CIE A Level Biology (9700) exams from 2022

Revision Notes

3.2.5 Rate: Inhibitor Concentration

Rate: Inhibitor Concentration

  • There are two types of inhibitors:
    • Competitive inhibitors have a similar shape to that of the substrate molecules and therefore compete with the substrate for the active site
    • Non-competitive inhibitors bind to the enzyme at an alternative site, which alters the shape of the active site and therefore prevents the substrate from binding to it
  • Both types of inhibitors slow down or stop enzyme activity
  • Increasing the concentration of an inhibitor, therefore, reduces the rate of reaction and eventually, if inhibitor concentration continues to be increased, the reaction will stop completely
  • For competitive inhibitors, countering the increase in inhibitor concentration by increasing the substrate concentration can increase the rate of reaction once more (more substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes)
  • For non-competitive inhibitors, increasing the substrate concentration cannot increase the rate of reaction once more, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form

The effect of inhibitor concentration on an enzyme-catalysed reaction, downloadable AS & A Level Biology revision notes

The effect of inhibitor concentration on the rate of an enzyme-catalysed reaction

Exam Tip

While a competitive inhibitor will lower the initial rate of reaction (by occupying some of the available active sites), eventually the same amount of product will be produced as would have been produced without the competitive inhibitor (the maximal rate is not affected).

Non-competitive inhibitors lower the initial rate of reaction and the maximal rate of reaction (a lower amount of product is produced than would normally be produced).


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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