Final answer:
During hard exercise when not enough oxygen is available, the body uses anaerobic respiration to produce energy by converting pyruvic acid to lactic acid. This process allows for the recycling of NAD+ and continued glycolysis. Glycolysis breaks down glucose to produce ATP and lactic acid, but it is not sustained for a long time and is less efficient than aerobic respiration.
Step-by-step explanation:
During hard exercise when not enough oxygen is available, the body uses anaerobic respiration to produce energy. In this process, pyruvic acid is converted to lactic acid, which allows for the recycling of the enzyme NAD+ and continued glycolysis. Glycolysis, which does not require oxygen, breaks down glucose to produce ATP (energy) and lactic acid. However, glycolysis is not sustained for a long time and is less efficient compared to aerobic respiration.