Substrate Depletion Slows Reaction Rate

Steps:

• Examine data: Total O₂ rises from 15 cm³ at 30 s to 251 cm³ at 60 s (236 cm³ increase), then to 285 cm³ at 90 s (34 cm³ increase).
• Calculate rate: Initial rate high (236 cm³/30 s), then slows (34 cm³/30 s), showing decreasing production.
• Identify pattern: Cumulative volume increases but at diminishing rate, indicating substrate (H₂O₂) limits reaction.
• Link to enzyme kinetics: As substrate converts to products, fewer molecules available, reducing enzyme-substrate complexes and rate.

Why D is correct:

• In enzyme-catalyzed reactions, rate decreases when substrate concentration falls (Michaelis-Menten kinetics), as less substrate means fewer complexes form, slowing product (O₂) formation.

Why the others are wrong:

• A: Rate decreases, not increases toward Vmax; Vmax is maximum rate at saturating substrate.
• B: Total O₂ volume increases overall; denaturation would halt reaction entirely.
• C: Rate decreases, not increases; complex formation declines with substrate depletion.

Final answer: D