In the two-stage model of photosynthesis, what occurs in stage 2 (the light-independent reactions/Calvin cycle)?

• A. Glucose is broken down into pyruvate to produce ATP and NADH (glycolysis)
• B. CO2 is fixed and reduced to form carbohydrates using ATP and NADPH produced in stage 1
• C. O2 serves as the final electron acceptor to drive ATP synthesis in mitochondria (oxidative phosphorylation)
• D. Light energy is absorbed to split water, releasing O2 and generating ATP and NADPH

Answer: (B)

Verified step by step guidance

1. Identify that the problem asks about the events occurring in stage 2 of photosynthesis, also known as the light-independent reactions or the Calvin cycle.
2. Recall that stage 1 (the light-dependent reactions) involves capturing light energy to produce ATP and NADPH, and splitting water to release O2.
3. Understand that stage 2 uses the ATP and NADPH generated in stage 1 to fix carbon dioxide (CO2) into organic molecules.
4. Recognize that during the Calvin cycle, CO2 is incorporated into a 5-carbon sugar (ribulose bisphosphate) and then reduced through a series of enzyme-mediated steps to form carbohydrates.
5. Conclude that stage 2 does not involve breaking down glucose or using oxygen as an electron acceptor; instead, it focuses on synthesizing carbohydrates from CO2 using energy from ATP and NADPH.