Thus, the hydrogen so released reduces the carbon dioxide to carbohydrate, and molecular oxygen is produced. 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. Synthesis common to all cells there are numerous compounds not obtained by cells as prefabricated nutrients; rather, they are synthesized within the cells themselves.
These are primarily the organic macromolecules which constitute the bulk of cell contents exclusive of water. In all cells except the photo synthetic cells discussed above, and the chemosynthetic bacterial cells mentioned previously, the raw materials employed in synthetic reactions comes from the digestion of prefabricated materials taken into the organism.
The energy necessary to drive these endergonic reactions also comes from these prefabricated materials, in their respiration. Preserve Articles is home of thousands of articles published and preserved by users like you. Here you can publish your research papers, essays, letters, stories, poetries, biographies, notes, reviews, advises and allied information with a single vision to liberate knowledge.
Before preserving your articles on this site, please read the following pages: Short Essay on Photosynthesis Akhila Mol. What is Bursa of Fabricus? 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 and dark reactions are therefore mutually dependent on one another. 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.
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) .
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.
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.
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.