The ATP yield is higher in aerobic respiration than in anaerobic respiration because aerobic respiration can fully oxidize glucose, whereas anaerobic respiration uses alternative electron acceptors that are less energetically favorable.
During aerobic respiration, glucose is oxidized to carbon dioxide and water in a series of enzyme-catalyzed reactions that take place in the mitochondria of eukaryotic cells or in the cytoplasm and cell membrane of prokaryotic cells. These reactions include glycolysis, the Krebs cycle, and the electron transport chain. The electron transport chain generates a proton gradient across the inner mitochondrial membrane or cell membrane, which drives the synthesis of ATP via oxidative phosphorylation. The net yield of ATP from the complete oxidation of one molecule of glucose is 36-38 molecules of ATP.
In contrast, during anaerobic respiration, the final electron acceptor is an inorganic molecule other than oxygen, such as nitrate or sulfate. These alternative electron acceptors have lower electron affinities than oxygen, which means that less energy is released during their reduction. As a result, the proton motive force generated by the electron transport chain is weaker, which results in a lower yield of ATP. For example, during fermentation, the net yield of ATP from the oxidation of glucose is only 2 molecules of ATP.
In summary, the ATP yield is higher in aerobic respiration than anaerobic respiration because aerobic respiration fully oxidizes glucose to carbon dioxide and water, whereas anaerobic respiration uses less energetically favorable alternative electron acceptors.