Final answer:
Lactic acid is produced during lactic acid fermentation, a process used by certain bacteria and animal muscle cells under low-oxygen conditions, catalyzed by lactate dehydrogenase. In muscles, lactic acid is transported to the liver for conversion back into pyruvic acid for energy.
Step-by-step explanation:
Lactic Acid Fermentation
Lactic acid is produced during a process known as lactic acid fermentation. This process is carried out by certain bacteria, such as those found in yogurt, and by animal muscle cells, like those in humans, during intense physical activity when there is not enough oxygen available for aerobic respiration. The key chemical reaction in this process is Pyruvic acid + NADH → lactic acid + NAD+, and it is catalyzed by the enzyme lactate dehydrogenase (LDH). In muscle cells, this process allows them to continue producing energy even when oxygen levels are low, although it can lead to the temporary accumulation of lactic acid in the muscles. Ultimately, the lactic acid must be transported via the blood to the liver, where it can be converted back into pyruvic acid and used for further energy production.