If we compare both anaerobic respiration and anaerobic fermentation, we can say that both of them are completely separate from each other as both follow different pathways. In anaerobic respiration, the process involves the transfer of electrons through a system of chain system in the membrane of the cell.
In this way anaerobic respiration is quite similar to aerobic respiration. In this type of respiration, the process goes through glycolysis, oxidation of the pyruvate, Kerb cycle and then ultimately the transfers the chain of electrons, just like the aerobic respiration. The only difference between these two types of respiration is that anaerobic respiration do not need oxygen (nitrite, nitrate etc) while in aerobic respiration, oxygen is absolutely necessary.
Fermentation takes place when a co-enzyme, NADH reduces the pyruvate to form the organic compound. It is the process of getting energy by the oxidation of some compounds like carbohydrates, and by using an endogenous electron-acceptor that is usually an organic compound. The common products of fermentation are lactic acid, ethanol and hydrogen etc. But respiration is the process where electrons are given to an exogenous electron-acceptor, such as oxygen, through the electron transport chain. Another difference between fermentation and respiration is that it is not necessary for fermentation to occur in environments suitable for anaerobic respiration. A good example of this is yeast, which ferments even if oxygen is present, or if sugar is present. Thus, we can easily say that fermentation can occur when the electrons that are present in the coenzymes (NADH derived after glycolosis) are turned back partly to pyruvate. It should also be noted that the electrons are donated to things that have come from the pyruvate cells.
Fermentation is processed through the following path: Glycolysis – then donating the electron back to the pyruvate or product of pyruvate (which is electron or accepter coming from the internal source). However, anaerobic respiration proceeds through the following path: Glycolysis – oxidation of the pyruvate – Kerb cycle – transfers chain of the electron, which has the electron acceptor at the terminal end (without the requirement of oxygen).
Fermentation also occurs in some muscle cells, which are also called twitch muscles, because these muscles cannot store or use much oxygen in comparison to the other muscles. When we run the oxygen, supply of these muscles gets short as a result of which the twitch muscles starts using the fermentation of lactic acid. Through this process, the muscles can go on functioning as ATP is produced by the Glycolysis.
There is yet another difference between anaerobic respiration and anaerobic fermentation is the electron acceptor, which is also known as the final electron acceptor. For fermentation, pyruvate is the final electron acceptor. This can be seen in yeast fermentation, which gives alcohol as the final acceptor and does not break further for releasing energy. However, the main purpose of anaerobic respiration is to produce ATP (Adenosine Triphosphate), which is used by a cell for energy purposes.