Hp Carbon Fixation In Plants

We have already described how chloroplasts use the energy of light to oxidize H2O to give O2 and to generate ATP and NADPH (page 271). A series of so-called dark reactions in the chloroplast stroma uses ATP and NADPH in a process that grabs carbon dioxide from the air and builds it into larger molecules, a process called carbon fixation. These reactions are often called the Calvin cycle in honor of their discoverer (Fig. 13.13). The initial step is carried out by the enzyme ribulose bisphosphate carboxylase, said to be the most abundant protein on earth. CO2 is combined with the five-carbon sugar ribulose bisphosphate to give a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. Further reactions use NADPH and ATP to convert these three-carbon

complex sugar intraconversions

Figure 13.13. The dark reactions of photosynthesis capture carbon dioxide from the air.

complex sugar intraconversions

Figure 13.13. The dark reactions of photosynthesis capture carbon dioxide from the air.

units to fructose-6-phosphate. It is interesting that many of these reactions are similar to those of the pentose phosphate pathway (page 289).

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