Fermentation is the metabolic process by which organisms acquire energy in the absence of oxygen or with very little oxygen. However, it is not the only way to get energy anaerobically from fuel. Another pathway is anaerobic respiration. People often confuse them, and some classify fermentation as anaerobic respiration.
Fermentation
Both are survival strategies for organisms under oxygen-deficient conditions. Fermentation does not involve an electron transport chain or ATPase. The mechanism for synthesizing ATP is substrate-level phosphorylation. Glucose is broken down into two pyruvates, two ATP, and two NADH during glycolysis. If there is no mechanism to recycle NADH, glycolysis will quickly deplete the cell's NAD+ pool and shut down due to the lack of oxidizers. In fermentation, pyruvate or its breakdown products are the internal electron acceptors that consume NADH. Fermentation is utilized by almost all eukaryotes to produce ATP in an oxygen-free environment, but fermentation can only sustain life for a short time. The lack of respiratory chain means it produces less ATP than anaerobic respiration. Common examples include alcoholic and lactic acid fermentation. For instance, yeast ferments barley into beer, or intense exercise causes muscle soreness. Additionally, fermentation produces short-chain fatty acids and alcohols.
Anaerobic respiration
Anaerobic respiration primarily occurs in prokaryotes. In fact, its mechanism for recycling NADH is the electron transport chain. Electrons from NADH are transferred to other electron acceptors via the electron transport chain, such as nitrates, sulfates, sulfur, carbonates, ferric iron, and fumarate. Transport of electrons is coupled to energy-conserving translocation of protons across cell membranes, but the ATP produced is very little, usually only slightly more than fermentation, and much less than aerobic respiration. Even nitrate respiration whose free energy release is comparable to aerobic respiration, can only make about 10 ATP per glucose. One of the reasons is that free energy released by the decomposition of these organic compounds in non-oxygen substances is less than the free energy released in oxygen. Another reason is that specialized mitochondria of eukaryotes have very abundant and complex cristae to increase area and prevent proton leakage, whereas prokaryotes do not: protons are stored in the space between cell membrane and cell wall, and there are very few cristae in their cell membrane.
Glucose or other organic substances (short-chain fatty acids or alcohols) can also be completely broken down into water and carbon dioxide (oxygen elements come from electron acceptors). In addition, nitrogen gas and hydrogen sulfide are additional by-products. The smell of rotten eggs in sewers and mudflats indicates the presence of sulfate-reducing bacteria. Some ancient bacteria also consume inorganic matter instead of organic matter to get energy, such as methanogens producing ATP from hydrogen gas and carbon dioxide. Methane is their by-product, just like carbon dioxide in aerobic respiration.
Some bacteria are strictly anaerobic, and their metabolism is inhibited, or they may even die when oxygen is present. Facultative anaerobes have branched electron transport chains: they perform aerobic respiration when oxygen is abundant and switch to anaerobic respiration when oxygen levels are low or absent. There are also very few eukaryotes that can survive and reproduce in oxygen-free environments. For example, nitrite is metabolized to Nâ‚‚O by certain fungi; electrons are accepted by fumarate in roundworms that live in human intestine.