Skip Nav


Essay Writing Guides

❶Thus, it is at this point that the products of the light phase enter into the reduction of carbon dioxide.

An overview of photosynthesis


The light reactions of photosynthesis take place in the thylakoid membrane, whereas the dark reactions are located in the chloroplast stroma. 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. 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. This scheme is known as the linear electron transfer pathway or Z-scheme Figure 4.

Generally, electrons are transferred from redox couples with low potentials good reductants to those with higher potentials good oxidants e. 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 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. 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.

Relationship between redox potentials and standard free energy changes. 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. 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.

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. 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. 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.

The energy of the excited electron in the S 1 state can have one of several fates: 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. Two chlorophyll molecules with resonant S 1 states undergo a mirror transition resulting in the non-radiative transfer of excitation energy between them.

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. A photosystem consists of numerous LHCs that form an antenna of hundreds of pigment molecules. 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. 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 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.

Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P within PSII by light and is known as the S-state cycle Figure After the fourth turnover of P, 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 P provides the thermodynamic driving force, whereas the manganese cluster acts as a catalyst for the water-splitting reaction. The organization of PSII and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P 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. 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.

PSI is a light-driven plastocyanin—ferredoxin oxidoreductase Figure The organization of PSI and its light-harvesting antenna. 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.

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.

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 is a small soluble electron carrier protein that resides in the thylakoid lumen. Ferredoxin is a small soluble electron carrier protein that resides in the chloroplast stroma. The FNR complex is found in both soluble and thylakoid membrane-bound forms. According to the structure, 4. The enzyme is a rotary motor which contains two domains: The cyt b 6 f complex, in contrast, is evenly distributed throughout the grana and stromal lamellae. 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.

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. 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.

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.

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

It is noteworthy that, despite the complexity of the dark reactions outlined above, the carbon fixation step itself i. Rubisco is a large multisubunit soluble protein complex found in the chloroplast stroma.

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.

Thus the electrons emitted from chlorophyll are returned to it unchanged and energy being released as heat and fluorescence. This cyclic process is known as cyclic photophosphorylation. It is concerned with synthesizing ATP molecules but the source of energy is different. In photophosphorylation energy comes from sunlight via chlorophyll.

We have already seen that when light strikes chlorophyll an electron is emitted. Now not all the electrons are returned via the electron carrier system to chlorophyll. Sometimes an electron, together with a hydrogen ion, is taken up by a hydrogen acceptor, nicotinamide adenine dinucleotide phosphate NADP , which is thus reduced.

The reduced NADP i. The chlorophyll molecule which has lost an electron is in an unstable state. The hydroxyl ion donates an electron to chlorophyll and the OH resulting from this forms water and oxygen. By acting as an electron donor the hydroxyl ion restores the stability of the chlorophyll molecule. The oxygen is given off in photosynthesis.

The electron derived from the hydroxyl ion is conveyed from one chlorophyll molecule to another via an electron carrier system. This results in the formation of ATP. This pathway is non-cyclic phosphorylation. The passage of electrons in non-cyclic phosphorylation is complex, involving two distinct molecules or groups of molecules of chlorophyll a.

These are known as photosystems I and II. They are at different energy levels, and under the influence of light the energy state of an electron can be raised, enabling it to be transferred from one system to the other.

In the course of its journey it passes through electron, carriers with resulting ATP synthesis. One of the electron carriers is an iron-containing protein called ferredoxin. The various pigments [Chlorophyll a blue-green , chlorophyll b yellow-green , xanthophyll yellow , carotene orange and phaeophytin grey is the breakdown product of chlorophyll.

It appears that electrons are moved from one pigment to another. Chlorophyll a is the final pigment in the series, receiving electrons from other pigments and handing them on via electron carriers to NADP. In the Figure 5, photosystem I groups of molecules of chlorophyll a is at a higher energy level than photosystem II. When light strikes a chlorophyll molecule belonging to photosysiem I, an electron is raised to a higher energy state and is taken up by an electron acceptor and then passed to NADP.

Meanwhile the electron which has been lost from photosystcm I is replaced by an electron from photosystem II. This later electron, lost from photosystem II under the influence of light is taken up by an electron acceptor and then passed to photosystem I via the electron carrier system with the production of ATP. The electron lost from photosystem II is replaced by an electron from the hydroxyl ion derived from the splitting of water.

Dark reactions involve the reduction of carbon dioxide to form carbohydrate. This is an endergonic process requiring energy. The reduction of CO 2 , and subsequent synthesis of carbohydrate, takes place in a series of small steps, each controlled by a specific enzyme.

The individual steps have been analysed by Melvin Calvin and his associates. The chain of reactions is cyclical and known as the Calvin cycle. This is carbon dioxide acceptor and fixes the CO 2 , i. The enzyme needed for this is called RUBP carboxylase. The combination of carbon dioxide with ribulose biphosphate gives an unstable 6-carbon compound which splits immediately into two molecules of a 3-carbon Compound, phosphoglyceric acid PGA.

The hydrogen for the reduction comes from reduced NADP of light reaction, which also supplies most of the energy, the rest coming from ATP. The 3-carbon sugar is now built up to a 6-carbon sugar which can be converted into starch for storage. Not all the 3-carbon sugar PGAL is converted into 6-carbon sugar.

Some of it majority infact enters a series of reactions which results in the regeneration of ribulose biphosphate. Its continuous supply is important for the continued fixation of CO 2. It represents a storage of , calories per mole. The starch is not the only end-product of photosynthesis. For amino acids formation, nitrates are required.

These are converted to ammonium ions which are used for the formation of glutamine. Fnm this other amino acids are made by transamination. In transaminat on, the amino group is removed from amino acid and transferred to a carbohydrate derivative which is thereby converted into a new amino acid. Some plants like cane-type plants such as sugar cane and maize, use another compound, phosphoenol pyruvic acid PEP instead of ribulose biphosphate, as the substrate for carbon dioxide fixation in certain of their cells.

The immediate product of CO 2 fixation is not 3-carbon PGA but the 4-carbon compound oxaloacetic acid.

Main Topics

Privacy Policy

Photosynthesis involves a complex series of reactions, some of which take place only in the presence of light, while others can also be carried out in the dark. (1) .

Privacy FAQs

Photosynthesis Essay: Photosynthesis is the process of production of organic elements from carbon dioxide, water and energy of the sun by plants. Photosynthesis is the most essential process which occurs on our planet, due to which exists life on Earth.

About Our Ads

In summary, photosynthesis is an extremely complex process involving many separate reactions. Like virtually all reactions which within occur living systems, they . Essay is provided by US essay writers Photosynthesis is the process through which green plants and other specific living organisms utilize light energy to convert water and carbon dioxide in to simple sugars.

Cookie Info

Photosynthesis is process by which green plants and certain other organisms use the energy of light to convert carbon dioxide and water into the simple sugar glucose. In so doing, photosynthesis provides . Background information: Photosynthesis Photosynthesis is the process of autotrophs turning carbon dioxide and water into carbohydrates and oxygen, using light energy from sunlight. Autotrophs are organisms that are able to produce nutrients and organic compounds using inorganic materials.