CIE A Level Biology (9700) 2019-2021

Revision Notes

3.2.2 Rate: pH Concentration

Rate: pH

  • All enzymes have an optimum pH or a pH at which they operate best
  • Enzymes are denatured at extremes of pH
    • Hydrogen and ionic bonds hold the tertiary structure of the protein (ie. the enzyme) together
    • Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH ions (alkaline solutions) can cause these bonds to break
    • This alters the shape of the active site, which means enzyme-substrate complexes form less easily
    • Eventually, enzyme-substrate complexes can no longer form at all
    • At this point, complete denaturation of the enzyme has occurred
  • Where an enzyme functions can be an indicator of its optimal environment:
    • Eg. pepsin is found in the stomach, an acidic environment at pH 2 (due to the presence of hydrochloric acid in the stomach’s gastric juice)
    • Pepsin’s optimum pH, not surprisingly, is pH 2

_The effect of pH on an enzyme-catalysed reaction, downloadable AS & A Level Biology revision notesThe effect of pH on the rate of an enzyme-catalysed reaction for three different enzymes (each with a different optimum pH)

  • When investigating the effect of pH on the rate of an enzyme-catalysed reaction, you can use buffer solutions to measure the rate of reaction at different pH values:
    • Buffer solutions each have a specific pH
    • Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
    • A measured volume of the buffer solution is added to the reaction mixture
    • This same volume (of each buffer solution being used) should be added for each pH value that is being investigated

Exam Tip

Temperature can both affect the speed at which molecules are moving (and therefore the number of collisions between enzyme and substrate in a given time) and can denature enzymes (at high temperatures). pH, however, does not affect collision rate but only disrupts the ability of the substrate to bind with the enzyme, reducing the number of successful collisions until eventually, the active site changes shape so much that no more successful collisions can occur.


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.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Go to Top