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2.3.7Structure & Functions in Living Organisms

Practical: Investigating pH and Enzyme Activity — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

5 min read 4 parts Lesson 14 of 21 Unit quiz

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision
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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Part 1 of 4The Investigation

2.3.7 Practical: Investigating pH and Enzyme Activity

Amylase digests starch to maltose; iodine solution is blue-black with starch and yellow-brown without it.

1
Place a drop of iodine solution in each well of a spotting tile so that it is ready for sampling.
2
Mix amylase solution with a buffer of a chosen pH, such as pH 3, in a test tube.
3
Add starch solution, start the stopwatch at once and mix the contents with a pipette.
4
Every 10 seconds transfer a drop of the mixture into a fresh well of iodine and note whether blue-black appears.
5
Continue until the iodine stays yellow-brown, which shows all the starch has been digested, and record the time taken.
6
Repeat with buffers at other pH values, such as 5, 7, 9 and 11, keeping everything else the same.

Part 2 of 4Results and Explanation

Results and Explanation
Fastest time
  • The pH with the shortest time for the iodine to stay yellow-brown is closest to the optimum pH of the enzyme.
Slower away from optimum
  • At higher or lower pH values the starch takes longer to disappear because fewer enzyme molecules work effectively.
Denaturation
  • Extreme pH changes the shape of the active site, so the substrate no longer fits and enzyme-substrate complexes cannot form.
Rate
  • Rate can be compared as 1 divided by time taken, so a shorter time means a faster rate of reaction.

Part 3 of 4Planning and Control (CORMS)

Exam questions expect you to identify the variables in any practical.

Change
  • The independent variable is the pH of the buffer solution added to the reaction mixture.
Organism or enzyme
  • The same concentration and volume of amylase is used in each test so the enzyme is not a variable.
Repeat
  • Repeating each pH several times and calculating a mean improves reliability and helps identify anomalous results.
Measure
  • The dependent variable is the time taken for the starch to be fully digested, so that no blue-black colour remains.
Same
  • Temperature, volumes and concentrations of starch, amylase and iodine are controlled, for example by using a water bath.

Part 4 of 4Exam Checklist & Tips

2.3.7 – Key Points for the Exam

  • Optimum pH The pH at which an enzyme works fastest.
  • Denaturation A permanent change in the shape of an enzyme, including its active site, so it can no longer work.
  • Buffer solution A solution that holds the pH at a fixed value.
  • Amylase An enzyme that digests starch into maltose.
  • Iodine solution A reagent that turns blue-black in the presence of starch.

Common Mistakes

  • Saying the enzyme is killed by extreme pH; enzymes are not alive, they are denatured.
  • Forgetting to keep temperature constant, so two variables change at once.
  • Reading the endpoint wrongly; the endpoint is when iodine stays yellow-brown, not when it turns blue-black.
  • Stating that a lower pH always slows an enzyme; the effect depends on the optimum pH of that enzyme.

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