Final answer:
Pyruvate is produced during glycolysis and consumed in the mitochondria during the Krebs cycle under aerobic conditions, or transformed into lactic acid or ethanol via fermentation under anaerobic conditions.
Step-by-step explanation:
Under aerobic conditions, pyruvate is produced during a process called glycolysis, which is the first step of cellular respiration. Once produced, the pyruvate is then consumed in the presence of oxygen in the mitochondria of eukaryotic cells. It undergoes a transformation into an acetyl group that combines with coenzyme A (CoA) to form acetyl CoA. This acetyl CoA enters the Krebs cycle (also known as the citric acid cycle), contributing to the production of ATP, NADH, and FADH2. These latter two molecules are electron carriers that subsequently pass their high-energy electrons to the electron transport chain, driving ATP synthesis. As a terminal step, oxygen acts as the final electron acceptor, creating water as a byproduct.
During the Krebs cycle, the energy from the initial glucose molecule, now stored in pyruvate, is transferred to energy carriers. NADH and FADH₂ are produced, along with a small amount of ATP. This process happens in a loop, feeding back intermediates that ensure its continuation as long as substrates are present and oxygen is available.
In the event oxygen is scarce or unavailable, pyruvate is instead converted through anaerobic respiration, or fermentation. This produces lactic acid in human muscles or ethanol in yeast, allowing for a small quantity of ATP to be produced and the regeneration of NAD+ from NADH, ensuring glycolysis can continue.