Biology 9700/33 — October/November 2012

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Use of the Light Microscope

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

An enzyme catalyses the hydrolysis (breakdown) of sucrose to reducing sugars. This enzyme is essential in a seed to provide reducing sugars so that the seed can grow.

A student suggested the hypothesis:

“seeds soaked in a sucrose solution release the enzyme which will hydrolyse the sucrose solution into reducing sugars”.

You are required to:

soak seeds in a sucrose solution to enable the enzyme to hydrolyse the sucrose to reducing sugars
show the release of the enzyme by testing for reducing sugars in the solution surrounding the seeds
make different concentrations from a solution of reducing sugars
carry out Benedict’s test on the reducing sugars and on the solution surrounding the seeds and use the results to estimate the concentration of reducing sugars in the solution surrounding the seeds.

You are provided with:

labelled	contents	hazard	volume / $\text{cm}^3$
S	sucrose solution	none	15
R	0.4% reducing sugars solution	none	30
W	distilled water	none	100
Benedict’s solution	Benedict’s solution	harmful irritant	60

labelled	hazard	details
seeds	none	dried

Proceed as follows:

(a)

(i)

You will need to select 10 seeds to use in the investigation.

State which variable you will need to standardise (keep the same) when selecting the seeds.

Select your 10 seeds and put them into a large test-tube. Put $3\ \text{cm}^3$ of S into the same test-tube and leave for 20 minutes.
Set up a water-bath and heat the water to a suitable temperature to test for reducing sugars using the Benedict’s test.

(ii)

State the temperature you will need to maintain in the water-bath to carry out the Benedict’s test.

temperature = ______

(iii)

You will need to make dilutions of the 0.4% solution of R, so that the concentration of R is reduced by 0.1% between each successive dilution.

You will need $10\ \text{cm}^3$ of each concentration.

Prepare the space below to show the

concentration of R
volume of R
volume of distilled water, W.

(iv)

You will need to test the different concentrations of R and $2\ \text{cm}^3$ of the solution surrounding the seeds.

State the volume of Benedict’s solution and the volume of each of the concentrations of R.

volume of Benedict’s solution = ______
volume of each concentration of R = ______

Make the dilutions of R you decided in (a) (iii).
Using the volumes you decided in (a) (iv) carry out the Benedict’s test on your concentrations of R.
Record the time taken for the first appearance of a colour change. If there is no colour change after 180 seconds record ‘more than 180’.
After the seeds have been soaked for 20 minutes, carry out the Benedict’s test on the solution surrounding the seeds.

(v)

Prepare the space below and record your results.

(vi)

Estimate the concentration of reducing sugars in the solution that surrounded the seeds.

(vii)

The student’s hypothesis was:

“seeds soaked in a sucrose solution release the enzyme, E, which will hydrolyse the sucrose solution into reducing sugars”.

State whether your results support this hypothesis.

Use your results to explain your answer.

(b)

Describe how you could modify this procedure to investigate the effect of temperature on the release of the enzyme from the seeds.

(c)

Scientists investigated the effect of pH on the activity of the same enzyme found in leaves and in the bark of rubber trees.

A standard volume and concentration of sucrose solution was mixed with the same volume of extract from either leaves or bark.

The activity of the enzyme was measured by finding out the concentration of reducing sugars produced in an hour.

The scientist’s results are shown in Table 1.1.

Table 1.1

pH	activity of enzyme concentration of reducing sugars / $\text{mmol hour}^{-1}$
	in leaves	in bark
4.0	8	10
5.3	53	19
6.4	33	30
7.2	18	32
7.8	7	18

(i)

Plot a graph of the data shown in Table 1.1.

Fig. 1.1

(ii)

Use the data to describe the trend for the activity of the enzyme in the leaves.

(iii)

Explain the change in the activity of the enzyme in the leaves between pH 5.3 and pH 6.4.

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Q2Use of the Light Microscope · Presentation of Data and Observations18M

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