What builds up during anaerobic metabolism?

• A. Lactic acid
• B. Pyruvic acid
• C. Acetyl-CoA
• D. Glucose

Answer: (A)

During anaerobic metabolism, oxygen is limited, so cells can’t fully oxidize glucose through the mitochondria. Glycolysis still happens, producing pyruvate and NADH. To keep glycolysis working, pyruvate is converted to lactate by lactate dehydrogenase, which regenerates NAD+ needed for glycolysis. This regeneration of NAD+ allows a continued though limited supply of ATP without oxygen. As a result, lactic acid (lactate) builds up in the muscles and bloodstream when intense activity outpaces oxygen delivery. Pyruvate itself isn’t accumulating because it’s quickly converted to lactate; acetyl-CoA formation and the full aerobic pathways aren’t active without oxygen, and glucose serves as the substrate—it's used up rather than built up. When oxygen becomes available again, lactate can be taken up by the liver and reconverted to glucose via the Cori cycle.