Under aerobic conditions, the liver can covert lactic acid back into pyruvate and then use pyruvate to regenerate glucose in a multi-step process called gluconeogenesis. In that way, lactose fermentation is reversible.Continue Reading
In humans, lactic acid is produced by muscles when glucose is broken down under anaerobic conditions (little or no oxygen). Since anaerobic glycolysis yields only 2 units of adenosine triphosphate (ATP) for each molecule of glucose consumed (compared to 36 ATP generated when oxygen is present), the body tends to rely on it for short bursts of physical activity.
Metabolically, lactic acid is a dead end. Rather than dispose of it, however, the body conserves carbon by recycling lactic acid into the three-carbon molecule pyruvate. The liver, and to a lesser extent, the kidneys, convert pyruvate into glucose, making it available for the brain, skeletal muscles and other organs to use.
Students often ask why the body cannot dispose of food and instead synthesize all of the glucose it needs through gluconeogenesis alone. The answer is that gluconeogensis consumes 6 ATP for each glucose molecule produced as opposed to a net gain of 2 ATP generated by metabolizing glucose into pyruvate. Relying on gluconeogenesis as the sole source of glucose would slowly deplete the body's reserves of ATP, culminating in energy starvation and metabolic collapse. .Learn more about Organic Chemistry