Nearly all the energy used by cells comes from the bonds that make up complex, organic compounds. These organic compounds are broken down into simpler molecules, such as glucose. Subsequently, cells extract energy from glucose over many chemical reactions—a process called cellular respiration.

Cellular respiration can take place in the presence or absence of oxygen, referred to as aerobic and anaerobic respiration, respectively. In the presence of oxygen, cellular respiration starts with glycolysis and continues with pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Both aerobic and anaerobic cellular respiration start with glycolysis. Glycolysis yields a net gain of two pyruvate molecules, two NADH molecules, and two ATP molecules (four produced minus two used during energy-requiring glycolysis). In addition to these major products, glycolysis generates two water molecules and two hydrogen ions.

In cells that carry out anaerobic respiration, glycolysis is the primary source of ATP. These cells use fermentation to convert NADH from glycolysis back into NAD⁺, which is required to continue glycolysis. Glycolysis is also the primary source of ATP for mature mammalian red blood cells, which lack mitochondria. Cancer cells and stem cells rely on aerobic glycolysis for ATP.

Cells that use aerobic respiration continue to break down pyruvate after glycolysis via pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. Pyruvate oxidation converts pyruvate from glycolysis into acetyl-CoA—the primary input for the citric acid cycle. NAD⁺ for continued glycolysis is replenished during oxidative phosphorylation, when NADH shuttles and donates electrons to the electron transport chain, becoming NAD+.

The energy-carrier ATP is the main product of cellular respiration. Although oxidative phosphorylation produces most of the ATP generated by aerobic respiration, ATP is also produced during glycolysis and the citric acid cycle.