Final answer:
Lactic acid fermentation occurs in certain bacteria and muscle cells when there is insufficient oxygen. Lactic acid accumulation is removed by the blood and brought to the liver for further metabolism.
Step-by-step explanation:
Lactic acid fermentation occurs in certain bacteria, like those found in yogurt, as well as in muscle cells when they are worked hard and fast. This process is used when there is insufficient oxygen supply for aerobic respiration to continue, such as in muscles used to the point of fatigue. Lactic acid fermentation helps provide energy for short-duration, intense activities, like sprinting.
During lactic acid fermentation, pyruvate is converted into lactic acid by the enzyme lactate dehydrogenase. This reaction allows for the regeneration of NAD+ so that glycolysis can continue to produce ATP. In the human body, lactic acid accumulation must be removed by the blood circulation and brought to the liver for further metabolism. In the liver, lactic acid can be converted back into pyruvic acid and utilized for energy.