Why does the reaction rate touch the x-axis at approximately 20°C and 45°C?

Why does the reaction rate touch the x-axis at approximately 20°C and 45°C?

• A. Substrate availability is exhausted.
• B. The enzyme is present in insufficient quantity.
• C. At these temperatures, the enzyme is either too cold or too hot to function effectively.
• D. The reaction reaches equilibrium.

Answer: (C)

The main idea here is how temperature controls enzyme activity. Enzymes work within a narrow temperature window: as temperature rises, molecules move faster and collisions between enzyme and substrate become more frequent, so the reaction rate increases up to the enzyme’s optimal temperature. But outside that window, things go wrong. If it’s too cold, the molecules don’t have enough kinetic energy to reach the activation energy quickly, so the rate drops toward zero. If it’s too hot, the enzyme’s folded structure can unfold orIts active site can be distorted, preventing substrate binding and catalysis. When the rate line touches the x-axis at about 20°C and 45°C, it shows there’s effectively no catalytic activity at those temperatures because the enzyme is either too cold or denatured by heat. The other ideas don’t fit as neatly: running out of substrate would pause the reaction for a while, but wouldn’t create two specific temperature points where activity vanishes; simply having not enough enzyme would lower the maximum rate but wouldn’t drive the rate to zero; and reaching equilibrium is about the balance of forward and reverse reactions, not about the rate being zero due to enzyme inactivity.