[Campbell Biology P.241] A student asserts that the light reactions can continually produce ... | Practice Question

A student asserts that the light reactions can continually produce ATP and NADPH as long as light is available, regardless of the Calvin cycle's activity. Based on the information provided, how would you respond to this assertion?

  • A: The student is correct, as light reactions directly use light energy; their function is solely dependent on light availability.
  • B: The student is partially correct; ATP production might continue, but NADPH production would cease without the Calvin cycle to regenerate NADP+.
  • C: The student is incorrect; the light reactions require the regeneration of ADP and NADP+ from the Calvin cycle to continue producing ATP and NADPH.
  • D: The student is incorrect because the light reactions would run out of CO2 if the Calvin cycle stopped.

Explanation

The light reactions produce ATP and NADPH, which are then used by the Calvin cycle. For the light reactions to continue, they need a continuous supply of ADP and NADP+ to pick up phosphate and electrons, respectively. These molecules ADP and NADP+ are regenerated when ATP and NADPH are used in the Calvin cycle. Therefore, if the Calvin cycle stops, the supply of ADP and NADP+ would eventually deplete, causing the light reactions to slow down or stop, even in the presence of light. The text states: "molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and NADPH and ATP are then released to the stroma, where they play crucial roles in the Calvin cycle." This strongly implies the interdependence.