Anaerobic respiration or fermentation occurs when oxygen is unavailable or cannot be used by the organism.

Two common types of fermentation are alcohol fermentation and lactic acid fermentation. In alcohol fermentation, pyruvate gives off carbon dioxide and is converted to ethyl alcohol (ethanol) in a two-step process. In lactic acid fermentation, pyruvate is converted to lactate (lactic acid).

Humans capitalize on both of these fermentation processes. Yeast undergo alcohol fermentation in the production of beer and wine. Certain bacteria and fungi undergo lactic acid fermentation, and are used to make cheese and yogurt. Thus, metabolism and respiration balance is augmented by the control of other enzymes at different key locations in cellular respiration.

Cells are thrifty, expedient, and responsive in their metabolism. Cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, cells make energy–rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation (All the reactions are balanced chemical equations occurs when the number of the atoms involved in the reactants side is equal to the number of atoms in the products side).

Any excess energy is then stored in larger, energy-rich molecules such as polysaccharides (starch and glycogen) and lipids. Thus, from glycolysis (lysis of glucose) to aerobic respiration (or through the anaerobic process of fermentation), ATP is produced as the prized power molecule.