pH and enzymes — answer key

1. PQ1 · Practice

Model answer. a. A buffer keeps the pH of the mixture at one value for the whole run.

A full answer includes

• I chose a.

Also accept

• a

Watch for

• “A buffer speeds up the enzyme.” — A buffer only fixes the pH; it is not a catalyst.

2. PQ2 · Practice

Model answer. c. pH 7: the shortest time to the end point, so the fastest reaction of those tested.

A full answer includes

• I chose c.

Also accept

• c

Watch for

• “pH 9, because it has the biggest number.” — The biggest time means the slowest reaction.

3. PQ3 · Practice

Model answer. d. If the temperature changed as well as the pH, you could not tell which one caused a difference in rate.

A full answer includes

• I chose d.

Also accept

• d

Watch for

• “40 °C is needed for the buffer to work.” — Buffers work at any temperature; the point is to change only one variable.

4. PQ4 · Practice

Model answer. pH 9 is away from amylase’s optimum pH. Some of the bonds that hold the protein in shape change, so the active site changes shape. Starch fits it less well, fewer enzyme–substrate complexes form each second, and the reaction is slower.

A full answer includes

• At pH 9, away from the optimum, bonds holding the enzyme’s shape change.
• The active site changes shape, so starch fits less well.
• Fewer enzyme–substrate complexes form, so the rate is lower.

Also accept

• pH away from optimum changes active site shape; fits less well; fewer complexes

Watch for

• “pH 9 is too hot for the enzyme.” — pH is about acidity or alkalinity, not temperature.

5. PQ5 · Practice

Options as numbered on the pupil sheet: 1 — About pH 2; 2 — About pH 7; 3 — About pH 12.

Model answer. A protease from the stomach works best in strong acid, about pH 2. Amylase from saliva works best near neutral, about pH 7.

A full answer includes

• Stomach protease: about pH 2.
• Amylase from saliva: about pH 7.

Also accept

• stomach protease: 2; salivary amylase: 7

Watch for

• “Every enzyme’s optimum pH is 7.” — Each enzyme has its own optimum, matched to where it works.

6. PQ6 · Practice

Model answer. (a) pH 7: the tallest foam, so the fastest release of oxygen. (b) Any two of: the mass and size of the potato pieces, the volume and concentration of hydrogen peroxide, the temperature, and the time before the foam is measured.

A full answer includes

• (a) pH 7.
• (b) Any two of: mass (and size) of potato pieces; volume and concentration of hydrogen peroxide; temperature; time before measuring.

Also accept

• (a) pH 7; (b) two controlled variables such as mass of potato, volume of hydrogen peroxide, temperature, time

Watch for

• “The pH must be kept the same.” — pH is the independent variable here; it is the one you change.

7. PQ7 · Practice

Model answer. b. Away from the optimum pH, the active site changes shape, so the substrate fits less well and fewer enzyme–substrate complexes form.

A full answer includes

• I chose b.

Also accept

• b

Watch for

• “The enzyme is used up faster.” — Enzymes are not used up; the change is in the shape of the active site.

8. PT1 · Apply

Model answer. Pepsin stops working in the small intestine. pH 8 is far from its optimum of about pH 2, so bonds holding the protein in shape change, the active site changes shape and pepsin is denatured. Protein no longer fits its active site, so no enzyme–substrate complexes form.

A full answer includes

• Pepsin’s activity falls to almost nothing in the small intestine.
• pH 8 is far from its optimum (pH 2): the bonds holding its shape change and its active site changes shape (it is denatured).
• Protein no longer fits the active site, so no enzyme–substrate complexes form.

Also accept

• activity falls; far from optimum pH 2; active site shape changed, denatured; protein no longer fits

Watch for

• “Every enzyme works best at pH 7, so pepsin works better in the intestine.” — Pepsin’s optimum is about pH 2; pH 8 is far from it.

9. PT2 · Apply

Model answer. Put the same volume of each buffer, from pH 4 to pH 9, into labelled tubes with the same volume and concentration of hydrogen peroxide. Add the same mass of potato, cut to the same size, start a stop-clock, and measure the height of the foam with a ruler after 2 minutes. Keep the temperature the same (for example in a water bath at 25 °C). Repeat each pH three times and calculate a mean. Plot mean foam height against pH and find the best tested pH. Wear eye protection: hydrogen peroxide irritates eyes.

A full answer includes

• Change: pH, using buffers (for example pH 4, 5, 6, 7, 8, 9).
• Measure: foam height after a fixed time (for example 2 minutes), with a ruler.
• Keep the same: mass and size of potato, volume and concentration of hydrogen peroxide, volume of buffer, temperature.
• Repeat each pH (at least three times) and calculate a mean; wear eye protection.

Also accept

• buffers to change pH; foam height after fixed time; same potato, peroxide and temperature; repeats and mean; safety

Watch for

• “Use different amounts of potato at each pH to see more foam.” — The amount of potato must be the same, or you cannot tell what caused a difference.