(a) In animals anaerobic respiration occurs in the situation of deficiency of oxygen during heavy exercise when pyruvic acid is reduced to lactic acid by the enzyme lactate dehydrogenase.

(b) In yeast, the incomplete oxidation of glucose occurs under anaerobic conditions, where pyruvic acid is converted to $\mathrm{CO}_2$ and ethanol by the action of enzyme pyruvic acid decarboxylase and alcohol dehydrogenase.

The ascending order of substrate that will release more energy on oxidation will be as follows

1 gm protein $<0.5 \mathrm{gm}$ In protein $<1 \mathrm{gm}$ glucose $<1 \mathrm{gm}$ fat +0.5 gm glucose

The product of aerobic glycolysis in skeletal muscles is pyruvic acid while in anaerobic fermentation in yeast ethanol and $\mathrm{CO}_2$ are produced.

The glucose as absorbed and reaches blood, giving instant energy. Whereas, cheese sandwich will require time for digestion, and absorption. Sick person requires immediate energy supply, so glucose or fruit juices contains glucose are given to them.

In the process of aerobic respiration, a single molecule of glucose can yield up to 36 ATP molecules. However, in fermentation or anaerobic respiration there is net gain of only 2 molecules of ATP from each glucose molecule, which is comparatively much less than that of aerobic respiration.

Hence, aerobic respiration is more efficient process.