Key Concepts of Section 12.8

• In the Calvin cycle, CO₂ is incorporated, or fixed, into organic molecules in a series of reactions that occur in the chloroplast stroma. The initial reaction, catalyzed by rubisco, forms a three-carbon intermediate, 3-phosphoglycerate. Some of the glyceraldehyde 3-phosphate generated in the cycle is transported to the cytosol and converted to sucrose (see Figure 12-48).

• The light-dependent activation of several Calvin cycle enzymes and other mechanisms increase fixation of CO₂ in the light. The redox state of the stroma plays a key role in this regulation, as does regulation of the activity of rubisco by rubisco activase.

• In C₃ plants, a substantial fraction of the CO₂ fixed by the Calvin cycle can be lost as the result of photorespiration, a wasteful reaction catalyzed by rubisco that is favored at low CO₂ and high O₂ levels (see Figure 12-49).

• In C₄ plants, CO₂ is fixed initially in the outer mesophyll cells by reaction with phosphoenolpyruvate. The four-carbon molecules so generated are shuttled to the interior bundle sheath cells, where the CO₂ is released and then used in the Calvin cycle. The rate of photorespiration in C₄ plants is much lower than it is in C₃ plants.