Anaerobic respiration occurs in the absence of oxygen, unlike aerobic respiration. Some organisms such as certain types of bacteria carry out anaerobic respiration as the main or only kind of respiration, while other organisms such as ourselves only carry out anaerobic respiration under special circumstances.
An example is muscle strength workouts. When a muscle is worked beyond the available oxygen in the cells, anaerobic respiration takes over aerobic respiration, and its products eventually result in the muscle becoming sore, twitching and being unable to carry on contracting.
What are these products and how do they get made in anaerobic respiration?
Let’s continue with the muscle workout example. Under aerobic conditions, the NADH produced from glycolysis of partially broken down hexoses like glucose joins the Krebs cycle where the final electron acceptor following the electron transport chain is oxygen.
In anaerobic respiration there is no oxygen available, so what happens? Instead of getting oxidised by oxygen, NADH is oxidised by pyruvate. This being a redox reaction, pyruvate is at the same time reduced by NADH. This results in the formation of lactic acid which causes all the symptoms of strenuous exercise in the muscle.
Lactate is the ion formed when lactic acid dissolves… For the purpose of talking about anaerobic respiration they’re equivalent!
The more lactic acid is formed as a result of increasing oxygen starvation, the larger the oxygen debt becomes. This is the amount of extra oxygen needed to make up for the lactic acid.
Yeast Fermentation and the Production of Ethanol
Another anaerobic respiration scenario occurs in yeasts. Instead of producing lactic acid from the reduced pyruvate, they produce ethanol. This is widely used as the alcohol in beer, wine, etc.