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7.2.2.3. Fermentations

Fermentation

The fermentation is a catabolic process of incomplete oxidation, which does not require oxygen (anaerobic conditions), in which the final electron acceptor is an organic compound.

Fermentation can occur in the cytoplasm of bacteria, yeast, muscle cells, and plant cells.

Depending on the end products, there are different types of fermentation.

In the cells of the same organism, both cellular respiration and fermentation can occur , depending on the environmental conditions of the cells. For example, some plant cells, which when they have little oxygen oxidize the NADH obtained in glycolysis, producing ethyl alcohol . Another example is that of the shoelaces, which although they are produced by micro-tears of muscle fibers, some authors believe that they are produced by the crystallization of lactic acid. However, they are the microorganisms (yeasts and bacteria) in which we find the greatest variety of types of fermentation.

In fermentation, the incomplete oxidation of organic compounds occurs, so not all the contained energy is released. The energy yield obtained is very low, since only ATP molecules are formed by phosphorylation at the substrate level, in glycolysis, without the participation of the respiratory chain.

Although it seems that fermentation is not energy-efficient due to the little ATP produced, it allows the regeneration of NAD+, which is necessary for glycolysis to continue.

Fermentations are probably the oldest catabolic pathways that have ever existed, since the early atmosphere did not have oxygen.

The fermentation of carbohydrates has a common first stage with respirationglycolysis, and then, from the pyruvic acid obtained, different final products are obtained. The stages of fermentation are:

  • Glycolysis : oxidation of glucose to pyruvic acid. NAD+ is consumed and NADH and 2 ATPare produced.
  • Pyruvic acid reduction  to obtain final products. NAD+ is obtained , which will be used again in glycolysis. If the originating product is lactic acidlactic fermentation has occurred, while if ethanol and CO2 have been obtained, it is alcoholic fermentation.

According to the final product obtained, several types of fermentations are distinguished :

Most important types of fermentation

The main types of fermentation are:

Fundamental ideas about fermentation

Fermentation

  • The fermentation is a catabolic process of incomplete oxidation, which does not require oxygen (anaerobic conditions), in which the final electron acceptor is an organic compound.
  • It is produced in the cytoplasm  of bacteria, yeast, muscle cells, and plant cells . 
  • Incomplete oxidation of organic compounds.
  • Stages of fermentation:
    • Glycolysis: oxidation of glucose to pyruvic acid. NAD+ is consumed and NADH and 2 ATP are produced. Only these 2 ATPs are obtained.
    • Pyruvic acid reduction to obtain final products. 
  • Main types of fermentation: