What is Catabolism?

Catabolism is the break down of complex molecules. Catabolism is the breakdown of complex substances to their constituent parts (glucose, amino acids and fatty acids) which form substrates for metabolic pathways.

Metabolism comprises two major parts; one is anabolism and the other catabolism. The Catabolism process is the action of the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions.



The process involves the breakdown of large molecules such as polysaccharides, lipids, nucleic acids and proteins into smaller units like monosaccharides, fatty acids, nucleotides, and amino acids, respectively.

Catabolism in Different Organisms

The exact nature of catabolic reactions differs from organism to organism and can be classified based on their sources of energy and carbon which are as given below-

  • Organotrophs use organic sources as a source of energy.
  • Lithotrophs use inorganic substrates.
  • Phototroph uses sunlight as chemical energy.

Stages of Catabolism

Catabolism can be broken down into 3 main stages. The three stages are as explained as follows-

Stage 1 – Stage of Digestion

The large organic molecules of organic chemistry like proteins, lipids, and polysaccharides are digested into their smaller components outside cells. This stage acts on starch, cellulose or proteins that cannot be directly absorbed by the cells.

Stage 2 – Release of energy

Once the molecules are broken down, these molecules are taken up by cells and converted to yet smaller molecules, usually acetyl coenzyme A, which releases some energy.

Stage 3 – Energy Stored

The released energy is stored by reducing the coenzyme nicotinamide adenine dinucleotide into NADH.

This process provides the chemical energy necessary for the maintenance and growth of cells. Some examples of the catabolic processes include the glycolysis, the citric acid cycle, the breakdown of muscle protein in order to use the amino acids as substrates for gluconeogenesis, the breakdown of fat in adipose tissue to fatty acids, and oxidative deamination of neurotransmitters by monoamine oxidase.


Catabolic Reactions - Catabolism

Catabolic Reactions

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