pH and enzymes — questions

1. state

Why is a buffer solution added to each tube in a pH investigation?

Circle one answer.

• To keep the pH at one value during the run
• To speed up the reaction
• To stop the starch reacting with iodine
• To keep the temperature steady

Nudge (only if you are stuck): What must stay steady during each run?

2. identify

Amylase at 40 °C, time to the end point with each buffer: pH 5: 120 s; pH 6: 90 s; pH 7: 60 s; pH 8: 90 s; pH 9: 180 s. Which was the best tested pH?

Circle one answer.

• pH 5
• pH 6
• pH 7
• pH 9

Nudge (only if you are stuck): The fastest reaction has the shortest time.

3. explain

In the pH investigation, all runs are done at 40 °C. Which statement explains why the temperature is kept the same?

Circle one answer.

• Iodine only works at 40 °C
• Buffers only work at 40 °C
• Amylase is denatured at every other temperature
• So that only the pH changes, and any difference is caused by the pH

Nudge (only if you are stuck): How many variables should change on purpose in a fair test?

4. explain

The reaction is slower at pH 9 than at pH 7. Explain why, in terms of the active site.

Nudge (only if you are stuck): What does a change of pH do to the bonds that hold the enzyme in shape?

5. identify

Match each enzyme to its optimum pH.

Write the number of the matching option.

1. About pH 2
2. About pH 7
3. About pH 12

• Protease from the stomach: ______
• Amylase from saliva: ______

Nudge (only if you are stuck): Where in the body does each enzyme work?

6. identify / describe

Catalase in potato breaks down hydrogen peroxide, giving off oxygen that makes a foam. Equal masses of potato were added to hydrogen peroxide with buffers. Foam height after 2 minutes: pH 5, 10 mm; pH 7, 45 mm; pH 9, 25 mm. (a) Which tested pH was best? (b) Name two variables that must be kept the same.

(a) Best tested pH

(b) Two controlled variables

Nudge (only if you are stuck): The highest foam shows the fastest reaction. Think about what else could change the height of the foam.

7. identify

An enzyme is put in a solution a little away from its optimum pH. Which change explains why it works more slowly?

Circle one answer.

• The enzyme molecules are used up
• The active site changes shape, so the substrate fits less well
• The substrate changes into a different molecule
• The molecules stop moving

Nudge (only if you are stuck): Think about shape and fit.

8. predict / explain

Pepsin is a protease made in the stomach, where the pH is about 2. Its optimum pH is about 2. Some pepsin travels with the food into the small intestine, where the pH is about 8. Predict and explain what happens to the activity of pepsin there.

Nudge (only if you are stuck): How far is pH 8 from pepsin’s optimum, and what does that do to its active site?

9. plan

Plan an investigation into the effect of pH on catalase from potato, using foam height. Include what you change, what you measure, what you keep the same and how you make the results reliable.

Nudge (only if you are stuck): Use the amylase method as a model: one variable changed, one measured, the rest kept the same.