A student investigated the effect of pH on the rate of starch hydrolysis by amylase. At pH 7 they measured the concentration of maltose in the reaction mixture every 10 seconds and plotted it against time. The tangent they drew to the curve at time zero passes through (0 s, 0 mmol dm−3) and (40 s, 2.4 mmol dm−3).
(a)
Calculate the initial rate of reaction at pH 7. Give your answer in mmol dm−3 min−1.
[2 marks]
(b)
Give two variables, other than temperature, that the student should keep the same at every pH.
[2 marks]
Give 2. Extra wrong answers cancel right ones.
(c)
Explain why the student compared initial rates at each pH rather than the amount of maltose made after 5 minutes.
[2 marks]
1 marksCore biology3.1.4.2
An enzyme binds its substrate and the shape of its active site changes slightly. This brings the groups involved in the reaction into a better arrangement.
Which explanation is consistent with these observations?
[1 mark]
1 marksCore biology3.1.4.2
An inhibitor binds reversibly to the active site of an enzyme. The enzyme concentration is kept the same.
Which change is most likely to reduce the effect of the inhibitor?
[1 mark]
4 marksCore biologyPractical: RP013.1.4.2
A student investigated the effect of pH on the initial rate of starch hydrolysis by amylase. The student planned to use a different buffer for each pH and to run each pH at a different temperature.
(a)
Explain why running each pH at a different temperature is unsuitable.
[2 marks]
(b)
Describe how the student should keep two other variables the same.
[2 marks]
Give 2. Extra wrong answers cancel right ones.
3 marksCore biology3.1.4.2
At a fixed enzyme concentration, a reversible inhibitor binds to the enzyme at a site away from the active site. Its binding changes the shape of the active site. Increasing the substrate concentration does not restore the maximum rate.
Explain why increasing the substrate concentration does not restore the maximum rate.
[3 marks]
1 marksUnfamiliar contextPractical: RP013.1.4.2
Two enzyme preparations, P and Q, were tested at the same temperature and pH. Their recorded total enzyme concentrations were the same. The table shows their initial rates. The maximum rate was not reached and the amount of active enzyme was not measured.
Table of initial rates: at substrate concentration 1 arbitrary unit, P 20 and Q 10; at 4, P 50 and Q 25; at 16, P 80 and Q 40, all in micromoles per minute.
Substrate concentration / arbitrary units
Initial rate of P / µmol min⁻¹
Initial rate of Q / µmol min⁻¹
1
20
10
4
50
25
16
80
40
Which conclusion is justified?
[1 mark]
5 marksUnfamiliar contextPractical: RP013.1.4.2
A coloured product was measured during an enzyme reaction. The graph shows absorbance against time for the complete reaction and for a blank with no enzyme. The tangent to the complete-reaction curve at time zero passes through (0 s, 0.06) and (40 s, 0.62). The blank line passes through (0 s, 0.04) and (40 s, 0.12). The blank reaction adds to the signal of the complete reaction.
Calculate the initial rate of the enzyme-catalysed reaction alone, in absorbance units per second.
[2 marks]
(b)
An absorbance increase of 1.00 corresponds to 2.50 mmol dm−3 of product. The reaction volume is 3.00 cm3. Calculate the initial rate of product formation in µmol min−1.
[3 marks]
6 marksUnfamiliar contextPractical: RP013.1.4.2
A student incubated an enzyme with its substrate for five minutes at pH 5, 7 and 9, stopped each reaction, added a colour reagent and measured the absorbance. The colour reagent gives a different absorbance for the same amount of product at different pH values, so the student also processed a 1.0 mmol dm−3 product standard at each pH. At each pH, absorbance is proportional to product concentration. The student concluded that the optimum pH is 7.
Table: at pH 5, reaction absorbance 0.18 and standard absorbance 0.50; at pH 7, 0.32 and 1.00; at pH 9, 0.14 and 0.50.
pH
Absorbance after 5 minutes
Absorbance of 1.0 mmol dm⁻³ product standard
5
0.18
0.50
7
0.32
1.00
9
0.14
0.50
(a)
Calculate the product concentration after five minutes at pH 5, in mmol dm−3.
[1 mark]
(b)
Evaluate the student's conclusion. Use the data in your answer.
[3 marks]
(c)
Describe two improvements that would locate the optimum pH for the initial rate.
[2 marks]
Give 2. Extra wrong answers cancel right ones.
5 marksUnfamiliar context3.1.4.2
The graph shows the initial rate of an enzyme-controlled reaction at different substrate concentrations, with no inhibitor and with each of two inhibitors, X and Y, at the same concentration.