Rank the ATP production processes from most to least ATP produced per substrate.

The ranking of ATP production processes from most to least ATP produced per substrate correctly identifies the electron transport chain as the most efficient mechanism. This is because the electron transport chain, which occurs in the inner mitochondrial membrane, generates the majority of ATP through a process called oxidative phosphorylation. Here, electrons are transferred through complexes, leading to the pumping of protons and the formation of a proton gradient that drives ATP synthase to produce ATP.

Following the electron transport chain, glycolysis contributes a net gain of 2 ATP molecules per glucose molecule via substrate-level phosphorylation. This process occurs in the cytoplasm and breaks down glucose into pyruvate, producing ATP and NADH in the process.

The Krebs cycle, which takes place in the mitochondrial matrix, produces ATP (or its equivalent GTP) directly as well, along with several high-energy electron carriers (NADH and FADH2) that are used in the electron transport chain. However, overall, it yields fewer ATP than glycolysis when not accounting for the subsequent processes that utilize the electron carriers.

Fermentation, which occurs in the absence of oxygen, results in the least ATP production. It allows for the regeneration of NAD+ so that glycolysis can continue, but it only produces a total