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C1.1.6—Molecular motion

Molecular motion enables substrates to collide with enzyme active sites, making successful enzyme-substrate complex formation possible during catalysis in reactions.

Syllabus
First assessment 2025
Objective
C1.1.6
Level
HL

Molecular Motion Enables Enzyme Encounters

Enzyme catalysis requires random molecular motion to bring a substrate into a productive collision with an active site.

Higher kinetic energy increases motion and collision frequency up to the point where enzyme structure becomes unstable. A collision must also have suitable orientation and enough energy for binding and reaction.

Sometimes a large substrate is effectively immobilized, so enzyme molecules diffuse to exposed sites. In other systems the enzyme is immobilized in a membrane, and moving substrate molecules collide with its fixed active sites.

A membrane-embedded enzyme remains in one location while dissolved substrate diffuses through the membrane environment and collides with the active site.

Immobilized does not mean inactive, and faster motion alone cannot guarantee catalysis: molecular complementarity, orientation and enzyme conformation still matter.

Molecular motion

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: Substrates must collide with enzyme active sites for catalysis.

Representative question

Question 1

[Maximum number: 1]

Which statement applies to enzymes?

A

Enzyme function depends on collisions between substrate and active sites.

B

One active site typically binds to a broad range of substrates.

C

The active site on the substrate is specific to one enzyme.

D

When enzymes are immobilized they stop working.

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

  • Substrates must collide with enzyme active sites for catalysis.
  • Successful collisions can form enzyme-substrate complexes.
  • Molecular motion affects how often enzyme and substrate meet.
  • Immobilization does not automatically stop enzyme activity.