Biology 9700/51 — October/November 2017

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

The enzyme urease catalyses the breakdown of urea into carbonate ions and ammonium ions as shown in Fig. 1.1.

Fig. 1.1

A solution of urea does not conduct electricity, but a solution of the two ions does conduct electricity.

An increase in ions increases conductivity, so the rate of reaction is proportional to the conductivity.

A meter measures conductivity in microsiemens per centimetre ( $\mu\text{Scm}^{-1}$ ). The conductivity meter also records temperature.

$K_m$ shows the affinity of an enzyme for its substrate. The lower the $K_m$ the greater the affinity.

A student carried out an investigation to find the $K_m$ of the enzyme urease at different temperatures.

The student:

made $500\ \text{cm}^3$ of $0.2\ \text{mol dm}^{-3}$ solution of urea, molar mass $60.1\ \text{g mol}^{-1}$
used serial dilution to make a total of five urea solutions from $0.2\ \text{mol dm}^{-3}$ stock solution
used $1\ \text{g}$ per $10\ \text{cm}^3$ urease solution
used a conductivity meter to measure the initial rate of reaction at each temperature.

Fig. 1.2 shows the experimental set-up during the course of a reaction.

Fig. 1.2

(a)

(i)

Describe how the student could make $500\ \text{cm}^3$ of $0.2\ \text{mol dm}^{-3}$ solution of urea.

(ii)

Describe how the student made a further four solutions of urea by serial dilution.

(b)

(i)

Identify the independent and dependent variables in this investigation.

independent = ______
dependent = ______

(ii)

Suggest a suitable control for this investigation.

(iii)

Describe a method the student could use to find the $K_m$ value of urease at different temperatures. The solutions were made as described in (a)(i) and (a)(ii) and the apparatus shown in Fig. 1.2 was used.

Your method should be set out in a logical way and be detailed enough to let another person follow it. You should not include details of how to prepare the urea or urease solutions.

(c)

(i)

Use the axes below to sketch a graph to show the effect of substrate concentration on the initial rate of reaction at one temperature.

Indicate on your graph how the student could find the $K_m$ of urease at that temperature.

Fig. 1.3

(ii)

Table 1.1 shows the results that the student obtained for $K_m$ at different temperatures.

Table 1.1

temperature	A	B	C	D	E
$K_m$ / arbitrary units	1.8	1.3	0.9	0.8	1.2

State which of these temperatures, A to E, is closest to the likely optimum temperature of urease.

Explain your answer.

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