The effect of pH on catalase is a favourite enzyme practical in IGCSE Biology Paper 5 and Paper 6. By hand, measure 5.0 cm³ of each buffer into its labelled boiling tube with a measuring cylinder (rinsing it between buffers), add 5.0 cm³ of hydrogen peroxide, tare the balance and weigh 2.00 g of chopped potato, tip it in and time two minutes, then hold a ruler against the tube and read the foam. Head the table, choose the right kind of graph for a continuous variable, find the optimum pH from your own results and explain it with the shape of the active site. Every attempt uses a different set of buffers and a different potato. Allow about 35 minutes.
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Headings pH (no unit) and height of foam / mm, every height read from your own ruler to within 1 mm and recorded in whole millimetres, and marks for your technique on the bench: 5.0 cm³ of buffer and of hydrogen peroxide in every tube, 2.00 g of potato on a tared balance, and each tube read 2 minutes after its potato went in.
Axes labelled from the table headings, the smallest y-axis scale that fits your tallest foam, every cross within half a small square, and a line graph (ruled lines or a smooth curve) because pH is continuous: a bar chart loses the mark.
The optimum pH from your own results, the rise-then-fall trend, the most enzyme-substrate complexes where the active site is most complementary to hydrogen peroxide, and denaturation (a changed active site) at the extremes. Enzymes are never “killed”.
Two variables kept constant (hydrogen peroxide, mass of potato, time) and a boiled-potato control, which shows that the foam is caused by the enzyme.
A specific source of error (hand-chopped pieces with different surface areas), an improvement that deals with it (identical cork-borer cylinders, repeats and a mean) and eye protection because hydrogen peroxide is an irritant.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.