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Metabolism is simply defined as the summation of the chemical reactions that occurs in the cell at each point in time. It is the processes of catabolism (i.e. breaking down of molecules) and anabolism (i.e. the building up of newer molecules) that occur in the cell. Metabolism is critical for the management of an organism’s energy sources and other cellular materials or products. The term metabolism was originally invented by the famous German physiologist, Theodor Schwann (1810-1882) to mean all of an organism’s chemical processes. It is noteworthy that all living cells depend on a series of biochemical reactions that go on in the cell in order to maintain homeostasis (i.e. a constant internal environment). Metabolic reactions in the form of oxidation and reduction reactions always occur in microbial cells; and these activities direct the synthesis of important molecules for the cells growth, reproduction and development via various metabolic pathways catalyzed by enzymatic reactions. Oxidation-reduction reaction (or redox reaction) is a type of reaction that occur in living systems in which electrons are transferred from one substance or molecule to another especially in scenarios where energy is either released for cell activities or for storage purposes.

In redox reactions, electrons flow from reducing agents (the electron donors or reductants) to oxidizing agents (the electron acceptors or oxidants); and the entire oxidation reaction (i.e. redox reaction) is reversible in nature. Free energy in the form of ATP is always released for cellular activities each time electrons flow from reductants to oxidants during redox reactions in the cell.Metabolic reactions in the cell ensure that complex organic molecules (e.g. carbohydrates and lipids) are broken-down to simpler molecules (e.g. CO2 and NH4+) that can be utilized by the cell for its normal activities. In majority of these reactions, molecules known as electron acceptors are reduced while the electron donors become oxidized. Generally, chemical reactions in the cell i.e. redox reactions can either require energy or release energy; and these processes are generally known as oxidation-reduction reactions as earlier highlighted.  And the energy currency of the cell is known as adenosine triphosphate (ATP), a nucleotide molecule that has three phosphate groups linked to a pentose sugar by phosphodiester bonds. On hydrolysis, ATP (the principal energy-rich chemical of the cell) is converted to adenosine diphosphate (ADP) and inorganic phosphate (Pi), and this process marks the release of energy in the cell (Figure 1).

Figure 1. Energy cycle of the cell showing the reversible reaction between ATP and ADP. During catabolic reactions, complex molecules such as proteins, starch or carbohydrates and lipids are broken down to simpler molecules including amino acids, glucose and glycerol or fatty acids respectively. The energy required for this hydrolytic reaction is from ATP; and anabolic reactions produce energy which is transferred to catabolic pathways for the breakdown of complex molecules in the cell. On the other hand, the energy released during catabolic reactions is stored in the cell as ATP (the energy currency of the cell). 

The energy released during the oxidation of carbon molecules and other complex organic molecules is captured and utilized for the synthesis of ATP from ADP and inorganic phosphate molecules. And the energy required for chemical reactions in the cell as well as those released during redox reaction is stored in the form of ATP. And once energy is needed, ATP is hydrolyzed and free energy is released for metabolic activities in the cell. ATP is the major link between catabolism and anabolism. Just as money is earned and spent in an economy, ATP (which is the energy currency of the cell) is also produced or earned in catabolic reactions and expended or consumed (i.e. utilized) in anabolic reactions for the overall growth and development of the cell.   


Metabolic reactions help to maintain a state of balance or equilibrium in the cell. And the energy released during these processes is used by the cell for growth and other cellular activities. The growth of microorganisms requires the polymerization of building blocks (e.g. amino acids and nucleotides) into proteins, lipids, carbohydrates and nucleic acids; and these molecules are garnered to synthesize important cell components such as peptidoglycan/cell wall, microbial capsules, food vacuoles, cell membrane and other vital cell organelles. Energy required for reproduction and for the overall development of the microbial cell is also provided for by metabolic reactions that go on in the cell. Metabolic reactions are usually divided into two main interconnected phases: anabolism (anabolic reaction) and catabolism (catabolic reaction). 


Anabolism is the processes by which energy and raw materials are used to build new macromolecules (e.g. nucleic acids and proteins) and other cellular structures during biosynthetic activities in the cell. It is an energy requiring-process that utilizes the free energy released by catabolic reactions in the cell. ATP is utilized in anabolic reactions. In anabolic reactions, larger complex molecules such as proteins, carbohydrates and lipids amongst others are produced from simpler and smaller molecules such as amino acids, glucose molecules and fatty acids or glycerol respectively. The formation of nucleic acids (inclusive of DNA and RNA) from nucleotides is also a typical example of anabolic reaction.


Catabolism is the cellular breakdown of complex organic molecules such as proteins into simpler and less-complex molecules (e.g. NH4+, CO2, O2, H2O, PO4 and SO4) utilized by the cell. It is an energy-releasing process; and it is the processes by which a living organism obtains its energy and raw materials from nutrient breakdown. ATP is produced during catabolic reactions. Catabolic reactions provide the energy that is required to drive anabolism in the cell. The reactions that go on in the glycolytic pathway (Glycolysis), Krebs cycle and during fermentation are all typical examples of catabolism. These processes produce precursor molecules such as pyruvate (pyruvic acid) and acetyl CoA amongst others which are used to drive anabolic reactions. Nutrients are usually channeled via a variety of metabolic pathways where they are efficiently catalyzed by enzymes for usage by the cell.


Amphibolism is another phase of metabolic reactions that occur in microbial cells. Amphibolism is simply defined as the combination of catabolism and anabolism. The term “amphibolism” is usually used to describe the metabolic pathway that participates in both anabolic and catabolic reactions. A typical example of an amphibolic pathway is the tricarboxylic acid (TCA) cycle or Kreb’s cycle in which many of the reaction within the pathway is reversible in nature, and has stages that operate anabolically and catabolically. Though the TCA cycle catalyzes the breakdown of some complex biological molecules including carbohydrates, proteins or amino acids and fatty acids during catabolism or catabolic reactions, it can also function in anabolism because some of the products of the Kreb cycle also serve as synthetic precursors for the biosynthesis of other important biological molecules.

The TCA is a typical example of an amphibolic pathway because it can take part both in anabolism and catabolism. Depending on the particular metabolic needs of a microbial cell at any point in time, a cell can decide which way to go in the citric acid cycle or TCA cycle which operates anabolically and catabolically. Amphibolic pathways refer generally to metabolic pathways that functions both catabolically and anabolically. It is noteworthy that during anabolism, microbial cells utilizes energy released during catabolism to synthesize complex biological molecules such as proteins from simpler molecules e.g. amino acids. 

Based on their energy and nutritional requirements as well as an organism’s carbon sources, microorganisms may be classified as autotrophs and heterotrophs; phototrophs and chemotrophs; and as lithotrophs and organotrophs. Autotrophs utilize CO2 as their sole source of carbon while heterotrophs acquire their carbon from reduced, preformed organic molecules from other living organisms. Phototrophs get their energy from the sunlight while chemotrophs acquire theirs from the oxidation of organic or inorganic compounds. Lithotrophic microorganisms obtain their energy from reduced inorganic compounds while organotrophic organisms obtain their energy from organic compounds.   


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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.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of microorganisms. 12th edition. Pearson Benjamin Cummings Publishers. USA. Pp.795-796.

Nelson, David L.; Cox, Michael M. (2005). Lehninger Principles of Biochemistry (4th ed.). New York: W.H. Freeman.

Verma P.S and Agarwal V.K (2011). Cytology: Cell Biology and Molecular Biology. Fourth edition. S. Chand and Company Ltd, Ram Nagar, New Delhi, India.


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