Cellular respiration, photosynthesis and fermentation are three processes that all occur in living organisms. They are all different types of oxidation-reduction reactions, meaning they involve the transfer of electrons from one molecule to another. All three of these processes have an important role in the energy production and metabolism within cells.

Cellular respiration is a metabolic process which involves taking in oxygen and converting glucose into energy. The end product is usually carbon dioxide, water, adenosine triphosphate (ATP) molecules, and other products such as reduced NAD+ or FADH2 molecules. Cellular respiration typically takes place in the mitochondria of eukaryotic cells and in the cytoplasm for prokaryotic cells. During cellular respiration two pathways can be used: aerobic or anaerobic. When using the aerobic pathway oxygen is consumed during the reaction while producing more ATP than when using the anaerobic pathway without oxygen as a reactant. This is why aerobic respiration is generally considered more efficient at producing ATP than anaerobic respiration which only produces half as much ATP per glucose molecule degraded compared to aerobic pathways.