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Electron Transport Chain

The light-driven reactions of photosynthesis, referred to as electron transport chain, were first formulated by Robert Hill in 1939. The electron transport chain of photosynthesis is initiated by the absorbance of light by the photosystem II (P680). When P680 absorbs light, it is excited and its electrons are transferred to an electron acceptor molecule. By doing so, P680 becomes a strong oxidizing agent and splits a molecule of water to release oxygen. This light-dependent splitting of the water molecule is called photolysis. Manganese, calcium and chloride ions play prominent roles in the photolysis of water. With the breakdown of water, electrons are generated, which are then passed to the oxidized P680. Thus, the electron-deficient P680 (because it had transferred its electrons to an acceptor molecule before) is able to restore its electrons from the water molecule. After accepting electrons from the excited P680 the primary electron acceptor (pheophytin in plants) is reduced. The reduced acceptor (a strong reducing agent) now donates its electrons to the downstream components of the electron transport chain.

Similar to the photosystem II (P680), photosystem I (P700) is excited on absorbing light and gets oxidized. It transfers its electrons to the primary electron acceptor, which in turn gets reduced. While the oxidized P700 draws electrons from photosystem II, the reduced electron acceptor of photosystem I, transfers electrons to ferredoxin and ferredoxin-NADP reductase to reduce NADP to NADPH2. The NADPH2 a powerful reducing agent, is then utilized in the reduction of CO2 to carbohydrates in the carbon reaction of photosynthesis. The reduction of CO2 to carbohydrates also requires energy in the form of ATP, produced via electron transport chain discussed above. This process of ATP formation from ADP in the presence of light in chloroplasts is called photophosphorylation.

Thus, the light reactions of photosynthesis result in the formation of three products: Reduced NADP, ATP and molecular oxygen. The reduced NADP and ATP are used in the dark reactions of photosynthesis to bring about the reduction of carbon dioxide to glucose.

Dark Reactions of Photosynthesis (Biosynthetic Phase)

The photosynthetic reactions that lead to the reduction of carbon dioxide to glucose using ATP and reduced NADP are called dark reactions. These reactions takes place very fast and can occur in complete darkness, if the plant has been previously exposed to sunlight.

In 1945, Melvin Calvin and his colleagues started a series of investigations that resulted in the overall understanding of the dark reactions of photosynthesis. They used the unicellular green alga Chlorella in their work. Their aim was to determine the pathway by which carbon dioxide became fixed into carbohydrates. They used radioactive 14C to trace the fate of carbon dioxide. After injecting 14C into all illuminated suspension of algae that had been carrying out photosynthesis with normal CO2, the algae were killed after preselected time. The radioactive compounds were separated and identified by the technique of two dimensional paper chromatography. The findings of Calvin and his colleagues proved to be true for a wide variety of photosynthetic organisms ranging from photosynthetic bacteria to higher plants. The pathway of CO2 fixation that Calvin and his colleagues discovered is now known as Calvin Benson cycle or C3 cycle.

Calvin-Benson Cycle (C3 Cycle)

The features of Calvin-Benson cycle are illustrated in the figure. It occurs in the chloroplast with most of the involved enzymes dissolved in the stroma. The first step in the carbon dioxide reduction consists of the reaction of CO2 with a phosphorylated 5-carbon sugar, ribulose 1,5-diphosphate, to form two molecules of 3-phosphoglyceric acid. The two molecules of 3-phosphoglyceric acid so formed now undergo enzymatic reduction to two molecules of 3-phosphoglyceraldehyde at the expense of reduced NADP (NADPH) and ATP formed in the light reactions of photosynthesis. Then the two molecules of 3-phosphoglyceraldehyde are converted to glucose essentially by the reversal of the reaction involved in glycolysis. Thus the addition of one carbon atom in the form of a CO2 molecule to a 5-carbon sugar ultimately leads to the formation of a six carbon sugar. This process requires the input of two molecules of ATP and two molecules of reduced NADP. This cycle of events is repeated over and over, and in a cycle, a molecule of CO2 reacts with the 5-carbon sugar, ribulose 1,5-diphophate to yield two molecules of phosphoglycerate. These then combine to make glucose at the expense of ATP and reduced NADP.
The Calvin-Benson cycle of carbon dioxide fixation looks very simple the way we have described it. However it is to be noted that the pathway of carbon in photosynthesis is actually very complex. The enzymatic mechanisms by which the 5-carbon sugar, ribulose 1.5-diphosphate, is regenerated at each revolution of this cycle, are very complex. In this pathway the reaction of carbon dioxide with the 5-carbon sugar leads to the formation of two molecules of 3-carbon compounds (3-phosphoglycerate); hence this cycle is also called C3 -cycle of carbon dioxide fixation.

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