Cellular Energy Practice Test

Chloroplast thylakoids are illuminated while suspended in a solution containing ADP and Pi. ATP is produced under illumination. When the surrounding solution is buffered so that the pH outside the thylakoids becomes equal to the pH inside the thylakoid lumen, ATP production decreases even though light intensity and electron transfer through photosystems remain unchanged. The thylakoid membrane remains intact and impermeable to protons except through ATP synthase. This focuses on how light-driven electron transfer establishes a proton gradient used to power ATP formation.

In isolated mitochondria, adding ADP and $P_i$ increases oxygen consumption; adding oligomycin (ATP synthase inhibitor) returns oxygen use to baseline. Which explanation best accounts for the change in oxygen consumption? Electron transport and ATP synthesis are coupled through a proton gradient across the inner membrane. When ADP is available, ATP synthase allows protons to flow back to the matrix, reducing the gradient and permitting continued electron transfer to oxygen. Oligomycin blocks proton flow through ATP synthase, so the gradient builds and electron transfer slows. Oxygen is the terminal electron acceptor, so its consumption reflects electron transport rate. Which process most directly links ADP availability to increased oxygen consumption in these mitochondria?