Final answer:
Pyruvic acid can either enter aerobic respiration, becoming acetyl-CoA for the Citric Acid Cycle, or undergo anaerobic fermentation to form lactic acid. The fate of pyruvic acid depends on the presence of oxygen. During aerobic respiration, pyruvic acid is decarboxylated and enters the Krebs cycle, whereas, without oxygen, it is reduced to lactic acid.
Step-by-step explanation:
Outcomes of Pyruvic Acid Metabolism
Pyruvic acid, which is formed during glycolysis, can undergo two different metabolic fates depending on the availability of oxygen. Aerobic respiration occurs when oxygen is present, and anaerobic fermentation takes place in the absence of oxygen. In the presence of oxygen, pyruvic acid releases a carbon dioxide molecule and gets converted into a two-carbon acetyl group which combines with oxalic acid to form citric acid. This citric acid then enters the Citric Acid Cycle (Kreb's Cycle), where it will ultimately be oxidized to produce ATP, the energy currency of the cell. Under anaerobic conditions, such as during intense exercise, pyruvic acid is instead reduced to lactic acid by lactic acid fermentation, regenerating NAD+ needed for glycolysis to continue and provide energy though less efficiently.
In aerobic situations, the transformation starts with pyruvic acid decarboxylation, in which the three-carbon pyruvate molecule loses a carbon dioxide, becoming a two-carbon acetyl group that is then attached to coenzyme A, forming acetyl-CoA, which enters Krebs cycle. The anaerobic pathway, conversely, involves the conversion of pyruvate to lactic acid, which can cause muscle fatigue. The produced lactic acid can be reverted back to pyruvate in well-oxygenated muscle cells or can travel to the liver to be converted into glucose as part of the Cori Cycle once oxygen is available again.