All the biochemical reactions taking place inside a living system together constitute metabolism. E.g.
- Kreb's cycle
- Removal of CO2 from amino acids to form amine.
- Removal of amino group in a nucleotide base.
- Hydrolysis of a glycosidic bond etc.
- Primary metabolites: They are the compounds which have identifiable functions and roles in physiological processes. They are not not essential to maintain 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 not not essential to maintain life. E.g.
Pigments: Carotenoids, Anthocyanins etc.
Alkaloids: Morphine, Codeine etc.
Terpenoides: Monoterpenes, Diterpenes etc.
Essential oils: Lemon grass oil etc.
Toxins: Abrin, Ricin etc.
Lectins: Concanavalin A.
Drugs: Vinblastin, curcumin etc.
Polymeric substances: rubber, gums, cellulose etc.
In metabolism, there is a series of linked reactions (multistep chemical reaction) called metabolic pathways.
Metabolic pathways are similar to automobile traffic in a city. Flow of metabolites through the metabolic pathways has a definite rate and direction like automobile traffic. This metabolic flow is called dynamic state of body constituents.
Metabolic pathways are 2 types:
1. Anabolic (biosynthetic) pathways: In this, simpler molecules form complex structures. It consumes energy. E.g. formation of acetic acid from cholesterol, assembly of amino acids to protein, photosynthesis etc.
2. Catabolic pathways: In this, complex molecules become simple structures (degradation). It releases energy. E.g. formation of lactic acid from glucose (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).