SUMMARY

• 23.1 The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water

  ATP and NADPH formed in the light reactions of photosynthesis are used to convert CO₂ into hexoses and other organic compounds. The dark phase of photosynthesis, called the Calvin cycle, starts with the reaction of CO₂ and ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate. The steps in the conversion of 3-phosphoglycerate into fructose 6-phosphate and glucose 6-phosphate are like those of gluconeogenesis, except that glyceraldehyde 3-phosphate dehydrogenase in chloroplasts is specific for NADPH rather than NADH. Ribulose 1,5-bisphosphate is regenerated from fructose 6-phosphate, glyceraldehyde 3-phosphate, and dihydroxyacetone phosphate by a complex series of reactions. Three molecules of ATP and two molecules of NADPH are consumed for each molecule of CO₂ converted into a hexose. Starch in chloroplasts and sucrose in the cytoplasm are the major carbohydrate stores in plants.

• 23.2 The Calvin Cycle Is Regulated by the Environment

  The activity of the Calvin cycle is coordinated with the light reaction of photosynthesis. Reduced thioredoxin formed by the light-driven transfer of electrons from ferredoxin activates enzymes of the Calvin cycle by reducing disulfide bridges. The light-induced increase in pH and Mg²⁺ levels of the stroma is important in stimulating the carboxylation of ribulose 1,5-bisphosphate by rubisco.

  Rubisco also catalyzes a competing oxygenase reaction, which produces phosphoglycolate and 3-phosphoglycerate. The recycling of phosphoglycolate leads to the release of CO₂ and further consumption of O₂ in a process called photorespiration. This wasteful side reaction is minimized in tropical plants, which have an accessory pathway—the C₄ pathway—for concentrating CO₂ at the site of the Calvin cycle. This pathway enables tropical plants to take advantage of high levels of light and minimize the oxygenation of ribulose 1,5-bisphosphate. Other plants, including some that grow in arid ecosystems, employ crassulacean acid metabolism to prevent dehydration. In CAM plants, the C₄ pathway is active during the night, when the plant exchanges gases with the air. During the day, gas exchange is eliminated and CO₂ is generated from malate stored in vacuoles.