Final answer:
Lactic acid fermentation is an anaerobic process used by animals and certain bacteria to convert pyruvic acid into lactic acid, enabling ATP production when oxygen is low.
Step-by-step explanation:
The lactic acid fermentation process is an anaerobic form of respiration utilized by certain bacteria and animals, including those found in yogurt and mammalian muscle cells under low oxygen conditions. This biochemical process involves the conversion of pyruvic acid, produced from glycolysis, into lactic acid. It enables cells like red blood cells, which lack mitochondria, and skeletal muscles, working to the point of fatigue and beyond their oxygen supply, to continue generating energy in the form of ATP. Lactate dehydrogenase (LDH) is the enzyme catalyzing this reaction. The lactic acid produced must eventually be transported to the liver via blood circulation for further metabolism. During intensive physical activity, when oxygen is scarce, this process also occurs in muscles, allowing for continued ATP production. This is summed up by the chemical reaction: Pyruvic acid + NADH → lactic acid + NAD+.