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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
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2018
Most common paperPaper2
Typical marks3–4

Common command terms

  • Outline
  • Explain

Recent exam appearances

May 2018Paper2 ["HL"] · TZ24(c)[ 3 ]C1.1.1—Enzymes as catalysts
November 2016Paper2 ["HL"] · TZ03(b)[ 4 ]C1.1.1—Enzymes as catalysts
May 2016Paper2 ["HL"] · TZ05(a)[ 4 ]C1.1.1—Enzymes as catalysts
Practice this objective

Coverage 2016–2018 · Updated 15 Jul 2026

Enzymes Speed Reactions as Catalysts

Enzymes are biological catalysts that increase reaction rate without being consumed.

They provide an alternative pathway with lower activation energy, so more substrate molecules can react at a given temperature. The enzyme is regenerated after products leave its active site.

For a catalyst claim, check:

  • reaction rate increases
  • activation energy decreases
  • enzyme is not used up
  • equilibrium position is unchanged

Adding catalase to hydrogen peroxide makes oxygen bubbles appear faster, but the catalase remains available for further reactions.

An enzyme changes kinetics, not the overall energy difference or equilibrium constant.

Enzymes as catalysts

Assessment in practice

4 marks
How it is assessed

This objective is assessed through structured response, commonly using Outline / Explain.

Command terms

Outline / Explain

What earns marks

Build the answer around this relationship: Enzymes speed biological reactions without being used up.

Representative question

Question 1

[Maximum number: 3]

Explain how enzymes catalyse chemical reactions.

Enzyme Catalysis and Rate

Enzymes are biological catalysts that lower activation energy and remain unchanged. Their globular protein shape creates active-site specificity; induced fit aligns substrates; molecular motion and collisions affect rate; temperature, pH, and substrate concentration change activity; assays measure substrate loss or product formation over time.

  • Define enzyme as biological catalyst, effective in small amounts and unchanged.
  • Use active site, specificity, induced fit, ES complex, and activation energy in mechanism answers.
  • Use curve shapes: temperature optimum/denaturation, pH optimum, and substrate saturation plateau.
  • For practicals, state what is measured per unit time and use initial rate, controls, and replicates.

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.
ConceptIB Biology HL