth Res. doi: 10.1007/s11120-013-9874-6

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School of Biological Sciences, Washington State University, Pullman, WA, 99164-4236, USA

Asaph B. Cousins

Department of Molecular Biology & Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield, S10 2TN, UK

Matt Johnson

Department of Plant Biology, Institute for Genomic Biology, University of Illinois at Urbana-Champaign, 1402 Institute for Genomic Biology, 1206 W Gregory Dr, Urbana, IL, 61801, USA

Andrew D. B. Leakey

Department of Crop Sciences, University of Illinois at Urbana-Champaign, 1402 Institute for Genomic Biology, 1206 W Gregory Dr, Urbana, IL, 61801, USA

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Cousins, A.B., Johnson, M. & Leakey, A.D.B. Photosynthesis and the environment. Photosynth Res 119 , 1–2 (2014). https://doi.org/10.1007/s11120-013-9958-3

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Published : 14 December 2013

Issue Date : February 2014

DOI : https://doi.org/10.1007/s11120-013-9958-3

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Home — Essay Samples — Science — Light — Photosynthesis Process

Photosynthesis Process

• Categories: Light Photosynthesis

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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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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.

© 2016 The Author(s).

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

10. Explain the significance of photosynthesis?

Significance of photosynthesis are as follows:- 1. provide food:- photosynthesis helps to prepare food in plants and they prepare their food not only for themselves but all living beings which are lived on this earth because each and every organism is dependent on plants, either directly or indirectly. photosynthesis play vital role for the growth and sustenance of the biosphere. 2. provide oxygen:- photosynthesis is the only process by which oxygen is evolved which is used by living organisms photosynthesis in plants is necessary to maintain the oxygen levels in the atmosphere..

Write two significances of photosynthesis.