What is the major source of ATP in aerobic organisms?

The major source of ATP in aerobic organisms is oxidative phosphorylation. This process occurs within the mitochondria and takes advantage of the electron transport chain and chemiosmosis. During oxidative phosphorylation, electrons are transferred through a series of protein complexes in the inner mitochondrial membrane, which creates a proton gradient across the membrane. As protons flow back into the mitochondrial matrix through ATP synthase, the energy released from this movement is used to convert ADP and inorganic phosphate into ATP.

Although glycolysis and the Krebs cycle (also known as the citric acid cycle) do generate some ATP, they contribute significantly less to the total ATP yield. Glycolysis, for instance, produces a net gain of only 2 ATP molecules per glucose molecule, while the Krebs cycle yield is also relatively modest. Oxidative phosphorylation, however, can produce up to 34 ATP molecules from the oxidation of a single glucose, making it the primary and most efficient pathway for ATP production in aerobic conditions. This is why oxidative phosphorylation is identified as the major source of ATP in aerobic organisms.