USA Biology Olympiad (USABO) Practice Exam

The reduction phase of the Calvin cycle is essentially the reverse of which other process?

1. Citric acid cycle

2. Fermentation

3. Energy payoff phase of glycolysis

4. Photorespiration

The correct answer is: Energy payoff phase of glycolysis

The reduction phase of the Calvin cycle involves converting 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. This phase is similar to the energy payoff phase of glycolysis, where the conversion of glyceraldehyde-3-phosphate (or similar molecules) includes reduction reactions that produce ATP and NADH through the oxidation of glucose. During glycolysis, specifically in its energy payoff phase, G3P is further processed and eventually leads to the production of ATP and NADH, indicating that both processes share a fundamental mechanism of utilizing energy carriers to transform carbon compounds. Thus, the reduction phase of the Calvin cycle essentially mirrors this phase of glycolysis, where the change in energy state and electron transfer is central to both metabolic processes. This relationship highlights the interconnectedness of photosynthesis and cellular respiration in biological energy transformations.