Enzymes - GCSE Science Required Practical

Main Idea

This practical investigates how pH affects the rate at which amylase digests starch into sugar.

Iodine is used to test for starch. A dark blue-black or purple colour indicates that starch is still present. As digestion proceeds, the colour fades. When iodine no longer gives a positive result, the starch has been digested.

Method

1. Set up a water bath and use it to bring the starch, buffer and amylase solutions to about 30°C. Monitor the bath during the experiment and adjust it with hot or cold water as needed.
2. Measure the solutions into separate, labelled tubes:
   • 2 cm³ starch solution
   • 2 cm³ buffer solution
   • 1 cm³ amylase
3. Let the solutions reach the target temperature before mixing. Check the temperature of a solution inside a tube rather than relying only on the water-bath temperature.
4. Put iodine into the wells of a spotting tile, and prepare a test-tube rack and timer.
5. Combine all three solutions in one test tube and start the timer immediately, since the enzyme begins acting on the starch once the solutions are mixed.
6. Take an immediate, zero-time sample, then test another sample with iodine every 10 seconds.
7. Record how long it takes until the iodine test is no longer positive for starch. Avoid taking overly large samples so that enough mixture remains to complete the test.
8. Repeat the experiment at different pH values. The demonstration uses pH 5; the other tests are at pH 6, 6.4, 7 and 8.

Results and Interpretation

• In the pH 5 demonstration, the reported time for the starch to be digested is 170 seconds.
• Compare digestion times across pH values. A shorter time means the starch was digested faster, indicating a faster reaction.
• The presenter says the expected optimum is around pH 7, but the tested range does not identify the exact optimum conclusively. The results suggest it lies somewhere between pH 6.4 and pH 8.
• Further testing at pH values within that interval could narrow down the optimum.
• Results can be graphed as time against pH. Alternatively, plot reaction rate against pH. Reaction rate is inversely related to reaction time, with the fastest reaction expected around pH 7.

Speaker

• Presenter: Demonstrates and explains the practical; identified in the video context as Mr Edy. No other speakers are featured.