Biomolecules - Notes | Class 11 | Part 3: Metabolism



All the biochemical reactions taking place inside a living system together constitute metabolism. E.g. 
  • Removal of CO2 from amino acids to form amine.
  • Removal of amino group in a nucleotide base.
  • Hydrolysis of a glycosidic bond etc.
The intermediate products of metabolic reactions are called Metabolites.
Flow of metabolites in metabolic pathway has a definite rate & direction like automobile traffic. This metabolite flow is called dynamic state of body constituents.

Metabolites are 2 types:
  • Primary metabolites: They have identifiable functions in physiological processes and necessary for life. E.g. amino acids, sugars, nucleic acids, lipids, vitamins etc.
  • Secondary metabolites: They are not directly involved in normal growth, development or reproduction. They are found in plant, fungal and microbial cells. E.g.
    • Pigments: Carotenoids, Anthocyanins etc.
    • Alkaloids: Morphine, Codeine etc.
    • Terpenoids: Monoterpenes, Diterpenes etc.
    • Essential oils: Lemongrass oil etc.
    • Toxins: Abrin, Ricin etc.
    • Lectins: Concanavalin A.
    • Drugs: Vinblastine, curcumin etc.
    • Polymeric substances: Rubber, gums, cellulose etc.
In metabolism, there is a series of linked multistep chemical reaction called metabolic pathways. It is 2 types:

Anabolic (Biosynthetic) pathway

Catabolic pathway

Simpler molecules form complex structures (Constructive process).

Complex molecules become simple structures (destructive process).

It consumes energy.

It releases energy.

E.g. acetic acid becomes cholesterol, assembly of amino acids to protein, photosynthesis etc.

E.g. glucose becomes lactic acid (glycolysis), respiration etc.

The energy released through catabolism is stored in the form of chemical bonds. When needed, this bond energy is utilized for biosynthetic, osmotic and mechanical works.

The most important energy currency in living system is the bond energy in adenosine triphosphate (ATP).


In organisms, the metabolites are present in different concentrations. E.g. Blood concentration of glucose in a normal person is 4.2 - 6.1 mmol/L. Concentration of hormones is nanograms/mL.

Systems at equilibrium cannot perform work. As living organisms work continuously, they cannot reach equilibrium. i.e. “The living state is a non-equilibrium steady-state to be able to perform work”.

Living process is a constant effort to prevent falling into equilibrium. This is achieved by energy input obtained from metabolism. So, no living state without metabolism.


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