DISCUSS THE FIRST TWO REACTION (CARBOXYLATION AND REDUCTION PHASES) INVOLVED IN THE CALVIN-BENSON CYCLE. CLEARLY INDICATE THE SUBSTRATE ENZYMES AND THE PRODUCTS OF THESE TWO REACTIONS?

Answer 1

The Calvin-Benson cycle is the process by which plants fix CO2 from the atmosphere and convert it into glucose. This cycle consists of several reactions, with the first two being carboxylation and reduction phases.

1. Carboxylation phase:
During the carboxylation phase, the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the addition of carbon dioxide to a 5-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This reaction forms an unstable 6-carbon intermediate that immediately splits into two 3-carbon molecules called 3-phosphoglycerate (3-PGA). The substrate enzyme in this phase is Rubisco.

The reaction can be represented as:
CO2 + RuBP → 2 (3-PGA)

2. Reduction phase:
In the reduction phase, the energy and electrons required for the conversion of 3-PGA into the sugar glyceraldehyde-3-phosphate (G3P) are provided by ATP and NADPH. The enzyme phosphoglycerate kinase transfers a phosphate group from ATP to 3-PGA, forming 1,3-bisphosphoglycerate (1,3-BPGA). Then, the enzyme glyceraldehyde-3-phosphate dehydrogenase uses NADPH to reduce 1,3-BPGA to G3P. One out of every six G3P molecules produced is used for glucose synthesis, while the rest are recycled back into the cycle.

The reactions can be represented as:
3-PGA + ATP → 1,3-BPGA + ADP
1,3-BPGA + NADPH + H+ → G3P + NADP+ + Pi (inorganic phosphate)

In summary, the first two reactions in the Calvin-Benson cycle involve the carboxylation of CO2 and the subsequent reduction of the resulting 3-PGA to G3P. These reactions are essential for fixing carbon dioxide and producing the building blocks for glucose synthesis in plants.