Which of the following is a direct product of the light-dependent reactions that is consumed by the Calvin cycle?
AGlucose (C₆H₁₂O₆)
BCarbon dioxide (CO₂)
CNADPH
DOxygen (O₂)
The light reactions produce ATP, NADPH, and O₂. The Calvin cycle consumes ATP and NADPH to fix CO₂ into organic molecules. Glucose is the end product of the Calvin cycle, not an input. O₂ is released as a byproduct of water splitting, not passed to the Calvin cycle.
Question 2 True / False
The Calvin cycle (light-independent reactions) only occurs at night when light is absent.
TTrue
FFalse
Answer: False
The Calvin cycle is called 'light-independent' because it does not directly use light, not because it requires darkness. In practice, it runs simultaneously with the light reactions during the day, fueled by the ATP and NADPH the light reactions continuously produce. At night, when light reactions stop producing these inputs, the Calvin cycle also slows or stops.
Question 3 Short Answer
Explain how the two stages of photosynthesis are functionally coupled — what would happen to the Calvin cycle if the light reactions were suddenly blocked?
Think about your answer, then reveal below.
Model answer: The light reactions supply the ATP and NADPH that the Calvin cycle needs to fix CO₂. If light reactions stopped, these inputs would be depleted and the Calvin cycle would halt, even if CO₂ remained available.
This tests understanding of the dependency between stages rather than just memorizing inputs/outputs. The light reactions are the energy-capturing stage; the Calvin cycle is the carbon-fixing stage. Their coupling means photosynthesis as a whole depends on both stages working in sequence.