The raw material of energy is glucose, which can be obtained by eating sugars and starches in the diet, or from stored sugar (glycogen). The liver can convert glycerol (from fat), and some of the amino acids into glucose.

Glycolysis is the first pathway involved in converting glucose into energy, and, like all the energy-producing reactions, takes place in the part of the cell known as the mitochondrion. Unlike the Krebs cycle and oxidative phosphorylation, glycolysis requires no oxygen, and can even produce a little energy itself (two molecules of the 'energy currency' known as ATP - adenosine triphosphate - for every one molecule of glucose). So as well as starting off the rest of the process, glycolysis is important when the oxygen supply to the body is inadequate - as for instance under intense exercising conditions when exertion exceeds the capacity of the heart and lungs to clear carbon dioxide from the muscles.

In glycolysis a phosphate molecule is attached to glucose, producing glucose-6-phosphate. A series of chemical reactions then takes place, resulting in the production of pyruvate. If enough oxygen is present, pyruvate can then form acetyl CoA which then enters the next pathway, known as the Krebs cycle, citric acid cycle or tricarboxylic acid (TCA) cycle. If oxygen is in short supply, pyruvate is instead converted to lactic acid, which accumulates until oxygen becomes available and can then be converted back to pyruvate or can enter the Cori cycle to be converted back to glucose. Lactic acid accumulation in the muscles causes the burning pain normally associated with over-exertion. Glycolysis is dependent on vitamin B3, which it uses in its coenzyme form known as NAD.

The Krebs cycle consists of a complex series of chemical reactions which use oxygen, the vitamin B5-dependent CoA, and vitamins B1, B2 and B3 (in their coenzyme forms TPP, FAD and NAD) to gradually convert acetyl CoA to carbon dioxide and water, releasing electrons in the process.

The final pathway in energy production is known as oxidative phosphorylation, using the 'electron transport chain'. This chain is a series of proteins mounted in sequence on a membrane inside the mitochondria. These proteins receive the electrons released by the Krebs cycle, and pass them to one another, releasing a little energy at each step of this process. Some of the energy is released as heat, and some as ATP molecules. When all the energy has been released, the remaining hydrogen atoms are combined with oxygen to form water.

One of the molecules involved in oxidative phosphorylation is coenzyme Q10 often taken as a dietary supplement to assist energy production.

If glucose is not available for use as fuel, the body can use fatty acids to form acetyl CoA. Some amino acids can form pyruvate or acetyl CoA, or can even enter the Krebs cycle directly.