Muscle cells burn oxygen in organelles called mitochondria. Oxygen serves as the final acceptor in a series of reactions called the electron transport chain. Cytochrome proteins embedded in the inner mitochondrial membrane use high energy electrons to drive adenosine triphosphate (ATP) production. Oxygen combines with these electrons to form water.
The overall process of liberating energy from fuel molecules in the presence of oxygen is called aerobic respiration. One molecule of glucose yields approximately 34 ATP when metabolized aerobically. When glucose is broken down by anaerobic glycolysis, it yields only 2 ATP. This vast difference in ATP production means that certain organs, such as the brain and heart, require a continuous supply of oxygen in order to function.