Question

Predict the effect of an uncoupler such as dinitrophenol (Fig. 18-29) on the production of NADPH in a chloroplast.

Short answer

The production of NADPH will increase in the presence of an uncoupler such as dinitrophenol.

Step-by-step solution

Dinitrophenol as an uncoupler

2,4 dinitrophenol is an uncoupler, that disconnects oxidative phosphorylation from the electron transport. It is achieved by discharging the electrochemical gradient which is generated by the electron transport.

Effect of dinitrophenol on NADPH production

In the light reactions of photosynthesis, the series of electron transport reactions terminate with the reduction reaction of ${\mathrm{NADP}}^{+}\to \mathrm{NADPH}$. This generates a transmembrane protein gradient which is used to produce ATP.

Hence, the presence of uncoupler like dinitrophenol would result in an increase in the production of $\mathrm{NADPH}$, as the conversion of ${\mathrm{NADP}}^{+}\to \mathrm{NADPH}$ inclines to create a transmembrane proton gradient required for ATP synthesis.