During aerobic respiration, the oxygen taken in by a cell combines with glucose to produce energy in the form of Adenosine triphosphate (ATP), and the cell expels carbon dioxide and water. This is an oxidation reaction in which glucose is oxidized and oxygen is reduced. This process is critical to all eukaryotes, which are large cells that contain a nucleus and other organelles and which form complex organisms, such as human beings. Respiration in most prokaryotes, such as certain bacteria, is anaerobic. It involves oxidation/reduction reactions that produce energy without oxygen.

Oxidation and reduction are words that refer to the way electrons are exchanged in a chemical reaction. When chemists first described oxidation/reduction reactions, they used the term "oxidation" to refer only to reactions in which other chemicals were bonded to oxygen. They referred to reactions which converted a chemical back to a pure form, such as one that removed the oxygen from magnesium and left only magnesium, as reduction reactions. As scientists discovered more about the underlying mechanisms, however, it became clear that in oxidation, an element was losing one or more electrons to oxygen, and in reduction, an element was gaining electrons.

The ATP produced in cellular respiration is a chemical fuel that powers every reaction in the cell, either directly or indirectly. Respiration happens in every cell in the human body, as well as the cells of almost every eukaryote. The fact that our cells depend on this reaction is the reason that humans breathe in oxygen and breathe out carbon dioxide.

The process of cellular respiration involves two main steps. In the first step, which scientists call glycolysis, glucose breaks down. In the second, aerobic respiration breaks the remains of the glucose down further. During aerobic respiration, oxygen is reduced, donating an electron to hydrogen to form water. The entire process of cellular respiration oxidizes glucose. This produces the majority of the energy released in cellular respiration.

Fermentation also involves oxidation and reduction, and it produces ATP, but it does so less efficiently. Some simple organisms, such as yeasts, use this process in the absence of oxygen. Even humans use fermentation as a kind of backup for cellular respiration in muscle cells deprived of oxygen. During fermentation, a chemical called nicotinamide adenine dinucleotide + hydrogen (NADH) is oxidized and a chemical called pyruvate is reduced. This process produces only two ATP molecules per glucose molecule, while cellular respiration produces 36 ATP molecules from a single glucose molecule.