Metabolism: Definition, Terminology, Feaction & Function

The metabolism applies to the assembly of biochemical reactions which are employed by the organisms for the synthesis of cell materials and for the utilization of energy from their environments.

  Metabolism may be defined as 'the sum total of all the enzyme-catalyzed reactions that occur in an organism'.

  The large number of reactions in a cell are organized into a relatively small number of sequences or pathways. It is a highly coordinated and purposeful cell activity, in which multienzyme systems co-operate. This obviously points out to the fact that the metabolism of even a simple unicellular organism is time variant.

Metabolism in microorganisms and the human beings:

a] Microorganisms like bacteria (e.g., Escherichia coli) can double in number every 40 minutes in a culture medium containing only glucose and inorganic salts, or in 20 minutes in a rich broth. 

  The components of the medium are depleted and very little is added to the medium by the cells. Each cell contains hundreds to thousands of molecules of each of about 2,500 different proteins, about 1,000 types of organic compounds and a variety of nucleic acids. It is, thus, apparent that the bacterial cells participate in a variety of metabolic activities.

b] Human adults maintain a constant weight for about 40 years, during which period a total-~1! about 60 quintals of solid food and 45,000 litres of water are metabolised. And yet boll body weight and body composition remain almost constant.


The various processes constituting metabolism may be divided, somewhat arbitrarily, into catabolism and anabolism. Those processes, whose major function is the generation of chemical energy in forms suitable for the mechanical and chemical processes of the cells, are termed as catabolism; whereas those processes, which utilize the energy generated by catabolism for the biosynthesis of cell components, are termed as anabolism.

The various activities powered by catabolism include mechanical movement, growth, reproduction, accumulation of foods, elimination of waste, generation of electricity, maintenance of temperature etc. The various anabolic activities may be exemplified by food manufacture etc.

Some processes can be either catabolic or anabolic, depending on the energy conditions in the cell. These are referred to as amphibolism.

Catabolism reaction
Fuels(carbohydrates, fats) ➞ CO₂+ H₂O + Useful energy
Anabolic reaction
Useful reaction + Small molecules ➞ Complex molecules


In all cells, metabolism enables the cell to perform its vital functions. Metabolism performs following 4 specific functions:
  1. to obtain chemical energy from the degradation of energy-rich nutrients or from captured solar energy.
  2. to convert nutrient molecules into precursors of cell-macromolecules.
  3. to assemble these precursors into proteins, lipids, polysaccharides, nucleic acids and other cell components.
  4. to form and degrade biomolecules required in specialized functions of cells.

Metabolism are closely interrelated since the synthesis of the molecules, that are a component of cell, requires an input of energy, while at the same time it is obvious that the cell components are needed to provide the energy supply and to control intracellular solute concentrations.

The specialized functions also require biosynthetic processes as well as a supply of energy. Terms catabolism and dissimilation are synonyins and refer to the pathways or routes breaking down food materials into simpler compounds and resulting in the release of energy contained in them.

The processes of anabolism or assimilation utilize food materials and energy to synthesize cell components.

The energy relations of the biological processes, the term excrgonic is used to denote a chemical reaction which liberates chemical-free energy. The term exothermic refers to the total energy liberated including heat.

  As the magnitude of heat energy is small and also that it cannot drive biological reactions, the biochemists are more interested in free energy changes and often use drive term exergonic.

The corresponding energy-consuming term endergonic refers to the processes which require an input of free energy while the term endothermic denotes a total energy requirement (including heat).