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C1.1.10—Effect of enzymes on activation energy

Enzymes lower activation energy by providing an alternative pathway while leaving reactant and product energy levels unchanged overall during catalysis.

Syllabus
First assessment 2025
Objective
C1.1.10
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1A
Typical marks1

Common command terms

  • Predict
  • Identify

Recent exam appearances

May 2025Paper1A ["SL"] · TZ35[ 1 ]C1.1.10—Effect of enzymes on activation energy

Coverage 2025–2025 · Updated 15 Jul 2026

Enzymes Lower a Barrier; They Do Not Rewrite the Reaction

An enzyme is a biological catalyst. Most are proteins, although some RNA molecules also catalyse reactions. A small amount can act repeatedly because the enzyme emerges unchanged after products are released.

Energy profile comparing uncatalysed and enzyme-catalysed routes, with the enzyme route having a lower activation-energy peak.

The enzyme provides an alternative pathway with lower activation energy, so more collisions can reach the transition state at the same temperature. It does not change the reactants, products or overall free-energy difference, and it does not remove the barrier completely.

Induced Fit Makes Binding Catalytic

The active site is flexible, not a rigid lock. Initial contact with the correct substrate changes the conformation of both enzyme and substrate: this is induced fit.

Sequence showing substrate approach, induced-fit closure and substrate strain toward the transition state.

Induced fit can:

  • align catalytic groups with the bonds that must change
  • strain or polarize bonds in the substrate
  • stabilize the transition state

These effects lower the activation energy; complementary shape alone does not explain catalysis.

SL Checkpoint: Explain the Curve, Not Just Its Shape

globular fold → active-site chemistry → induced fit → lower activation energy → faster turnover → controlled metabolic pathways

When explaining a rate pattern, identify what changed, then connect it to:

  • molecular motion and successful collisions
  • active-site occupancy
  • active-site shape and charge
  • depletion or changing conditions during measurement

A strong investigation measures initial rate, changes one independent variable, controls alternatives, includes a relevant control and uses repeats to reveal variation.

Effect of enzymes on activation energy

[ 1 ]

The graph shows energy changes during a reaction both with and without an enzyme present.

Which statement correctly identifies two of the regions labelled X, Y and Z in the graph?

Concept essentials

  • Enzymes lower the activation energy required for reaction.
  • Catalysed and uncatalysed reactions have the same reactant and product energy levels.
  • Energy-profile graphs show a lower peak for the enzyme-catalysed pathway.
  • Lower activation energy increases reaction rate.