Introduction:

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:

• Photochemical reactions of photosynthesis begin at P680 and finish at NADP these reactions are linear rather than cyclic, and generate ATP and NADPH
• Photon hits end of reaction centre nearest inner surface of membrane, exciting the electron. This electron carries it energy to other end of reaction centre.
• Though reaction centre captures 98% -99% of photons which it absorbs, only about half of the energy is stored in change separation, therefore, photo system II is only about half as efficient as battery.
• Similar to all energy rich substances, excited electrons are unstable and release much energy as heat. Electrons ejected from P680 leave "holes" which are filled by electrons from Z protein, the manganese -having protein that get it electron from release oxygen, water, and uses electrons to replace those ejected by photons absorbed by reaction centre. All these happen in less than one billionth (10-9) of a second.
• Dividing two molecules of water releases the molecule of O2 and few electrons that entire one. Though, only one electron at time can be accepted by P680. Electrons stripped from water are handled by manganese, whose stable oxidation states ranging from +2 to +7 suit it perfectly for the function as charge accumulator. At saturating intensities of light, one molecule of oxygen is released per about 2,500 molecules of chlorophyll.
• Within a few trillionth of a second, electrons ejected from P680 of photo system II decrease phaeophyton, the pigment which accepts electrons from chlorophyll. Resulting separation of charge widen as electron moves across thylakoid membrane and are accepted by plastoquinone (Pq), that is embedded on outer, stromal side of membrane.
• Electrons then descend electron transport chain of molecule (like cytochromes) in thylakoids linking photo system II and photosytem I, that are joined in series in thylakoids membranes. As electrons move down this chain, they form the proton gradient which generates ATP.
• Final electron acceptor in electron transport chain is P700, reaction centre of photo system I. There, electrons contain higher energy than when they left P680 in split second earlier. At P700, four more photons absorbed by antennae transfer the energy to reaction centre, where energy is utilized to eject electrons from P700, electron donor, situated on thylakoid space side of membrane. The energized electrons cross membrane and reduce ferrodoxin. Small, iron-Containing protein which is electron acceptor of photo system I. Ferrodoxin then reduces NADP⁺ to NADPH.
• As ferrodoxin is on outer, stromal side of membrane, NADP⁺ forms on stroma, where it is utilized in biochemical reactions t to reduce carbon to carbohydrates.
• Light protons utilized in photochemical reactions make 3 molecules of ATP and reduce two molecules of NADP+.

2H₂O + 2NADP⁺ + 3ADP + 3Pi → (8 photons) O₂ + 2NADPH + 3ATP + 4e + 2H⁺.

• These ATPs and NADPs are utilized to reduce CO₂ in biochemical reactions of photosynthesis.

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.

• CO₂ diffuses into chloroplast stroma and is fixed; that is it is included into organic compound. This happens by covalent bonding of CO₂ to RuBP ribolose 1.5-phosphate aided by enzyme RUBP carboxylase/Oxygenase (Also called rubisco).
• Rubisco happens in all autotrophs except few species of bacterial, and is most plentiful protein on earth.
• Reaction of CO₂ and RuBP generates the unstable six-carbon compound which immediately breaks in two molecules of 3-phosphoglyceric and (3-PGA). This 3-carbon molecule is first stable product of photosynthesis that describes why botanists refer to the series of reactions as C₃ cycle or Calvin cycle.
• Plants which use only Calvin cycles to fix carbon dioxide are known as C₃ plants. Approx 85% of plants belong to this category like rice, oaks, peanuts, cotton, soyabeans, barley, tobacco, and most trees.
• The phosphate group is cleaved from every molecule of 3-PGA, and ATP and electrons from NADPH (produced by photochemical reactions) are then utilized to reduce diphosphoglycerate (DPGA) to glyceraldehydes-3-phosphate (G- 3-P), three-carbon sugar. Sugar, not glucose, is carbohydrate produced by Calvin cycle and is starting point for several other metabolic pathways in plant.
• Some G-3-P is utilized to reform RuBP. This can happen in dark and needs ATP made in photochemical reactions. The rest of the G-3-P moves through series of chemical reactions to form fructose diphosphate that is utilized to make starch, sucrose, glucose, and other compounds required by plant.

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

C₄ Photosynthesis:

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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