Question

As a research scientist, you measure the amount of ATP and NADPH consumed by the Calvin cycle in 1 hour. You find 30,000 molecules of ATP consumed, but only 20,000 molecules of NADPH. Where did the extra ATP molecules come from?

A. photosystem ll
B. photosystem I
C. cyclic electron flow
D. noncyclic electron flow
E. chlorophyll

Answer 1

C. cyclic electron flow

Step-by-step explanation:

Cyclic electron flow includes only PSI. Electrons passing from P700 of PS I to ferredoxin do not reduce NADP+. The electrons are channeled back through the cytochrome b6f complex to plastocyanin. Therefore, cyclic electron flow cycles the electrons out of and back to the P700 and is not accompanied by a reduction of NADPH or splitting of H2O molecule.

However, proton pumping by the cytochrome b6f complex during electron transfer generates the proton contraction gradient which in turn drives the phosphorylation of ADP to ATP. The cyclic photophosphorylation supplies the extra ATP required for the Calvin cycle.