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
- SL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2021
Most common paperPaper1
Typical marks1
Common command terms
- Identify
Recent exam appearances
May 2021Paper1 ["SL"] · TZ114[ 1 ]C1.1.6—Molecular motion
Practice this objective
Coverage 2021–2021 · Updated 15 Jul 2026
Molecular Motion Enables Enzyme Encounters
Molecules move randomly, and enzyme reactions depend on collisions that have enough energy and the correct orientation.
Temperature changes average kinetic energy and collision frequency. Diffusion brings substrate and enzyme together, while the active site filters collisions by shape and chemistry.
For a collision explanation, include:
- random motion and diffusion
- collision frequency
- orientation
- activation energy
At low substrate concentration, adding more substrate increases productive collisions until most active sites are occupied.
Faster motion alone does not guarantee reaction; collisions still need a suitable orientation and energy.
Molecular motion
Assessment in practice
1 marksHow 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
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.