Brief Notes on Carbon Reduction in Photosynthetic

Green plants and algae reduce C0₂ to carbohydrates in a scries of reactions in a cyclic manner which is known as Calvin cycle this cycle is named after Melvin Calvin, an American

Sucrose & biochemist, who headed a research team to elucidate this pathway for which he was awarded with a Nobel Prize in 1961. Since many other scientists of Calvin’s research team also contributed significantly for the elucidation of this cycle, it is often named as Calvin-Benson cyclc.

However, now people prefer to name this as photosynthetic carbon reduction (PCR) cycle or often as reductive pentose phosphate (RPP) cycle. Calvin cycle is now commonly referred to as the C₃ cycle because the first product of the C0₂ fixation reaction, 3-phosphoglyceric acid (3-PGA), is a three-carbon molecule. For ease of understanding, the Calvin cycle can be described under three reaction sequences: (a) carboxylation, (b) reduction, and (c) regeneration.

(a) Carboxylation Phase

The PCR cycle begins with the carboxylation reaction in which ribulose-i, 5-bisphosphate (RuBP) reacts with C0₂ to yield two molecules of 3-phosphoglyceric acid (3-PGA). This carboxylation reaction is catalyzed by the enzyme ribulose-i, 5-bisphosphate carboxylase- oxygenase (rubisco). 3-PGA is the first stable product of CO₃ fixation reaction in photosynthesis. As 3-PGA is a 3-carbon compound, Calvin cycle is also known as C3-cycle.

(b) Reduction Phase

In order for the Calvin cycle to continuously operate and reduce C0₂, the product of carboxylation reaction, 3-PGA, must be continuously removed and there should be a mechanism for continuous supply of CO, acceptor molecule, RuBP. The 3-PGA is removed by its conversion to a triosephosphate, glyceraldehydes-3-phosphate by a two step reaction. First 3-PGA is phosphorylated by reacting with ATP to form 1, 3-bisphosphoglyceric acid. This reaction is catalyzed by the enzyme phosphoglycerate kinase. In the second reaction, 1, 3-bisphosphoglyceric acid is reduced to glyceraldehyde-3-phosphate (G-3-P) by XADPII with the release of one molecule of inorganic phosphate (Pi). This reaction is catalyzed by the enzyme G-3-P dehydrogenase.

(c) Regeneration Phase

The product of reduction reaction, G-3-P, remains in equilibrium with its isorac dihydroxyacctone-3-phosphatc (DIIAP), an isomerization reaction catalyzed by the enzymj triosephosphate isomerase. G-3-P and DIIAP together are referred to as Iriosephosphales A portion of the triosephosphate molecules through a scries of reactions are used to regenerate RuBP which is needed for the initial carboxylation reaction of Calvin cycle. Regeneration a RuBP is crucial for the Calvin cycle to continue uninterrupted. Rest of the triosephosphates is used through another series of reactions to form the final product of photosynthesis, sucro and starch.

Nadp’Overall, 13 enzymes are required to catalyze the reactions of Calvin cycle each molecule of C0₂ reduced in Calvin cycle requires 2 molecules of NADPH and 3 molecules of ATP. To make one molecule of hexose-6-phosphate (Hexose-6-P), the cycle has to reduce 6 molecules of C0₂ using 12 molecules of NADPH and 18 molecules of ATP. The overall reaction of Calvin cycle for the production of one molecule of hexose phosphate can be written as:

Plants that assimilate C0₂ solely through the Calvin cycle are generally known as C₃ plants because the first stable product of photosynthetic CO. Fixation reaction is a 3-carbon compound, 3- PGA. In certain tropical plants such as maize, sugarcane, sorghum, amaranths etc, the first stable product of photosynthetic CO., fixation reaction is oxaloacetic acid (OAA), a 4-carbon compound.

These plants are known as C₄ plants and they assimilate CO, through a cyclc of reactions known as C₄ cyclc. Experiments initially conducted by 11. P. Kortschack and subsequently by M.D. Hatch and C.R. Slack led to the elucidation of this cycle for which it was earlier known as Hatch-Slack- Kortschack (HSK) cyclc or, in many texts, simply as Ilatch-Slack cycle.

C, plants exhibit a number of specific anatomical, physiological and biochemical characteristics not found in C_;i plants. Leaves of most of the C₄ plats have the presence of two distinct photosynthetic tissues. In the leaves, the vascular bundles are quite close together and each bundle is surrounded by a tightly fitted layer of cells called the bundle sheath.

The mesophyll cells are loosely arranged between the vascular bundles. This type of distinction between the mesophyll and bundle sheath photosynthetic cells is known as Kranz anatomy