Calvin cycle stages

The further complicated way of CO2 assimilation to form CH20 flows through series of intermediate compounds and reactions (Calvin cycle). Let us show some results of calculations of total spatial-energy assimilation processes of CO2. When C02 is reduced to the level of its structural formation in CH20, the chemical bonds are reconstructed on all stages of the cycle. Therefore, the additional activation energy from ATP and NADPN is required. 

FIGURE 20-4 The three stages of C02 assimilation in photosynthetic organisms. Stoichiometries of three key intermediates (numbers in parentheses) reveal the fate of carbon atoms entering and leaving the cycle. As shown here, three C02 are fixed for the net synthesis of one molecule of glyceraldehyde 3-phosphate. This cycle is the photosynthetic carbon reduction cycle, or the Calvin cycle. 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

To achieve the oxidation of acetyl CoA on a continuing basis, intermediates consumed in certain steps must be regenerated in others. Thus we have a situation similar to that in the Calvin cycle (Section 20-9), whereby the first stage of the cycle achieves the desired reaction (C02 formation) and the second stage is designed to regenerate intermediates necessary to perpetuate the cycle. 

The Calvin cycle comprises three stages (Figure 20.1) ... 

Figure 20.1. Calvin Cycle. The Calvin cycle consists of three stages. Stage 1 is the fixation of carbon by the carhoxylation of ribulose 1,5-hisphosphate. Stage 2 is the reduction of the fixed carbon to begin the synthesis of hexose. Stage 3 is the regeneration of the starting compound, ribulose 1,5-bisphosphate.

Calvin cycle Discovered by chemist Melvin Calvin (1911-97), it is the second major stage in photosynthesis after light reactions whereby carbon molecules from... 

CO2 are fixed into sugar (glucose) and mediated by the enzyme rubisco (ribulose-l-5-biphosphate carboxylase). It occurs in the stroma of chloroplasts. The Calvin cycle is also known as the dark reaction, as opposed to the first-stage light reactions. 

Fig. 2.8 Summary of the Calvin cycle. The cycle consists of three stages, culminating in the regeneration of ribulose 1 5-bisphosphate and a net increase in glyceraldehyde 3-phosphate. Numbers in parenthesis reveal the fate of carbon atoms entering and leaving the cycle (Modified from Fig. 20-04 in Lehninger Principles of Biochemistry. D.L. Nelson M.M. Cox, 4th Ed. 2005. W.H. Freeman Co., New York)...

Stage II Many of the reactions of the Calvin cycle also occur in the pentose phosphate pathway. The two F6P molecules from the last step of stage I react with the remaining 6 G3Ps as shown in Figure 17.22. Two of the G3Ps are isomerized to dihydroxyacetone... 

Ribulose-l,5-bisphosphate carboxylase (rubisco) is the central enzyme of the Calvin Cycle because it catalyzes the initial reaction is stage I, in which C02 is incorporated into ribulose-l,5-bisphosphate. This carboxylase activity occurs under conditions of low 02 and high C02. 

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