Question

If you add a compound to illuminated chloroplasts that inhibits the NADP+ reductase, NADPH generation ceases, as expected. However, ferredoxin does not accumulate in the reduced form because it is able to donate its electrons not only to NADP+ (via NADP+ reductase) but also back to the cytochrome b6-f complex. Thus, in the presence of the compound, a "cyclic" form of photosynthesis occurs in which electrons flow in a circle from ferredoxin, to the cytochrome b6-f complex, to plastocyanin, to photosystem I, to ferredoxin. What will happen if you now also inhibit photosystem II?

(a) Less ATP will be generated per photon absorbed.
(b) ATP synthesis will cease.
(c) Plastoquinone will accumulate in the oxidized form.
(d) Plastocyanin will accumulate in the oxidized form.

Answer 1

If both NADP+ reductase and photosystem II are inhibited in illuminated chloroplasts, ATP synthesis will cease because no electrons are supplied by PSII to the ETC, and cyclic photophosphorylation cannot compensate for this loss. Therefore, no proton gradient can be established to drive ATP synthesis.

Step-by-step explanation:

When inhibitors are added to illuminated chloroplasts, affecting both NADP+ reductase and photosystem II (PSII), the photosynthesis process undergoes significant changes. Normally, photosystem II captures light energy to excite electrons, which are then passed through the electron transport chain (ETC). However, if PSII is inhibited, electrons cannot be passed from water to PSII and subsequently to the ETC.

Therefore, when PSII is inhibited, cyclic photophosphorylation cannot replenish energized electrons, leading to a cessation of ATP generation since there are no electrons to flow through the cytochrome complex and create a proton gradient for ATP synthesis.