Microbial 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).
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. 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).
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.
Figure 1: Energy cycle of the cell showing the reversible reaction between ATP and ADP.
Microbial physiology is simply defined as the study of the cell structure, growth factors, metabolic activities, nutritional requirement and the genetic composition of microorganisms. It is generally the study of the metabolic activities of microorganisms at both the cellular and molecular levels. Physiology is defined as the study of life processes in living cells. Microbial physiology also encompasses the study of microbial genome and how microorganisms acquire substrates from their environment and metabolize same for their growth. The study of microbial physiology helps microbiologists to elucidate the cellular functions of both prokaryotic and eukaryotic cells and how changes in the environment of microorganisms affect their growth or genetic composition. In addition, microbial physiology also looks at the factors that support the growth of microorganisms inclusive of their requirements for oxygen, water activity, temperature requirement, salinity and other nutritional requirements.
Microbial physiology also helps microbiologists to gain basic knowledge about the reproduction patterns and processes of microorganisms; and the study of microbial physiology incorporates the discipline of genetics and biochemistry to make clear the biochemical and molecular basis of microorganisms. Microbial metabolism refers to all the chemical changes occurring in a microbial cell during its growth and development for optimal and stable maintenance. The study of microbial physiology and metabolism is critical to the study of microbiology because microorganisms are metabolizing entities that carry out different forms of metabolic activity including anabolism (anabolic reaction) and catabolism (catabolic reaction) that ensures proper biosynthesis and breakdown of macromolecules respectively in the cell.
Anabolism and catabolism are the two major types of metabolism occurring in every living system. However, there is still yet another type of metabolism, which is known as amphibolism or amphibolic pathway. These individual types of metabolism are described succinctly in this section.
Anabolism is the metabolic reaction or pathway that results in the synthesis of new cell molecules or structures. It can also be called anabolic reaction. Anabolism is an energy requiring process because it requires the input of energy required to form or build larger macromolecules from smaller ones. Many component of glycolysis and Kreb cycle are the starting points to make amino acids, fatty acids and nucleotides- which are building blocks used for the biosynthesis of proteins, fat/lipids and nucleic acids respectively. ATP is used in anabolic reactions. The biosynthesis of macromolecules is examples of anabolic reactions in the cell.
Catabolism is the metabolic process that results in the breakdown of bond in larger molecules (macromolecules) into smaller molecules. It can also be called catabolic reactions. In catabolic reactions, the energy stored in the complex molecules is made available to do work or transformed into ATP. The energy stored in ATP can be used to perform cellular work including movement of substances across cell membranes and for movement of cell organelles. Cellular respiration and fermentation are examples of catabolic reactions. ATP is produced in catabolic pathways reactions.
Amphibolism: The other type of metabolism or metabolic activity that may be obtainable in an organism is known as amphibolism. Amphibolism is defined as the metabolic process that combines both the catabolic and anabolic pathways. It is generally used to describe a biochemical pathway that involves both catabolism and anabolism. The citric acid cycle or the Kreb cycle is an example of an amphibolic pathway, because the Krebs cycle involves both the catabolism (breakdown) of carbohydrate moleculess and fatty acid molecules and the biosynthesis of anabolic precursors (such as α-ketogluturate and oxaloacetate) which are required for the synthesis of amino-acid. These two different activities (i.e., anabolism and catabolism) occurring in the Kreb cycle at different time points is what makes the citric acid cycle an amphibolic pathway.
Amphibolism is another phase of metabolic reactions that occur in microbial cells as aforementioned. 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; and thus 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.
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