Warm an enzyme and it works faster, up to a point. Go past its optimum and the rate collapses, because the protein loses the shape of its active site. pH does the same on either side of an enzyme's optimum. The explanations in terms of kinetic energy, collisions, shape and fit are Extended content.
Enzyme activity rises with temperature up to an optimum temperature, then falls quickly as the enzyme is denatured: the shape of its active site changes, so the substrate no longer fits. Each enzyme also has an optimum pH; on either side of it activity falls, and far from it the enzyme is denatured. Denaturation is permanent.
Fig. 1: the rate rises to a peak at the optimum temperature, then falls steeply as the enzyme is denatured
Section 01
Temperature and the optimum.
Mix amylase with starch in a tube in a water bath. At regular intervals a drop goes into iodine on a spotting tile: blue-black means starch is still there, orange-brown means it has all been digested. Choose a temperature from 10 °C to 70 °C, start the test and read the time for the starch to disappear from the results table. Then switch to the molecule view.
Temperature / °C
Time for starch to go / min
Rate = 1 ÷ time / min−1
10
16
0.06
20
8
0.13
30
4
0.25
40
3
0.33
50
6
0.17
60 and 70
over 20
about 0
Optimum temperature: the temperature at which the enzyme works fastest; about 40 °C for this amylase, close to human body temperature.
Denaturation: above the optimum the enzyme's shape, including its active site, is changed. The substrate no longer fits, so the rate falls steeply. This cannot be reversed by cooling.
Extended: below the optimum. As temperature rises, enzyme and substrate molecules gain kinetic energy and move faster, so there are more frequent effective collisions between substrate and active sites and more enzyme-substrate complexes form each second.
Extended: above the optimum. Molecules move even faster, but bonds holding the enzyme's shape break. The active site loses its complementary shape and fit, so fewer effective collisions happen and the rate falls to zero.
Fair test: keep the volume and concentration of starch and amylase, and the pH, the same; let both solutions reach the bath temperature before mixing.
Examiner trap
Low temperature does not denature an enzyme. At 10 °C the molecules simply move slowly and collide less often; warm it up and it works again. Only high temperatures (and extreme pH) denature. Also avoid saying the enzyme is "killed": enzymes are molecules, not living things.
Section 02
pH: every enzyme has its own optimum.
Choose pepsin, amylase or trypsin and drag the pH slider (or the dot on the graph) from pH 1 to pH 12. Watch the rate and the active site readout.
Enzyme (where it works)
Optimum pH in the sim
pepsin (stomach, acidic)
2
amylase (mouth)
7
trypsin (small intestine, alkaline)
8.5
Optimum pH: each enzyme works fastest at its optimum pH, which suits the place where it works. Pepsin works in stomach acid; trypsin in the alkaline small intestine.
Either side of the optimum: the rate falls. Far from the optimum the enzyme is denatured and the rate is zero.
Extended: shape and fit. A change in pH alters the shape of the active site, so the substrate no longer fits as well and fewer enzyme-substrate complexes form. Far from the optimum the bonds holding the shape break and the enzyme is denatured: the substrate cannot fit at all.
Investigating pH: use buffer solutions to set each pH, keep the temperature constant with a water bath, and time the disappearance of starch (iodine test) or measure the volume of oxygen from hydrogen peroxide.
Examiner trap
The optimum pH is not always 7. Pepsin works best in acid and is denatured at pH 7. Read the optimum off the graph for the enzyme in the question.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the temperature tab, at which temperature does the amylase digest all the starch in the shortest time?
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In the temperature tab, starch is still present after 20 minutes at 70 °C. Why?
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In the pH tab, pepsin is selected and the pH is set to 7. What does the active site readout show?
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Which enzyme in the pH tab has the highest optimum pH?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
What happens to an enzyme when it is denatured?
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Extended: the rate of an enzyme reaction doubles between 20 °C and 30 °C. The best explanation is that:
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An enzyme from the stomach is tested at pH 2, 7 and 10. Which result is most likely?
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Extended: why does an enzyme that has been heated to 80 °C not work again when it is cooled to 37 °C?
Skill unlocked
Temperature, pH and enzymes, mastered.
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