Limiting Factors Affecting Enzymes: pH (AQA A Level Biology): Revision Note

Exam code: 7402

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Cara Head

Updated on

The effect of pH

  • All enzymes have an optimum pH

  • Enzymes are denatured at extremes of pH

    • Hydrogen and ionic bonds hold the tertiary structure of the protein (i.e. 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:

    • E.g. pepsin is found in the stomach, an acidic environment at pH 2

    • Pepsin’s optimum pH is therefore pH 2

Graph showing enzyme activity vs pH: pepsin peaks at pH 2, urease at pH 7, and trypsin at pH 8, illustrating different optimum pH levels.
The effect of pH on the rate of an enzyme-catalysed reaction for three different enzymes (each with a different optimum pH)

Buffer solutions

  • 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

Examiner Tips and Tricks

Temperature affects both the movement of molecules (increasing collision rate) and can denature enzymes at high levels. pH does not affect collision rate, but changes in pH can disrupt substrate binding, reducing successful collisions until the active site is no longer functional.

Calculating pH

  • If the hydrogen ion (H+) concentration of a solution is known, the pH can be calculated using the equation:

pH = -log₁₀ [H⁺]

  • You can find the ‘log’ function on your calculator (‘log’ is the same as ‘log10’ so don’t worry if the calculator doesn’t say ‘log10’)

Worked Example

The hydrogen ion concentration of a solution is 1.6 x 10-4 mol dm-3. Find the pH of this solution.

The pH of the solution is:

pH = -log₁₀ [H⁺]

pH = -log₁₀ 1.6 x 10-4 = 3.796

pH = 3.8

Worked Example

The hydrogen ion concentration of a solution of sodium hydroxide is 3.5 x 10-11 mol dm-3. Find the pH of this solution.

The pH of the solution is:

pH = -log₁₀ [H⁺]

pH = -log₁₀ 3.5 x 10-11 = 10.456

pH = 10.5

Worked Example

Ethanoic acid (also known as acetic acid) is a weak acid produced by wood ants that they can spray at predators as a defence mechanism. The hydrogen ion concentration of a sample of ethanoic acid taken from some wood ants was 8.39 x 10-6 mol dm-3. Find the pH of the ethanoic acid produced by wood ants.

The pH of the solution is:

pH = -log₁₀ [H⁺]

pH = -log₁₀ 8.39 x 10-6 = 5.076

pH = 5.08

Examiner Tips and Tricks

Don’t forget the minus sign in the formula: pH = –log₁₀ [H⁺] as this is a common exam mistake. pH values should be between 0 and 14. If your answer falls outside this range, check your working. Use clues in the question to estimate if your answer is reasonable (e.g. alkalis should have high pH, weak acids around pH 4–6).

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Cara Head

Reviewer: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding