Cellular respiration

Cellular respiration is how cells release the chemical energy locked in fuel molecules, mainly glucose but also fats and amino acids, and store it in ATP, the molecule that powers most work in a cell. In its aerobic form, the overall reaction consumes glucose and oxygen and gives off carbon dioxide and water, like a slow, tightly controlled burning.

The process runs in stages. Glycolysis splits glucose into two pyruvate molecules in the cytoplasm for a small net gain of ATP. In eukaryotes, pyruvate then moves into the mitochondria, is converted to acetyl CoA and is fed into the citric acid cycle, which releases carbon dioxide and loads electrons onto the carriers NADH and FADH2. Finally, the electron transport chain in the inner mitochondrial membrane passes those electrons to oxygen, the final acceptor, which becomes water. The energy released pumps protons across the membrane, and their flow back through ATP synthase makes most of the cell's ATP, a step called oxidative phosphorylation.

When oxygen runs short, many cells switch to fermentation, which keeps glycolysis going but yields far less ATP. Some microbes respire without oxygen, handing electrons to nitrate or sulfate instead. Cellular respiration is not the same as breathing, although breathing supplies its oxygen and removes its carbon dioxide.