Please help!! Electron transport chain question

Im study for a test and dont understand the following:

Q The NADP reductase is the last step in the electron transport chain of photosynthesis. If a chloroplast is treated with a poison that prevents electron transfer to the NADP reductase, which of the following would result?


A. ATP synthesis continues for a time until the already existing proton gradient is depleted
B. the flow of electrons down the electron transport chain is blocked immediately, leaving all the other components in a reduced state
C. the Calvin-Benson cycle stops immediately
D. A and B
E. A and C

The correct answer is D. I understand that the NADPH red occurs before NADP+ turns into NADPH so b is correct, but the b6f complex & the splitting of water which occurs before NADPH red should still create the proton gradient needed to create ATP...right?? so why is A also true?

Yep, the answer is D. Here's why:

Let's say you have 4 complexes in your electron transport chain (I don't recall exactly how many there are in plants, but the concept is the same). As complex 1 transfers its electrons to complex 2, complex 1 becomes oxidized (loses electrons) and complex 2 becomes reduced (gains electrons). As complex 2 transfers electrons to complex 3, complex 2 is oxidized and complex 3 is reduced.

Essentially, every complex oxidizes the guy before it and reduces the guy after it. Now, let's pretend complex 4 is the NADP reductase. Once the NADP reductase takes electrons from complex 3, the NADP reductase is reduced and it can't accept any more electrons. It solves this problem by transferring it's electrons to NADP (converting NADP to NADPH). The NADP reductase is now oxidized and can receive electrons again.

If you prevent the transfer of electrons to the NADP reductase, complex 3 is reduced, but can't get rid of its electrons. Think of it as a full bathtub. If the water can't drain out, you can't pour new water into the tub. Since complex 3 has no where to transfer it's electrons, it can't accept any more from complex 2. That means that complex 2 can't get rid of it's electrons, so it can't take any more from complex 1. Complex 1 can't get rid of it's electrons, so it can't accept any from whatever electron carrier it gets its electrons from.