Let us make an in-depth study of the anaerobic respiration. As the reactions of glycolysis do not require oxygen, pyruvic acid is formed as in aerobic respiration.
When sufficient oxygen is not available, the citric acid cycle and ETS cannot operate. Under such conditions metabolism of pyruvic acid may proceed anaerobically (i.e., without oxygen). As the reactions of glycolysis do not require oxygen, hence, pyruvic acid is formed as in aerobic respiration.
It may be further metabolized anaerobically to ethyl alcohol or lactic acid. When carried out by bacteria and other microorganisms, such as yeast (saccharomyas), it is also called fermentation. Anaerobic respiration (also called intra-molecular respiration) is an incomplete respiration as it releases only part of the free energy of the substrate, and it has among its end products something that can be further oxidized to yield additional energy.
Most anaerobic respiratory processes follow EMP pathway up to the production of pyruvate (i.e., pyruvic acid). In this respect anaerobic respiration is similar to the most common kind of aerobic respiration. The differences lie in what happens to the pyruvate. In aerobic respiration it is fed into the TCA cycle, in which free oxygen is used.
In anaerobic respiration the pyruvate is disposed of in various ways, some of which yield a little more energy, but others of which use up some of the energy that had been liberated during the formation of pyruvate.
Pyruvic acid can be converted to a wide variety of other products by reactions summarized in Figure 7.10. In all of these cases the metabolism of pyruvic acid involves the same principles. The NADH, which is in limited supply, must give up hydrogen atoms to an organic molecule (reduce that molecule) so that it can participate further in glucose metabolism by picking up more hydrogen atoms.
The bacteria classified as aero-tolerant or indifferent bacteria do not have the enzymes necessary to go through the TCA cycle. Therefore, they can only carry out fermentation, even if they are growing under aerobic conditions.
Remember all anaerobic respiration are not fermentation:
An important distinguishing feature of fermentation is that here the substrate lies outside the living cells in a liquid medium and not inside the cell. It is always carried out by bacteria and other microorganisms, such as yeast only.