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Why does the level of Ribulose Bisphophate and Triose phosphate initially decrease and then level off at a lower level when the plant is tranferred from a bright light to a dim light? And why does the level of Glycerate 3-phosphate increase and then level off at a higher level when tranferred from a bright light to a dim light?
The Calvin cycle, Calvin–Benson–Bassham (CBB) cycle, reductive pentose phosphate cycle or C3 cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is also known as the light-independent reactions.Steps
In the first stage of the Calvin cycle, a CO
Calvin cycle step 1 (black circles represent carbon atoms)
Calvin cycle steps 2 and 3 combined
- The enzyme RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate, RuBP, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction. The product of the first step is enediol-enzyme complex that can capture COor O
2. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The CO
2that is captured by enediol in second step produces a six-carbon intermediate initially that immediately splits in half, forming two molecules of 3-phosphoglycerate, or 3-PGA, a 3-carbon compound (also: 3-phosphoglyceric acid, PGA, 3PGA).
- The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3-PGA by ATP (which was produced in the light-dependent stage). 1,3-Bisphosphoglycerate (1,3BPGA, glycerate-1,3-bisphosphate) and ADP are the products. (However, note that two 3-PGAs are produced for every COthat enters the cycle, so this step utilizes two ATP per CO
- The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyses the reduction of 1,3BPGA by NADPH (which is another product of the light-dependent stage). Glyceraldehyde 3-phosphate (also called G3P, GP, TP, PGAL, GAP) is produced, and the NADPH itself is oxidized and becomes NADP+. Again, two NADPH are utilized per COfixed.