Photosynthesis packages light in chemical bonds. It is evaluated that every year, photosynthesis generates approx 1.4 x 1014kg of carbohydrates. (This evaluates means that sufficient sugar to fill trucks which will go from here to moon and back 50 times is being produced every year). Packaging light in carbohydrates engages two main reactions - Photochemical reactions and biochemical reactions. Photochemical reactions are procedures which convert light energy in chemical energy from where photosynthesis takes off. Biochemical reactions now use products of photochemical reactions to go in biosynthesis.
The Photochemical Reactions:
Cycle Electron Flow:
Photosynthetic prokaryotes utilize only cyclic photophosphorylation, the process geared to energy production rather than to biosynthesis. Photophosphorylation is comparatively simple and engages only photo system I. Electrons energized by light are recycled through electron transport chain and return to same reaction centre from which they initiated. Electron carriers move electrons only in company of hydrogen ions, which are transported across membrane as elections move to next member of chain. In procedure energy-releasing flow of electrons is coupled from ADP and Pi.
Coupling mechanism is chemiosmosis, as in mitochondria. At every step of electron transport chain, electrons lose potential energy, lastly returning to the ground-state energy in P700. Absorption of more light excites reaction centre again, restarting cycle. Electrons which return to P700 have only approx half of the energy they had when they left remainder of energy is photosynthesis pay-off. Cyclic phosphorylation short-circuits -production reduced NADP+ by shuttling energized electrons from P700 to ferrodoxin and then to plastoguinone. As electron again moves down electron transport chain, ATP forms. Such processes produce no oxygen or reduce NADP+.
Noncyclic Electron Flow:
Cyclic photophosphorylaton alone happened for more than a billion years and is geared to energy production, not biosynthesis. It is enough for prokaryoler which use molecules like H2S as electron source. Though, plants strip electrons from water. This needs huge change in biochemistry, as voltage from one light-energized reaction centre of P700 does not have sufficient energy to strip electrons from water and provide them to carbon dioxide. Plants overcome the problem by grafting onto tile bacterial system a second, more powerful photo system which allowed the linear noncyclic electron flow. These two pumps are joined in series, much as we employ two batteries to increase voltage in flashlight. Second pump is photo system II, whose unique arrangement of pigments permits it to harvest shorter wavelength light. This new photo system acts first in what is now known Z scheme, model for noncyclic flow of electrons (and energy) in photochemical reactions of photosynthesis. Noncyclic photophosphorylation happens in green plants, algae, and photosynthetic bacteria, and engages two photosystem that:
i) Create ATP in electron transport chain linking photo system I and II: and
ii) Reduce NADP
Main points of Photochemical Reactions:
2H2O + 2NADP+ + 3ADP + 3Pi → (8 photons) O2 + 2NADPH + 3ATP + 4e + 2H+.
Evolution of photosystem I changed life on earth: it made photosynthetic organisms independent of H2S from decaying organic compounds as sources of electrons. It liberated oxygen in atmosphere, thus permitting evolution of aerobic respiration.
The Biochemical Reactions:
Biochemical reactions of photosynthesis reduce carbon dioxide to carbohydrate.
First step of Calvin cycle makes precursors for glucose and other carbohydrates, whereas later steps produce RuBP. As Calvin cycle takes in only one carbon (as CO2) at a time, it takes 6 turns to produce net gain of six carbons (that is two molecules of glyceraldehydes-3-photosphate, a three-carbon sugar). These 6 turns need eighteen ATPs (three per carbon) and 12 NADPHs (two per carbon). All of which come from photochemical reactions of photosynthesis.
6CO2 + 18ATP + 12 NADP + 12H2O → C6H12O6 + 18Pi + 18ADP + 12NADP+ + 6H2O + 6O2
In some groups of tropical grasser like sugar cane, CO2 is still fixed via Calvin cycle but only after fixing it by another set of reactions which take place before Calvin cycle. The explanation of what occurs in these plants is given below:
Carbon dioxide diffuses in the leaf through stomata and is fixed in mesophyll cells. These cells lack rubisco;
Carbon dioxide in mesophyll cells mixes with 3-carbon compound known as phosphonenolphyruvic acid (PEP), respiratory intermediate.
Four-carbon acid is rapidly converted to malic acid (Malate) or aspartic acid (as partate) that is moved (at expense of ATP) by psalmodiesmate to adjacent bundle-sheath cell. There these acids are divide in CO2 and three-carbon compound. Therefore malic acid and aspartic acid function as shortlived reservoirs of CO2.
Pumping by C4 plants of CO2 in bundle sheath cells keeps internal concentration of CO2 in bundle sheath cells 20-120 times greater than normal. The high concentration of CO2 permits rubisco to fix CO2 at maximal rates. This allows C4 plants to fix CO2 more capably than C3 plants do.
CO2 released in bundle-sheath cells is fixed by Calvin cycle. In the meantime 3-carbon compound is shuttled back to mesophyll cell (again at expense of ATP), where it is converted to PEP, and initial CO2 acceptor in C4 photosynthesis.
Plants like sugar cane are known as C4 plants as four-carbon acid is first stable product of their photosynthesis. Though none of photosynthesis reactions of C4 plant are unique, they generate remarkable differences in plant growth. Benefit of added set of reactions to C4 photosynthesis is that PEP carboxylase scarvenges CO2 and doesn't react with oxygen. This combined with pumping of CO2 by surrounding mesophyll cells in bundle-sheath cells, allow C4 plants to fix CO2 with rubisco which is insulated from high concentrations of oxygen.
C4 plants can fix CO2 until internal concentration of CO2 reaches zero. This permits them to carry on to fix CO2 even in hot, dry weather when stomata start to close. C4 plants are photo synthetically more proficient than C plants only in dry night conditions. Increased efficiency in the condition is because of absent of photorespiration in C4 plants. Consequently, they require only about half as much water as C3 plants for photosynthesis. C4 plants utilize nitrogen more proficiently than do C3 plants, largely as they saturate their rubisco with CO2, thus maximizing it's efficiently.
It is almost impractical to light-saturate C4 plants; as a result, they generally out-compete C3 plants in hot, dry weather.
C4 photosynthesis evolved in hot conditions of tropics. Today C4 plants are all angiosperms which are most common in hot, open ecosystem. They happen in at least seventeen families none of which comprises only C4 plants. Families are diverse, distantly associated and have no common C4 ancestors. Mainly are monocosts; with some dicots. Examples are com sorghum, sugarcane, millet and piginveed (Amarathus).
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