Respiration in Plants - Notes | Class 11 | Part 3: Fermentation (Anaerobic Respiration)



It is the incomplete oxidation of glucose under anaerobic condition.It occurs in many prokaryotes and unicellular eukaryotes.

It is 2 types:
  • Alcoholic fermentation: Here, the pyruvic acid formed from glucose is converted to CO2 and ethanol. The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions. E.g. Yeast. Yeasts poison themselves to death when the concentration of alcohol reaches about 13%.
  • Lactic acid fermentation: Here, pyruvic acid is converted to lactic acid. E.g. Some bacteria.
The reducing agent (NADH+H+) is reoxidised to NAD+ in both the processes.

In animals, when oxygen is inadequate during exercise, pyruvic acid in muscle cells is reduced to lactic acid by lactate dehydrogenase.

Net ATP production from fermentation of one glucose molecule = 2. (4 ATP from glycolysis – 2 ATP utilized).

The steps involved in fermentation are shown below:

Drawbacks of fermentation 
  • Energy production is limited. Less than 7% of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP.
  • Hazardous products (acid or alcohol) are formed.

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