Anaerobic Respiration Builds Up Lactic Acid in Muscles

Steps:

• Intense exercise demands energy faster than oxygen can supply for aerobic respiration.
• Muscles switch to anaerobic respiration to generate ATP quickly.
• In human muscle cells, anaerobic glycolysis converts glucose to pyruvate, then to lactic acid.
• Lactic acid accumulates, lowering pH and causing pain.

Why C is correct:

• Lactic acid forms via anaerobic glycolysis: glucose → 2 pyruvate + 2 ATP (aerobic), but under low oxygen, pyruvate → lactic acid to regenerate NAD⁺ for continued ATP production.

Why the others are wrong:

• A. Carbon dioxide: Main byproduct of aerobic respiration, exhaled without causing muscle pain.
• B. Ethanol: Produced in yeast fermentation, not human muscle metabolism.
• D. Water: Formed in aerobic respiration's electron transport chain, harmless and not pain-inducing.

Final answer: C