Question

In chloroplasts, ATP is synthesized from ADP plus inorganic phosphate (Pi) in a reaction catalyzed by ATP synthase molecules that are embedded in the thylakoid membrane.Which of the following statements provides evidence to support the claim that no ATP will be synthesized in the absence of a proton gradient across the thylakoid membrane?1) No ATP is synthesized when channel proteins that allow the free passage of protons are inserted into the thylakoid membrane.2) No ATP is synthesized while the Calvin cycle is synthesizing carbohydrates and using ATP and NADPH at a high rate.3) Increasing the proton concentration difference across the thylakoid membrane is not associated with a parallel increase in the rate of ATP synthesis.4) Blocking electron flow from onecarrier to the next in the electrontransport chains blocks formation ofa proton gradient in the thylakoid.

Answer 1

When we are talking of proton gradient in photosynthesis, we are referring to Photosystem II and photosystem I, which occur in the thylakoid membrane, where basically energetic levels change due to the photons in the light that start the whole system, the gradients are created on each side of the membrane, so analyzing each statement we have that option 2, actually has an absence of gradients all the other involve different parts of photosystems I or II. Option 2 states that no ATP will be synthesized during the Calvin cycle, this is true, as the Calvin cycle occurs in the thylakoid stroma, not in the membrane, furthermore is the next step after Photosystems I and II. Hence we can say that statement 2 supports that in the absence of gradient there will not be ATP production.