IGCSE Biology / Life processes

Practical: measuring enzyme activity

Edexcel International GCSE Biology (4BI1), chapter 1: Life processes, specification point 2.12. Free lesson.

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Investigate how temperature affects amylase by measuring how long it takes to break down starch, then calculate the reaction rate. Controlling other variables and plotting rate helps you identify the optimum temperature fairly.

How to measure starch breakdown

Amylase breaks down starch, and iodine solution shows whether starch remains. Iodine turns blue-black while starch is present. When no starch remains, it stays orange-brown. Put drops of iodine in the wells of a spotting tile, with one well for each half-minute sample you plan to take.

Which variables to change, measure and control

A fair test changes one independent variable, measures one dependent variable, and keeps other factors constant. To test temperature, use at least five temperature values and measure the time for starch to disappear. Keep the amylase concentration the same. More enzyme provides more active sites, so the reaction could be faster for a reason other than temperature. Keep the starch concentration and volume the same. More substrate takes longer to break down. Keep pH steady with a buffer. Amylase also has a pH optimum, so changing pH would mean testing two factors at once.

Why the solutions must reach temperature first

Bring both solutions to the test temperature before mixing. If you mix cold solutions in a warm water bath, the reaction starts while the mixture is still warming up. The recorded time would then not be for the intended temperature.

How to calculate and plot the rate

The rate is calculated as 1 ÷ time. The time is shortest at the enzyme's optimum temperature, so the rate is highest there. On either side, the time gets longer and the rate falls. Put temperature on the horizontal axis and rate on the vertical axis. Plot rate, not time. A time graph has a dip at the optimum, while a rate graph has a peak, which makes the optimum easier to identify. Draw a smooth curve of best fit. Do not join the points dot to dot or force the curve through every point.

What repeats tell you

Repeats help you judge repeatability: whether the method gives similar results when repeated. They can reveal an anomaly, which you can discard before calculating a mean. Repeats do not make a result more accurate.

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