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7.2.1. Catabolism and obtaining energy

Catabolism and obtaining energy

The catabolism is responsible for the oxidative degradation of organic molecules to obtain the energy needed for the vital functions of the cell.

Molecules are oxidized, giving up electrons (or hydrogens) until a last molecule that accepts them. Depending on the nature of this electron acceptor, two types of organisms are distinguished:

  • Aerobic organisms. The final electron acceptor is oxygen, so they have to live in oxygen-rich media.
  • Anerobic organisms. The final electron acceptor is a molecule other than O2, such as NO2- or SO42-, for example.

Redox reactions

In catabolism, all chemical reactions in which energy is released are oxidation reactions. Electrons are transferred from a molecule that gives them to another that accepts them. Therefore, for oxidation to occur, a reduction is necessary, which is why these reactions are called redox reactions (oxidation-reduction reactions).

In these reactions, electrons or hydrogen atoms are transferred from one compound to another. Molecules that give up hydrogen atoms are oxidized, while those that accept them are reduced.


Characteristics of redox reactions

Oxidation reactions

Reduction reactions

Loss of hydrogens

Addition of hydrogens

Loss of electrons

Electron addition

Release of energy

Energy storage


Redox reactions

Rusty compound

Reduced compound

A + BO → AO + B



AH + B → A + BH



A + B → A + + B -

 A +

 B -

When hydrogen atoms are released in oxidation reactions, much of the energy stored in those bonds is also released.

Some nucleotides such as NAD+NADP+ or FAD, are hydrogen transporters, capturing the hydrogen atoms that are released in the molecules that are oxidized and transferring them to the acceptor molecules, which will be reduced.

Catabolic processes under aerobic conditions

The aerobic catabolism is composed of various processes in which energy is obtained in the form of molecules of ATP, to be used in other processes required, as in the anabolism. You can only take advantage of the energy stored in ATP, because if it is not stored, it is dissipated in the form of heat.

Both carbohydrates and fatty acids are degraded by glycolysis and β-oxidation, respectively to acetyl-CoA. Proteins are also degraded to other compounds until they all enter the Krebs cycle and the respiratory chain, producing CO2, H2O and ATP. 


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