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Enzymes

The effect of substrate and enzyme concentration

  • 3 things to know
  • Unit AS 1

Overview

Why the rate stops rising at high substrate concentration and maximum rate of an enzyme-catalysed reaction.

What you need to know

Answer each one before you reveal it.

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Process

Why does the rate stop rising as substrate is added, and what is limiting it at each stage?

Key term

Describe the effect on reaction rate of increasing substrate concentration at low substrate levels.

Key term

Explain why the rate stops rising at high substrate concentration.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Describe the effect on reaction rate of increasing substrate concentration at low substrate levels.

    Model answer

    The rate increases in proportion to substrate concentration, because more active sites are occupied at any moment.

  2. 2

    Explain why the rate stops rising at high substrate concentration.

    Model answer

    Every active site is already occupied, so the enzyme concentration has become the limiting factor and adding more substrate has no effect.

  3. 3

    What is meant by the maximum rate of an enzyme-catalysed reaction?

    Model answer

    It is the rate reached when all the available active sites are saturated with substrate and cannot work any faster.

  4. 4

    Describe the effect on reaction rate of increasing enzyme concentration when substrate is in excess.

    Model answer

    The rate increases in direct proportion to enzyme concentration, because more active sites are available.

  5. 5

    Explain why a graph of rate against enzyme concentration eventually levels off in a closed reaction vessel.

    Model answer

    The substrate becomes the limiting factor once there is more enzyme than there is substrate to occupy it.

  6. 6

    Why does the rate of an enzyme reaction fall as the reaction proceeds even at constant temperature?

    Model answer

    The substrate is being used up, so its concentration falls and fewer enzyme–substrate complexes form per second.

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