There are two different methods of ATP formation in cell respiration. One method is the direct (also called substrate-level) phosphorylation that occurs in glycolysis (producing a net gain of 2 ATP) and the Krebs cycle (producing 1 ATP per cycle). These numbers are certain and constant. In the second method of ATP formation, oxidative phosphorylation, the numbers of ATP molecules produced vary under different conditions and for different kinds of cells. For many years, it was believed that 1 NADH yielded 3 ATP and that 1 FADH2 yielded 2 ATP by oxidative phosphorylation. This gave a grand total of 36 to 38 molecules of ATP per glucose through cell respiration (see the footnote in table 5.1). Newer biochemical information, however, suggests that these numbers may be overestimates, because, of the 36 to 38 ATP produced per glucose in the mitochondrion, only 30 to 32 ATP actually enter the cytoplasm of the cell.

Roughly three protons must pass through the respiratory assemblies and activate ATP synthase to produce 1 ATP. However, the newly formed ATP is in the mitochondrial matrix and must be moved into the cytoplasm; this transport also uses the proton gradient and costs one more proton. The ATP and H+ are transported into the cytoplasm in exchange for ADP and Pi, which are transported into the mitochondrion. Thus, it effectively takes four protons to produce 1 ATP that enters the cytoplasm.

To summarize: The theoretical ATP yield is 36 to 38 ATP per glucose. The actual ATP yield, allowing for the costs of transport, is about 30 to 32 ATP per glucose. The details of how these numbers are obtained are described in the following section.

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