Light stimulates the Calvin cycle
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As you have seen, the Calvin cycle uses NADPH and ATP, which are generated using energy from light. Two other processes connect the light reactions with this CO2 fixation pathway. Both connections are indirect but significant:
Light-
induced pH changes in the stroma activate some Calvin cycle enzymes. Proton transfer from the stroma into the thylakoid lumen causes an increase in the pH of the stroma from 7 to 8 (a tenfold decrease in H+ concentration) that favors the activation of rubisco. Light-
induced electron transport reduces disulfide bridges in four of the Calvin cycle enzymes, thereby activating them (Figure 10.13). When ferredoxin is reduced in photosystem I (see Figure 10.7), it passes some electrons to a small, soluble protein called thioredoxin, and this protein passes electrons to four enzymes in the CO2 fixation pathway. Reduction of the sulfurs in the disulfide bridges of these enzymes (see Figure 3.5) forms SH groups and breaks the bridges. The resulting changes in their three- dimensional shapes activate the enzymes and increase the rate at which the Calvin cycle operates.
