Every cell runs on ATP, a molecule that stores energy in its chemical bonds. Cells make ATP by breaking down glucose, fatty acids and, when needed, amino acids.
Glycolysis
In the cytoplasm, one glucose splits into two pyruvate and makes a small amount of ATP without oxygen. When oxygen is short, pyruvate becomes lactate.
Citric acid cycle
With oxygen, pyruvate enters the mitochondria and becomes acetyl-CoA, which feeds the citric acid (Krebs) cycle. The cycle releases carbon dioxide and loads the electron carriers NADH and FADH2.
Electron transport chain
The carriers pass electrons along proteins in the inner mitochondrial membrane, and oxygen accepts them at the end to form water. This step makes most of the ATP, about 30 to 32 per glucose.
Other fuels
Fats are broken into acetyl-CoA by beta-oxidation and give more than twice as much energy per gram as sugar. Amino acids can also be burned after the liver removes their nitrogen as urea.
At the bedside
When tissues lack oxygen, as in shock or sepsis, cells switch to anaerobic metabolism and lactate rises. A high lactate is a marker of poor perfusion.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).