Cellular Respiration Practice Test 2025 – Complete Exam Prep Guide

The phase of cellular respiration that produces the most ATP is the Electron Transport Chain. This process occurs in the inner mitochondrial membrane and is the final stage of cellular respiration. During the Electron Transport Chain, electrons are transferred through a series of protein complexes, which are embedded in the membrane. As these electrons pass through the complexes, they release energy. This energy is used to pump protons (H+ ions) across the membrane, creating a proton gradient.

The stored potential energy in this gradient is then harnessed by the enzyme ATP synthase, which synthesizes ATP as protons flow back into the mitochondrial matrix. This mechanism can generate a significant amount of ATP—typically around 26 to 28 molecules of ATP per glucose molecule, depending on the efficiency and conditions within the cell.

In contrast, glycolysis and the Krebs Cycle produce far fewer ATP molecules. Glycolysis generates a net gain of 2 ATP molecules, while the Krebs Cycle contributes just 2 ATP directly. Fermentation, which is an anaerobic process, produces even less ATP, usually yielding only 2 ATP molecules from glycolysis without utilizing oxygen. Therefore, the Electron Transport Chain is the most productive stage of cellular respiration in terms of ATP synthesis.