Enzyme activity varies with pH, peaking at an optimum

Steps:

• Enzymes like amylase function best at a specific pH optimum, typically around 7 for salivary amylase.
• Reaction rate is highest at this pH, so time for starch breakdown to maltose is shortest.
• Away from optimum, activity decreases due to altered enzyme shape, increasing time taken.
• Plot time vs. pH: expect a curve with minimum time at optimum, rising on both sides.

Why D is correct:

• D shows a U-shaped curve with minimum time at neutral pH, matching the bell-shaped activity rate inverted for time.

Why the others are wrong:

• A: Linear increase ignores pH symmetry around optimum.
• B: Steady decline suggests no optimum, unlike enzyme kinetics.
• C: Peak time at optimum contradicts faster reaction at peak activity.

Final answer: D