CATABOLIC PROCESSES IN MICROBIAL CELL (CATABOLISM)

RESPIRATION IN MICROBIAL CELL

Respiration in microbial cells is generally an energy-yielding type of metabolism in which an energy substrate (usually the electron donor) is oxidized using an exogenous or externally derived electron acceptor or oxidizing agent. Examples of the energy or respiratory substrates include organic and inorganic compounds and ions while the oxidizing agent or electron acceptors include CO₂, sulphate, oxygen and nitrate as the case may be.

Respiration in microbial cells can either occur aerobically or anaerobically. Some microbial cells also exhibit facultative type of respiration which is a combination of aerobic and anaerobic respiration. Microorganisms that exhibit facultative type of respiration can live in environments with oxygen or no oxygen concentration.

Aerobic respiration is a type of respiration that occurs in the presence of oxygen; and organisms that exhibit this type of respiration are generally known as aerobes. Aerobic microorganisms have the ability to grow in the presence of oxygen (O₂). Most microorganisms (including those that parasitize man, animals and plants) have the ability to live in a state or environmental condition in which O₂ is present. Environmental conditions in which O₂ is always present is generally known as aerobiosis. Oxygen is usually the final electron acceptor in aerobic respiration.

Anaerobic respiration occurs in the absence of oxygen. Microorganisms that thrive in the absence of oxygen are generally known as anaerobes. Anaerobes (inclusive of pathogenic and non-pathogenic organisms) live in habitats in which there is a complete absence of O₂. Such a state with no O₂ concentration is known as anaerobiosis. The final electron acceptor in anaerobic respiration is usually organic but the majority is inorganic, and they include CO₂, Fe₃⁺, SO₄^2- and NO₃^– amongst others.

Both aerobic and anaerobic respiration releases energy required for the formation of ATP, the energy currency of the cell. Microorganisms have many different metabolic reactions based on their energy sources; and they use either respiration (inclusive of aerobic and anaerobic respiration) or fermentation pathways to make their ATP according to their individual oxygen requirements. Glycolysis or the glycolytic pathway, TCA cycle and the electron transport chain (ETC) as shall be seen later in this chapter are all part of respiration in microbial cells.  

References

Alberts B, Bray D, Lewis J, Raff M, Roberts K and Watson J.D (2002). The molecular Biology of the Cell. Fourth edition. New York, Garland, USA.

Berg JM, Tymoczko JL, Stryer L (2002). Biochemistry (5th ed.). New York, NY: W. H. Freeman.

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23^rd edition. McGraw Hill Publishers. USA.

Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall.

Cooper G.M and Hausman R.E (2004). The cell: A Molecular Approach. Third edition. ASM Press.

Karp, Gerald (2009). Cell and Molecular Biology: Concepts and Experiments. John Wiley & Sons.