When does anaerobic respiration occur in man and yeast?

Anaerobic respiration occurs in man when there is intensive exercise or stress on a muscle which leads to inadequate oxygen in the cells, the pyruvic acid gets reduced to lactic acid, and this is known as homolactic fermentation.

Anaerobic respiration occurs in yeasts when there occurs incomplete oxidation of glucose, and there isn’t any presence of oxygen, i.e., anaerobic conditions. The pyruvic acid is reduced into C₂H₅OH, i.e., Ethanol and release CO₂. This is known as alcoholic fermentation.