Fermentation and Anaerobic Respiration
If the cell is on the moon, or some other environment without a lot of oxygen (like your bedroom – seriously, clean it up), say in the deep ocean, then it uses anaerobic respiration to break down food into chemical energy. This type of metabolism uses a reactant other than oxygen to accept electrons in its electron transport chain.
Microorganisms are the only things that use anaerobic respiration, as far as we know. Some bacteria that live in vents near the ocean floor are called sulfate-reducers, since they use sulfate instead of oxygen as their electron receptor. Microbes called methanogens also use anaerobic respiration—they use carbon dioxide as an electron receptor and make methane in the process.
Methanogens are from the third domain of life, the Archaea. Some methanogens can be found in the guts of cows, where they contribute to methane-filled cow burps.
Others can live in "extreme environments" where most other life cannot exist, such as hot springs, hydrothermal vents, and the Earth's crust.
If a cell has a short supply of oxygen because happens to be in a body running at full speed away from a lion, however, it will not use anaerobic respiration but will start fermentation. Fermentation extends glycolysis with extra reactions that replenish NAD+, so that glycolysis can keep running and keep producing small amounts of ATP. Fermentation comes in two types:
• Lactic acid fermentation
• Alcohol fermentation
Lactic acid fermentation happens in bacteria, fungi, and animal cells. It is a pretty simple follow-up to glycolysis: the pyruvate molecules are reduced to lactate, while NADH is oxidized to NAD+. In this way, NAD+ is replenished for glycolysis. Lactic acid fermentation happens in some fungi and bacteria, but also in animal muscle cells. The dairy industry uses bacteria and fungi for their fermentation to make cheese and yogurt.
Alcohol fermentation is pretty similar to lactic acid fermentation. Instead of the pyruvate being reduced to lactate, it is reduced to ethanol, and lets off two molecules of CO2 along the way.
Two kinds of organisms use alcohol fermentation: bacteria and yeast (yeast are fungi, btw). Humans "use" alcohol fermentation in another way, by using it to make bread, beer and wine. We'll talk more about that in another section.