The electron transport chain transfers electrons, which drives protons from the mitochondrial matrix to the intermembrane space, establishing an electrochemical potential difference. This difference, composed of an electrical potential (due to hydrogen ions) and a chemical potential (from pH differences), propels ATP synthase to convert ADP and inorganic phosphate into ATP. From a single glucose molecule, two pyruvate molecules are formed, leading to two cycles of breakdown that produce 6 NADH, 2 FADH₂, and 2 ATP.

Energy for cellular respiration can be sourced from fats, proteins, sucrose, other disaccharides, and starch. Glycolysis processes various carbohydrates; starch is hydrolyzed into glucose, which is further broken down through glycolysis and the citric acid cycle. Glycogen stored in the liver and muscles is hydrolyzed to glucose between meals, while disaccharides like sucrose provide glucose upon digestion. Proteins are digested into amino acids, which are partially used for new protein synthesis, with excess amino acids converted into intermediates for glycolysis and the citric acid cycle after deamination. Fats break down into glycerol and fatty acids; glycerol enters glycolysis, and fatty acids undergo beta-oxidation to produce acetyl CoA for the citric acid cycle, generating NADH and FADH₂ for ATP production.

Fats yield more than twice the ATP per gram compared to carbohydrates, making them superior energy sources, though they are more challenging to deplete due to their high energy density. In the absence of oxygen, pyruvate undergoes fermentation to recycle NADH + H⁺ back to NAD⁺, sustaining glycolysis. Alcohol fermentation involves decarboxylation of pyruvate to acetaldehyde, which is then reduced to ethanol, regenerating NAD⁺. In lactic acid fermentation, pyruvate is converted to lactic acid in muscle cells and certain bacteria, also regenerating NAD⁺. Fermentation enables anaerobic organisms or oxygen-deprived cells to continue ATP production. Applications of fermentation include bacterial lactic acid fermentation used in foods like yogurt, sour cream, and buttermilk, and alcohol fermentation performed by yeast under anaerobic conditions.