In the Calvin cycle, what is the purpose of RuBP regeneration?

In the Calvin cycle, what is the purpose of RuBP regeneration?

• A. To restore the CO2 acceptor so the cycle can continue.
• B. To export glucose out of the chloroplast.
• C. To split water to release oxygen.
• D. To form ATP directly.

Answer: (A)

The main idea is that the Calvin cycle must continually rebuild the five‑carbon CO2 acceptor, RuBP, so the cycle can keep fixing carbon. After CO2 is fixed by RuBP, the carbon goes through a series of steps powered by ATP and NADPH to become glyceraldehyde-3-phosphate. Most of that G3P is then used, with energy input from ATP, to rebuild RuBP. Without regenerating RuBP, there would be no molecule to capture new CO2 and the cycle would stop. Other options miss the function of RuBP regeneration: exporting glucose out of the chloroplast isn’t the purpose of this regeneration step, splitting water occurs in the light reactions, and ATP is not formed during RuBP regeneration—it is consumed to rebuild RuBP.

The main idea is that the Calvin cycle must continually rebuild the five‑carbon CO2 acceptor, RuBP, so the cycle can keep fixing carbon. After CO2 is fixed by RuBP, the carbon goes through a series of steps powered by ATP and NADPH to become glyceraldehyde-3-phosphate. Most of that G3P is then used, with energy input from ATP, to rebuild RuBP. Without regenerating RuBP, there would be no molecule to capture new CO2 and the cycle would stop.

Other options miss the function of RuBP regeneration: exporting glucose out of the chloroplast isn’t the purpose of this regeneration step, splitting water occurs in the light reactions, and ATP is not formed during RuBP regeneration—it is consumed to rebuild RuBP.