Metabolism: an overview and types


  • About
  • Catabolism
  • Anabolism
  • Metabolic pathways


  • Metabolism is the collection of chemical reactions takes place to sustain life of an organism.
  • The main purposes of metabolism is to convert food to energy to run cellular activity; to convert food to building blocks for nucleic acids, lipids, protein and carbohydrates; and to remove metabolic waste.
  • The metabolic reactions which are enzyme catalysed are responsible for the growth and reproduction of organism, to maintain their structure and interact with environment.
  • Metabolic reaction is of two types one is catabolic reaction, means the breaking down of compounds and another is anabolic reaction- the building up of the compounds. The catabolic reaction liberates energy and the anabolic reaction uses energy.
  • Metabolic pathways include the steps through which one chemical is transformed into another and each step is facilitating by an enzyme.
  • Enzymes are the key component of the metabolic reaction; they act as catalyst- allows the reaction to proceed more rapidly.


Metabolic reaction is of two types:

  1. Catabolism
  2. Anabolism


  • In catabolism the compound through the set of chemical reaction is broken down into simpler compound or molecules.
  • This is achieved by breaking down and oxidizing food molecules.
  • Catabolism is responsible to provide energy for working of the cell and component needed for the anabolic processes which build molecules.
  • The nature of these catabolic reactions based on the source of energy and carbon which is differ from organism to organism.

The chief metabolic processes in a cell are:

  • Glycolysis
  • Pentose-phosphate pathway
  • Entner-doudoroff pathway
  • Tricarboxylic acid cycle
  • Fermentation
  • Glyoxylate cycle
  • Lipid hydrolysis
  • Protein hydrolysis


  • Anabolism is the set of constructive reactions which used energy released by the catabolic pathway to synthesize complex molecules.
  • The complex molecule construct cellular structure step by step, make up from small and simple precursor.
  • The biomolecules are necessary for the growth and reproduction, some biomolecule serve as the central metabolic intermediates.
  • Some organisms can synthesis all the necessary organic compound like autotrophs. They can be grown on simple media. On the other hand, the organisms which cannot synthesize organic compounds from atmosphere are known as fastidious organisms.
  • Following anabolic process takes place in organism:
  • Synthesis of glucose, lipids, amino acid and protein, nucleic acids
  • Synthesis of other growth factors like vitamins, hormones etc.

Metabolic process:


  • In the glycolysis process glucose and other sugar are partially oxidized to the smaller molecule i.e. pyruvate
  • Embden-Myerhof pathway, pentose phosphate pathway and Entner-Doudroff pathway are the three routes for the conversion of sugar into pyruvate.
  • It is anaerobic process in which organism obtain energy in the absence of oxygen, also called anerobic fermentation.

Tricarboxylic acid pathway:

  • Given by H. A. kerbs in 1973
  • Also known as citric acid cycle. Because citric acid is the first product of the kerb cycle which is as known as TCA cycle as the citric acid has three carboxylic group.

Glyoxyalte cycle:

  • It is anaplerotic reaction which means one product of a cycle is taken up by the other cycle
  • Oxaloacetate is taken from TCA cycle and used for carbon source from the amino acid synthesis.

Pentose phosphate pathway:

  • It is an alternative pathway for the sugar degradation.
  • Its main function is to generate power in the form of NADH in extramitochondrial cytoplasm and the second function is to convert hexoses into pentose for the synthesis of the nucleic acids. The third function is complete degradation of pentose.