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Photosynthesis: Essay on Photosynthesis (2098 Words)

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❶The phosphoglycolate must be converted in a series of reactions that regenerate one molecule of 3-phosphoglycerate and one molecule of CO 2. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems, and leaves.

An overview of photosynthesis

Photosynthesis


Here again, we are involved in an oxidation-reduction, but one which is highly unlikely from a thermodynamic viewpoint. The problem is this: In other words, the C0 2 and H 2 0 are much more stable than the 0 2 and the carbohydrate. Carbon dioxide joins a five-carbon sugar, rib lose diphosphate, which is already present in the cell, to form a very unstable six-carbon compound.

The six-carbon compound has a very brief existence; almost immediately it breaks down spontaneously into two molecules of a three- carbon compound, 3-phosphoglyeerie acid. H 2 with the aid of a molecule of ATP. Thus, it is at this point that the products of the light phase enter into the reduction of carbon dioxide. PGAL is now at the reduction level of a carbohydrate which corresponds to that of an aldehyde, and it may travel any of several different pathways.

It may undergo a series of reactions and eventually be transformed to RDP, it may become modified into glycerol, or it may undergo condensation to form the six-carbon sugar fructose diphosphate, which can undergo dephosphioiylation and certain internal transformations to become glucose.

Glucose may then serve as a building block for such saccharine sugars as sunrise or such polysaccharides as starch. Although PGAL might justly be considered the end product of photosynthesis.

PGA is frequently involved in transformation. It may proceed along a pathway leading to the formation of amino acids, which subsequently become involved in protein synthesis, or it may become involved in the formation of fatty acids, which join with glycerol in the formation of fats.

Notice that this scheme shows the entrance of carbon dioxide and the products of the light phase into a cycle involving the compounds we have discussed.

Although we have mentioned only a few of the many possible synthetic pathways taken by PGA and PGAL, it should be obvious that the basic organic molecules which serve as nutrient materials for cells of green plants themselves and for the cells of other organisms are produced in photosynthesis. In summary, photosynthesis is an extremely complex process involving many separate reactions.

Like virtually all reactions which within occur living systems, they are catalyzed by a complex of specific enzymes. Although the light and dark phases of photosynthesis can be separated experimentally, they are closely interrelated in the overall metabolism of any given photiosynthetic cell.

In addition to photosynthesis, the plant cell carriers on respiration, during which large amounts of ATP are formed, and this ATP supplies energy for many of the synthetic reactions we have mentioned. In other words, the ATP formed during the light phase of photosynthesis is not nearly sufficient to drive the many endergonic reactions carried on in the plant. Nevertheless, our original equation is accurate as a summary equation, because every energetic reaction is driven by energy which is ultimately supplied by sunlight.

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.

These reactions consume additional ATP and thus result in an energy loss to the plant. Under normal conditions, the ratio of carboxylation to oxygenation is between 3: 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.

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 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. Plants use much of this glucose, a carbohydrate, as an energy source to build leaves, flowers, fruits, and seeds.

They also convert glucose to cellulose, the structural material used in their cell walls. Most plants produce more glucose than they use, however, and they store it in the form of starch and other carbohydrates in roots, stems, and leaves. The plants can then draw on these reserves for extra energy or building materials. Each year, photosynthesizing organisms produce about billion metric tons of extra carbohydrates, about 30 metric tons for every person on earth.

Photosynthesis has far-reaching implications. Like plants, humans and other animals depend on glucose as an energy source, but they are unable to produce it on their own and must rely ultimately on the glucose produced by plants. Moreover, the oxygen humans and other animals breathe is the oxygen released during photosynthesis.

Humans are also dependent on ancient products of photosynthesis, known as fossil fuels, for supplying most of our modern industrial energy. These fossil fuels, including natural gas, coal, and petroleum, are composed of a complex mix of hydrocarbons, the remains of organisms that relied on photosynthesis millions of years ago.

Thus, virtually all life on earth, directly or indirectly, depends on photosynthesis as a source of food, energy, and oxygen, making it one of the most important biochemical processes known.

Plant photosynthesis occurs in leaves and green stems within specialized cell structures called chloroplasts. One plant leaf is composed of tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. The chloroplast, an oval-shaped structure, is divided by membranes into numerous disk-shaped compartments.

These dislike compartments, called thylakoids, are arranged vertically in the chloroplast like a stack of plates or pancakes.


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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) .

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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.

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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.

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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.