Final answer:
When the body's demand for energy exceeds the capacity of aerobic respiration due to a lack of oxygen, it transitions to anaerobic processes like glycolysis followed by fermentation to produce ATP less efficiently.
Step-by-step explanation:
If the body requires more energy than can be supplied by aerobic respiration, it must resort to alternative methods such as anaerobic respiration or fermentation. Aerobic respiration, which occurs in the mitochondria, is dependent on a steady supply of oxygen and is a highly efficient process, yielding approximately 36 ATP molecules per glucose molecule. However, when oxygen is lacking, such as during intense exercise, the body shifts to producing ATP through processes like glycolysis followed by lactic acid fermentation, supplying energy in the short-term but at lower efficiency.
Typically, aerobic respiration is capable of providing ATP for several hours or longer, as long as the body has a consistent supply of oxygen and nutrients like glucose. But during vigorous activity, the muscles may use up oxygen faster than the bloodstream can deliver it. In such cases, the body can quickly generate energy through anaerobic pathways, though these are much less efficient and produce fewer ATP per glucose molecule, with lactic acid as a byproduct causing muscle fatigue.
Adaptations such as increased levels of myoglobin in muscles and improvements in the circulatory system efficiency can enhance the oxygen supply and thereby extend the duration of aerobic respiration. Nonetheless, when the demand for energy exceeds what aerobic processes can supply, anaerobic mechanisms must compensate.