CBSE Class 10 Answered

Certain worms and similar animals found in stagnant water or at the bottom of the sea, and mammalian muscle cells.

(In animal and human muscle cells that are working so hard that their blood circulation cannot bring in enough oxygen, aerobic respiration cannot occur. Anaerobic respiration then produces pyruvic acid that is converted to lactic acid. This situation is called oxygen debt, and the accumulating lactic acid may cause cramps. As the physical activity eases off and the oxygen level begins to rise again, the oxygen is used to break down the lactic acid aerobically)