Cellular respiration and fermentation both break down glucose to generate energy. Cellular respiration uses oxygen and occurs in three main stages: glycolysis, the Krebs cycle in the mitochondria, and the electron transport chain. It is the most efficient pathway, producing up to 36 ATP per glucose molecule. Fermentation is anaerobic and less efficient, producing only 2 ATP per glucose through substrate-level phosphorylation. The electron transport chain uses a series of electron carriers and proton pumping to establish a gradient that drives ATP synthesis through chemiosmosis.