How does substrate-level phosphorylation differ from oxidative phosphorylation?

Substrate-level phosphorylation is a process that directly generates ATP by transferring a phosphate group from a substrate to ADP, forming ATP. This occurs in specific steps during glycolysis and the citric acid cycle, where high-energy phosphate groups are transferred directly from phosphorylated substrates.

In contrast, oxidative phosphorylation involves a series of redox reactions occurring in the electron transport chain located in the inner mitochondrial membrane. This process does not directly transfer a phosphate group from a substrate to ADP; instead, it relies on the proton gradient generated by the electron transport chain to synthesize ATP via ATP synthase.

Thus, the correct choice highlights the fundamental distinction between the two processes, emphasizing that substrate-level phosphorylation generates ATP directly from a substrate, whereas oxidative phosphorylation relies on an electrochemical gradient and the coupling of electron transport to ATP synthesis.