Calvin Cycle Carbon Fixation by RuBisCO
Carbon fixation is the first stage of the Calvin Cycle, where RuBisCO catalyzes CO₂ attachment to RuBP, producing 3-PGA. This mechanism anchors AP Biology Unit 3 (Cellular Energetics) and is a frequent AP exam free-response topic. Use these cards to distinguish RuBisCO's carboxylase versus oxygenase roles and commit key intermediates to memory before test day.
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5 CardsWhat conditions favor RuBisCO's carboxylase over oxygenase activity?
Why do C4 plants outperform C3 plants in hot, dry climates?
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What is the difference between RuBisCO's carboxylase and oxygenase activity?
Carboxylase activity fixes CO₂ onto RuBP to produce 3-PGA, fueling the Calvin Cycle. Oxygenase activity fixes O₂ instead, producing 2-phosphoglycolate and triggering photorespiration — a process that wastes energy and releases CO₂.
How many CO₂ molecules are needed to produce one G3P in the Calvin Cycle?
Three CO₂ molecules are fixed (one per RuBisCO reaction) to produce one glyceraldehyde-3-phosphate (G3P). The full cycle requires 3 ATP and 2 NADPH per CO₂ fixed.
Why does photorespiration reduce plant productivity?
Photorespiration consumes ATP and NADPH without producing sugar or net CO₂ fixation. It can reduce photosynthetic efficiency by up to 50% in C3 plants under hot, dry conditions when stomata close and O₂ accumulates.