1. Anaerobic Respiration:
When food is oxidised without using molecular oxygen, as found in anaerobic bacteria, yeasts, many parasitic animals like Taenia (tapeworm), Fasciola (liver Fluke), Ascaris, etc., the respiration is called anaerobic respiration. Less energy is produced in anaerobic respiration.
In microorganisms the term fermentation is more commonly used in place of anaerobic respiration. Fermentation is defined as the anaerobic breakdown of carbohydrates and other organic compounds into alcohols, organic acids, gases, etc., with the help of microoganisms or their enzymes. In microoganisms the term fermentation is known after the name of product like alcoholic fermentation and lactic acid fermentation, for example, yeasts oxidize glucose to ethyl alcohol and carbon dioxide without utilizing oxygen.
In muscles, certain bacteria and parasitic worms, like Taenia and Ascaris, glucose is metabolised to lactic acid without utilizing oxygen and without the formation of carbon dioxide. In cellulose fermentation, cellulose is converted into volatile fatty acid by anaerobic bacteria and protozoa as found in the rumen and reticulum (parts of stomach of ruminant mammals). Mammalian erythrocytes, as they lack mitochondria, carry out anaerobic respiration. Lactic acid thus produced anaerobically as oxidised aerobically by other tissues (e.g., liver and cardiac muscles).
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2. Aerobic Respiration:
When oxygen is used for respiration, is called aerobic respiration. Aerobic respiration involves two phases:
(i) External respiration: It is simply the intake of oxygen from the surrounding medium (air or water) and giving out of carbon dioxide into that surrounding medium.
(ii) Internal respiration: This involves three steps: (a) uptake of oxygen by tissue cells; (b) oxidation of food inside the cells by oxidising enzymes; and (c) elimination of carbon dioxide from tissues.