Wed. Apr 1st, 2020

Catabolism And Anabolism

2 min read
Those reactions that transforms fuels into cellular energy are called catabolic reactions. Catabolism occurs in three stages. The biosynthetic reactions involving the formation of complex molecules from simple precursors are Anabolism reactions.

Catabolism  :  The  degradative  processes concerned with  the  breakdown of  complex molecules  to simpler  ones,  with a release  of energy. Those reactions that transforms fuels into cellular energy are called catabolic reactions.


Anabolism:   Those reactions that require energy are called anabolic reactions. E.g. Synthesis of DNA, glucose etc. The useful forms of energy that are produced in catabolism are employed in anabolism to generate complex structures from simple ones, or energy-rich states from energy-poor ones.

ATP is chemical link between catabolism and anabolism. It is infact the energy currency of living cell.

                                                                Fig: Catabolism and Anabolism



The very purpose  of catabolism  is  to trap  the energy  of  the biomolecules  in the form of ATP and  to  generate  the  substances  (precursors) required  for  the synthesis  of complex  molecules.


Catabolism occurs in three stages:

Stage 1: Conversion of complex molecules into their building  blocks:

  • Polysaccharides are broken down to monosaccharides, lipids to free fatty acids and glycerol, proteins to amino acids.

Stage 2: Formation of simple  intermediate :

  • The building blocks produced are degraded to simple intermediates such as pyruvate and acetyl CoA.
  • These intermediates are not readily identifiable as carbohydrates, lipids or proteins. A small quantity of energy (as ATP) is captured in this step.

Stage 3: Final oxidation of acetyl CoA :

  • Acetyl CoA is  completely  oxidized  to CO2,  liberating  NADH and FADH2  that finally get oxidized  to  release large  quantity of energy  (as  ATP).
  • Krebs  cycle (or citric  acid  cycle)  is  the  common  metabolic pathway  involved  in  the final oxidation  of  all energy-rich
  • This pathway accepts the carbon compounds (pyruvate, succinate etc.) derived from carbohydrates, lipids or proteins.
                                                                                            Fig: Stages in catabolic reaction



For the synthesis of a large variety of complex molecules, the starting materials are relatively few. These include pyruvate, acetyl CoA and the intermediates of citric acid cycle.  Besides  the availability  of precursors,  the anabolic  reactions are dependent  on  the supply of  energy (as  ATP )  and  reducing  equivalents  (as  NADPH +H+).

Anabolic pathways require energy input, generally in form of phosphoryl group transfer potential of ATP and reducing power of NADP, NADPH and FADH2.



Anabolic and catabolic pathways are not reversible and operate independently. As such, the metabolic  pathways  occur  in specific  cellular locations  (mitochondria,  microsomes  etc.) and are controlled  by different  regulatory  signals.

Download Link: Catabolism and Anabolism.pdf

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