Which are the three main phases of the Calvin cycle?

Which are the three main phases of the Calvin cycle?

• A. Carbon fixation, reduction, and RuBP regeneration.
• B. Light absorption, water splitting, NADP+ formation.
• C. Oxygenation, decarboxylation, regeneration of Ru5P.
• D. Photorespiration, carbohydrate export, starch synthesis.

Answer: (A)

The Calvin cycle has three main phases: carbon fixation, reduction, and RuBP regeneration. In carbon fixation, the enzyme Rubisco combines carbon dioxide with a five-carbon sugar called RuBP to form 3-phosphoglycerate. In the reduction phase, these 3-PGA molecules are phosphorylated and then reduced using NADPH to produce glyceraldehyde-3-phosphate, a sugar phosphate. In RuBP regeneration, most of the G3P is rearranged and used, with ATP, to rebuild RuBP so the cycle can fix more CO2. Together, these steps convert inorganic CO2 into energy-rich sugar precursors that can be used to make sugars and starches. Other options describe processes outside these three internal steps: light absorption and the light-dependent reactions generate the energy carriers for the cycle, photorespiration involves a competing pathway that reduces efficiency, and carbohydrate export or starch synthesis are outcomes of photosynthesis rather than the three-phase cycle itself.

The Calvin cycle has three main phases: carbon fixation, reduction, and RuBP regeneration. In carbon fixation, the enzyme Rubisco combines carbon dioxide with a five-carbon sugar called RuBP to form 3-phosphoglycerate. In the reduction phase, these 3-PGA molecules are phosphorylated and then reduced using NADPH to produce glyceraldehyde-3-phosphate, a sugar phosphate. In RuBP regeneration, most of the G3P is rearranged and used, with ATP, to rebuild RuBP so the cycle can fix more CO2. Together, these steps convert inorganic CO2 into energy-rich sugar precursors that can be used to make sugars and starches.

Other options describe processes outside these three internal steps: light absorption and the light-dependent reactions generate the energy carriers for the cycle, photorespiration involves a competing pathway that reduces efficiency, and carbohydrate export or starch synthesis are outcomes of photosynthesis rather than the three-phase cycle itself.