Introduction to metabolism

Introduction to metabolism

  • Hundreds of  reactions  simultaneously  takeplace  in a  living  cell, in a well-organized and  integrated
  • The entire spectrum  of chemical  reactions,  occurring  in  the  Iiving system,  are  collectively  referred  to  as metabolism.
  • A metabolic  pathway  (or  metabolic  map) constitutes  a  series  of  enzymatic reactions  to produce  specific
  • The term metabolite is applied  to a substrate  or an intermediate  or a product  in the metabolic
  • Metabolism is a highly coordinated and purposeful cell activity, in which many multienzyme systems cooperate.
  • Metabolism has four specific functions:
    • To obtain chemical energy from the degradation of energy rich nutrients from the environment or from captured solar energy.
    • To convert nutrient molecules into the building block precursors of cell macromolecules.
    • To assemble these building blocks into proteins, nucleic acids, lipids, polysaccharides and other cell components.
    • To form and degrade biomolecules required in specialized functions of cells. Although metabolism involves hundreds of different enzyme catalyzed reactions, the central metabolic pathways, are few in number and they are identical in most forms of life.
  • Metabolism is broadly  divided  into  two categories.

      1. Catabolism  :  The  degradative  processes concerned with  the  breakdown of  complex molecules  to simpler  ones,  with a release  of energy.

  1. Anabolism :  The  biosynthetic  reactions involving  the  formation of  complex molecules from simple  precursors.
  • A clear demarcation  between  catabolism  and anabolism  is  rather difficult.
  • since there are several  intermediates  common  to  both  the processes.
  • The term amphibolism is also in use for  reactions which  are  both  catabolic and anabolic  in nature.

Fig: Energy relationship between Catabolic and Anabolic Pathways


  • 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
  • Catabolism occurs in three stages.
  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
  1. Formation of  simple  intermediate :
  • The building blocks  produced  in  stage (1)  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 stage
  1. 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: The three stages of catabolism


  • Anabolism is the biosynthetic phase of metabolism in which small, simple, precursors are built up into larger and more complex molecules, including lipids, polysaccharides, proteins and nucleic acids.
  • 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
  • Besides the availability  of precursors,  the anabolic  reactions are dependent  on  the supply of  energy (as  ATP )  and  reducing  equivalents  (as  NADPH +H+, NADH +H+, FADH2)
  • As such, the metabolic  pathways  occur  in specific  cellular locations  (mitochondria,  microsomes  ) and are controlled  by different  regulatory  signals.

In general, catabolic pathways are convergent and anabolic pathways are divergent



  • The terms-intermediary  metabolism  and energy  metabolism-are  also  in
  • Intermediary metabolism  refers  to  the  entire range  of  catabolic  and anabolic  reactions,  not involving  nucleic
  • Energy metabolism deals with  the metabolic pathways  concerned with  the storage  and liberation  of energy.


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