Effect of pH on amylase (enzymes) — required practical

AQA GCSE Biology

Investigate the effect of pH on the rate at which the enzyme amylase breaks down starch.

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Method

  1. Mix amylase, starch and a pH buffer in a tube kept at a constant temperature in a water bath.
  2. Every 30 seconds, take a drop and add it to iodine solution on a spotting tile.
  3. Record the time when the iodine stays orange-brown (all the starch has been digested).
  4. Repeat at different pH values using different buffers.

Variables

  • Independent variable: pH (set by buffer solutions)
  • Dependent variable: Time for the starch to be digested (rate = 1/time)
  • Control variables: Temperature, amylase concentration, starch concentration and volume

Results & analysis

Calculate rate as 1 ÷ time. The rate is highest at the optimum pH (about pH 7 for amylase) and falls either side as the enzyme’s active site changes shape.

Risk assessment

  • Iodine and some buffers are irritants — wear eye protection.
  • Take care with hot water baths.

Improving accuracy & reliability

  • Keep the temperature constant in a water bath.
  • Test the iodine at regular, fixed intervals.
Exam tip: Rate = 1/time — a shorter time to digest the starch means a faster reaction.

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