IGCSE · Practice questions · Temperature, pH and enzyme activity

Find the optimum, explain the fall.

Six original Cambridge-style questions on the effects of temperature and pH on enzyme activity: planning the starch and iodine investigation, calculating rates, reading data and explaining the results with kinetic energy, collisions and denaturation.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Rise, peak, crash.

Below the optimum (Extended): more kinetic energy, more frequent effective collisions, more enzyme-substrate complexes.
Above the optimum: active site changes shape, substrate no longer fits, enzyme denatured; permanent.
pH: each enzyme has its own optimum; on either side, shape and fit get worse; far away, denatured.
Rate = 1 ÷ time taken. Low temperature slows an enzyme but does not denature it.

Recall

[2 marks]

State what is meant by the optimum temperature of an enzyme and what is meant by denaturation.

Optimum temperature: the temperature at which the enzyme works fastest (the rate of reaction is highest). ✓
Denaturation: a permanent change in the shape of the enzyme and its active site, so the substrate no longer fits and the enzyme stops working. ✓

Analysis

[5 marks]

Plan an investigation to find the effect of temperature on the activity of amylase, using starch solution, iodine solution and a spotting tile.

Put drops of iodine solution into the wells of a spotting tile. ✓
Place separate tubes of starch solution and amylase in a water bath at the first temperature (for example 10 °C) until they reach that temperature, then mix them and start a stop-clock. ✓
At regular intervals (for example every 30 s) put one drop of the mixture into a fresh well of iodine; record the time when the iodine stays orange-brown (no starch left). ✓
Repeat at other temperatures, for example 20, 30, 40, 50 and 60 °C, and calculate rate = 1 ÷ time. ✓
Control: volume and concentration of starch and amylase, pH (use a buffer); repeat each temperature and calculate a mean. ✓

Calculation

[3 marks]

A student found that the starch disappeared after 5 minutes at 25 °C, 2 minutes at 40 °C and 10 minutes at 55 °C. Calculate the rate at each temperature in min⁻¹, and state which temperature was nearest the optimum.

25 °C: 1 ÷ 5 = 0.20 min⁻¹; 40 °C: 1 ÷ 2 = 0.50 min⁻¹. ✓
55 °C: 1 ÷ 10 = 0.10 min⁻¹. ✓
40 °C gave the highest rate, so it was nearest the optimum. ✓

Explain

[5 marks]

Extended. Explain the shape of a graph of the rate of an enzyme-controlled reaction against temperature, from 10 °C to 70 °C, with an optimum at 40 °C.

From 10 °C to 40 °C the rate increases. ✓
Enzyme and substrate molecules gain kinetic energy and move faster, so there are more frequent effective collisions and more enzyme-substrate complexes form. ✓
The rate is highest at the optimum temperature, 40 °C. ✓
Above 40 °C bonds holding the enzyme's shape break, so the active site changes shape and is no longer complementary to the substrate. ✓
The substrate no longer fits, fewer effective collisions occur, and the rate falls steeply until the enzyme is fully denatured and the rate is zero. ✓

Data

[4 marks]

The table shows the activity of two digestive enzymes, X and Y, at different pH values (as a percentage of each enzyme's maximum).

pH	2	4	6	8	10
Enzyme X / %	100	15	0	0	0
Enzyme Y / %	0	2	45	100	40

(a) State the optimum pH of each enzyme. (b) Suggest where in the alimentary canal each enzyme works, with a reason. (c) Explain why enzyme X has no activity at pH 8.

(a) X: pH 2. Y: pH 8. ✓
(b) X works in the stomach, which is acidic (hydrochloric acid); Y works in the small intestine (duodenum), which is slightly alkaline. ✓
(c) At pH 8 the active site of X has changed shape: the enzyme is denatured. ✓
So its substrate no longer fits and no enzyme-substrate complexes form. ✓

Explain

[3 marks]

Extended. A tube of amylase and starch at 10 °C digests the starch slowly. A second tube is heated to 70 °C and digests almost no starch. Both are then moved to 37 °C. Predict and explain what happens in each tube.

The tube from 10 °C now digests starch quickly: at 10 °C the enzyme was not denatured, its molecules just had little kinetic energy and few effective collisions. ✓
At 37 °C the molecules have more kinetic energy, so effective collisions are more frequent. ✓
The tube from 70 °C still digests no starch: the amylase was denatured, and the change in the shape of the active site is permanent, so starch cannot fit. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.