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3.2.33 · Enzymes

Rate- Enzyme Concentration — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 7 parts Lesson 11 of 17 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

3.2.3 Rate: Enzyme Concentration — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how enzyme concentration affects the rate of reaction and when substrate becomes limiting.More active sites — a higher enzyme concentration gives more active sites.More complexe…

  • More complexes
  • Linear increase
  • Limiting factor
  • Initial rate
  • Reason — each extra enzyme molecule adds active sites that can all be used
  • Plateau — if substrate is limited, further enzyme has no effect on the rate

Part 1 of 5Effect of more enzyme

More active sites
  • More active sites a higher enzyme concentration gives more active sites.
More complexes
  • More complexes enzyme–substrate complexes are more likely to form.

Part 2 of 5Substrate in excess

Linear increase
  • Linear increase the initial rate rises in proportion to enzyme concentration.
Reason
  • Reason each extra enzyme molecule adds active sites that can all be used.

Part 3 of 5Substrate limiting

Plateau
  • Plateau if substrate is limited, further enzyme has no effect on the rate.
Reason
  • Reason many active sites are empty, so substrate concentration limits the rate.
Graph
  • Graph a straight rising line that levels off.

Part 4 of 5Explaining the graph

1
State that rate rises as enzyme concentration rises.
2
Explain that there are more active sites available.
3
State that this holds only while substrate is in excess.
4
Identify the plateau as the point where substrate is limiting.

Part 5 of 5More from the specification

More from the specification
Initial reaction velocity
  • Initial reaction velocity measuring initial product formation rates before substrate depletion provides a standard means to evaluate enzyme kinetics.
Enzyme-Substrate Complex Formation
  • Catalysis depends on substrate molecules colliding with and binding to the complementary active site shape of an enzyme, converting substrate into product.
Competitive vs Non-Competitive Inhibitors
  • Competitive inhibitors bind reversibly to the active site,
  • increasing the Michaelis-Menten constant (Km),
  • whereas non-competitive inhibitors bind allosterically, lowering Vmax.
Temperature Effects and Denaturation
  • Catalytic rate rises with increasing temperature due to greater kinetic energy, until thermal vibration disrupts hydrogen bonds, denaturing active site shape at extreme temperatures.
Advantages of Immobilised Enzymes
  • Enzymes immobilised onto inert beads exhibit greater thermal stability across higher temperatures and can be easily recovered from product streams.
Buffer solution
  • A solution that holds the pH at a fixed value.
Enzyme-substrate complex
  • The temporary structure formed when a substrate binds to an enzyme's active site.
Inhibitor
  • A molecule that reduces enzyme activity.
Lactose
  • A disaccharide of glucose and galactose.
Optimum
  • The temperature or pH at which activity is highest.
Affinity
  • How readily haemoglobin binds oxygen.
Buffer
  • A substance that resists pH change by absorbing or releasing H+.
Successful collision
  • A collision with enough energy and correct orientation.
Non-competitive inhibitor
  • An inhibitor that binds elsewhere and changes the active site.
Immobilised enzyme
  • An enzyme fixed to a surface or trapped in a matrix so it can be reused.

Key terms

TermMeaning
Enzyme concentrationThe amount of enzyme per unit volume.
Active siteThe region of an enzyme where the substrate binds.
Limiting factorThe factor in shortest supply that restricts the rate.
Initial rateThe rate at the start of a reaction.
SubstrateThe molecule an enzyme acts on.
PlateauThe levelling-off part of a graph.

Avoid these mistakes

1
Saying more enzyme always increases the rate — only while substrate is not limiting.
2
Forgetting to mention substrate availability — it explains the plateau.
3
Saying extra enzyme is used up — enzymes are not used up; the extra sites are simply unoccupied.
4
Describing the effect at the end instead of the initial rate — use initial rates.

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