Metabolic Reactions

Metabolism- Chemical processes within living organisms to maintain life.

Sum of all chemical reactions occurring in living organisms is Metabolism.

  • Hundreds of reactions simultaneously take place in a living cell, in a well-organized and integrated manner. The entire spectrum or sum of all chemical  reactions,  occurring  in  the  living system,  are  collectively  referred  to  as metabolism. Metabolism is set of life sustaining chemical transformations that occurs in living cells.
                                                                            Fig: Metabolism

Purposes of metabolism:

  1. Conversion of fuel to energy required for cellular processes.
  2. Conversion of food/fuel to building blocks such as proteins, lipids, carbohydrates etc.
  3. Elimination of Nitrogenous wastes.

Metabolic pathways are organized in such a manner that one substrate is transformed to another substrate/ chemical compound by the sequence/series of enzyme-catalyzed reactions. Enzymes in metabolic reactions acts as a catalyst.

  • A metabolic  pathway  (or  metabolic  map) constitutes  a  series  of  enzymatic reactions  to produce  specific   products
  • The term metabolite refers to a substrate or an intermediate or a product in the metabolic reactions.


Metabolism is broadly divided into two categories.

  1. Catabolism: Degradative processes concerned with the breakdown of complex molecules to simpler ones, with a release of energy.
  2. Anabolism: Biosynthetic reactions involving the formation of complex molecules from simple precursors.
                                                                       Fig: Catabolism and Anabolism

However, clear  demarcation  between  catabolism  and anabolism  is  rather difficult, since there are several  intermediates  common  to  both  the processes.

  • The term amphibolism is used for reactions which are both catabolic and anabolic in nature.


Metabolism is closely linked with nutrition and availability of nutrients as Nutrition is key to the metabolism (Pathways of metabolism rely upon nutrients they breakdown) in order to produce energy.

Metabolic system of the particular organism determines whether the substrate is nutritious or toxic towards them. H2S (Hydrogen Sulphide) is toxic to most of the animals while some organisms thrives in their presence i.e. use them as nutrient source.


Striking feature about metabolism is similarity of basic metabolic pathways and components between various species even if they are greatly different from each other.

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