Question: 1

Fig. 4.1 shows a representation of part of a carbohydrate molecule called agarose.

One of the subunits of agarose is a sugar called galactose.

(a) (i) Identify the type of carbohydrate molecule of which the carbohydrate agarose is an example.

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[1]

(ii) Starch contains a carbohydrate called amylose. Amylose does not contain galactose.

Using the information in Fig. 4.1, identify one similarity and one further difference in structure between agarose and amylose.

similarity

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difference

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[2]

(b) Agarose forms part of a more complex carbohydrate called agar, which is used as a growth medium for bacteria. Bacteria cannot break down the agarose in agar.

Suggest why bacteria cannot break down agarose.
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[1]

(c) A student wished to demonstrate experimentally that bacteria cannot break down agarose.

The student used a culture of E. coli bacteria which had been grown in a solution containing starch.

Two tubes, A and B, were set up as follows:

Tube A: contained 0.1cm3 of the E. coli culture and 5cm3 of a nutrient solution in which agarose was the only carbohydrate.

Tube B: contained 5cm3 of a nutrient solution in which agarose was the only carbohydrate.

Both tubes were incubated at 30°C for 2 hours.

A sample from each tube was then tested for the presence of reducing sugar.

The results are shown in Table 4.1.

(i) Explain the purpose of tube B.

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[2]

(ii) The student wrote the following conclusion:

My experiment showed that bacteria must be able to break down agarose. This is because reducing sugar was present in tube A.

Suggest an alternative explanation for the presence of reducing sugar in tube A that is not consistent with the student’s conclusion.

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[1]

(iii) Suggest two ways in which the reliability of the experiment could be improved.

1

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2

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[2]

(d) (i) The student did not have access to a colorimeter when testing solutions for the presence of reducing sugar.

Describe how the student could carry out a chemical test for reducing sugar and suggest how he could estimate the amount of reducing sugar in the sample from tube A.

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[5]

(ii) Another student suggested that the agarose may have been broken down to a non-reducing sugar.

Describe how the test for reducing sugar could be modified to investigate this hypothesis.

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[3]

[Total: 17]