Answer: When oxygen supply is insufficient, cells still need energy, so they respire anaerobically for a short time.
Higher plants and animals normally respire aerobically. But when oxygen is not available in sufficient amounts, for example in muscle cells during heavy exercise or in roots of plants in waterlogged soil, the cells break down glucose without oxygen to get energy quickly. In muscles this produces lactic acid (causing fatigue), and in plant cells ethanol and CO2.