Enzyme Activity Peaks at Optimal Temperature

Steps:

• Enzyme-catalyzed reactions speed up with rising temperature up to an optimum, consuming more substrate in 10 minutes and lowering its concentration.
• Beyond the optimum, enzymes denature, slowing the reaction and leaving more substrate unreacted after 10 minutes.
• Thus, substrate concentration decreases with temperature initially, reaches a minimum at the optimum, then increases.
• The graph must show a V-shaped curve for substrate concentration versus temperature.

Why D is correct:

• It depicts substrate concentration falling to a minimum then rising, matching the bell-shaped rate curve where maximum substrate depletion occurs at optimal temperature per enzyme kinetics.

Why the others are wrong:

• A shows steady substrate decrease, ignoring denaturation slowdown.
• B shows substrate increase, contradicting faster initial reaction rates.
• C shows no clear optimum, with monotonic or incorrect trend.

Final answer: D