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C1.1.1—Enzymes as catalysts

Enzymes act as reusable biological catalysts, usually proteins, that speed reactions by forming substrate complexes and lowering activation energy in cells.

Syllabus
First assessment 2025
Objective
C1.1.1
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2019
Most common paperPaper2
Typical marks4–8

Common command terms

  • Outline
  • Explain

Recent exam appearances

May 2019Paper2 ["SL"] · TZ25(c)[ 8 ]C1.1.1—Enzymes as catalysts
May 2016Paper2 ["SL"] · TZ05(a)[ 4 ]C1.1.1—Enzymes as catalysts

Coverage 2016–2019 · 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.

A Small Active Site Runs a Reusable Catalytic Cycle

A typical enzyme is a globular protein. Folding brings particular amino-acid side chains together to form a small active-site pocket; most of the protein maintains the three-dimensional environment that makes this pocket work.

Three-stage diagram showing substrate entering an enzyme active site, an enzyme-substrate complex, and products leaving the unchanged enzyme.
1

Substrate collides with and binds to the active site.

2

A short-lived enzyme–substrate complex forms and catalysis occurs.

3

Products no longer fit in the same way, so they leave; the enzyme can bind another substrate.

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.

Enzymes as catalysts

[ 3 ]

Explain how enzymes catalyse chemical reactions.

Concept essentials

  • Enzymes speed biological reactions without being used up.
  • Substrates bind to active sites to form enzyme-substrate complexes.
  • Catalysis lowers activation energy and increases reaction rate.
  • Specificity depends on the relationship between substrate and active site.