An illuminated suspension of chloroplasts is actively carrying outphotosynthesis

Question

An illuminated suspension of chloroplasts is actively carrying outphotosynthesis. What would happen to the levels of3-phosphoglycerate and Ribulose 1,5-Bisphosphate if the light wassuddenly switched off?



I think the answer would be that the levels of 3-phosphoglycerateand Ribulose 1,5-Bisphosphate would decrease because theCO2 that is fed into the Calvin Cycle to make thesecompounds comes from the light-dependent reactions. Also, 3 of theenzymes in the stroma that regulate the Calvin Cycle arelight-dependent as well.

Can anyone tell me if this is correct or if I am missing anything?Thanks!!

Explanation / Answer

You are thinking along the right path, however, you are wrongwith where the CO2 comes from. The products of the light-dependent reaction of photosynthesisare ATP and NADPH. The CO2 comes from the atmosphere, bythe activity of the RUBISCO enzyme being able to captureCO2. So, as long as you have ATP, NADPH, and CO2, theCalvin Cycle could go on in the absence of light. However, wheneveryou run out of ATP and NADPH, that is when you need the light tomake these products. So, the levels of 3-phosphoglycerate and Ribulose1,5-Bisphosphate would decrease only when you run out of ATP andNADPH in the absence of light Hope this helps!

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