In prokaryotic cells, all the metabolic pathways occur in the cytoplasm, except for chemiosmosis and oxidative phosphorylation, which occur on the plasma membrane. Prokaryotic cells are capable of anaerobic respiration using alternative electron acceptors such as nitrate and sulfate, although they prefer oxygen as the terminal electron acceptor to drive chemiosmotic ATP synthesis. In the absence of any suitable electron acceptor, they use fermentation pathways.
In eukaryotic cells, glycolysis and fermentation reactions occur in the cytoplasm. The remaining pathways, starting with pyruvate oxidation, occur in the mitochondria. Most eukaryotic mitochondria can use only oxygen as the terminal electron acceptor for respiration. In the presence of oxygen, pyruvate enters the mitochondrial matrix and is oxidized to acetyl-CoA, and then to CO2 via the citric acid cycle. The electron transport chain and ATP synthase are located on the mitochondrial inner membrane.