2 Biological molecules
Sample question 2.01 You are provided with two solutions: A and B. One of these contains a reducing sugar only and the other contains a mixture of a reducing sugar and a non-reducing sugar. Giving experimental details, explain how you would identify each of the two solutions A and B. [10 marks] [Mark points are shown in square brackets – to a maximum of 10 marks] Put equal volumes of the solutions into two separate test-tubes and label these A and B [1]. Then add the same volume of Benedict’s reagent to each test-tube and heat them in a water bath [1]. The blue Benedict’s reagent will change colour in both tubes, because they both contain reducing sugar [1]. Next, put the same volume of solutions A and B into another two test-tubes and add dilute hydrochloric acid to each tube [1]. After heating, the tubes will be allowed to cool then add sodium hydrogen carbonate to both tubes [1] to neutralise any remaining acid [1]. Heating with dilute acid will hydrolyse the non-reducing sugar and produce more reducing sugars [1]. Finally, add Benedict’s reagent to both tubes and heat them again [1]. By comparing the colour produced in the tubes [1], it should be possible to determine which solution contained the mixture of reducing sugar and non-reducing sugar, because this solution will produce a deeper coloured precipitate [1], or will change colour more quickly than the solution originally containing reducing sugar only [1].
Revision checklist Check that you know: chemical tests for reducing sugars, non-reducing sugars and starch
This is a detailed account of the experimental method and clearly explains the steps that should be taken to identify these two solutions. The answer is also set out in the correct sequence, showing good understanding of the method required to carry out a semi-quantitative Benedict’s test to estimate the concentration of reducing sugars. The answer includes details of the tests for both reducing and non-reducing sugars. Notice that the answer includes references to using the same volume of solutions A and B and the same volume of Benedict’s reagent, which is necessary to make suitable comparisons and to draw valid conclusions from this experiment.
the formation of glycosidic bonds by condensation reactions the breakage of glycosidic bonds by hydrolysis the structures of starch, glycogen and cellulose
the emulsion test for lipids and the biuret test for proteins
how the structures of polysaccharides are related to their functions in living organisms
the semi-quantitative test for reducing sugars using Benedict’s reagent
the structure of a triglyceride and the formation of ester bonds
the structure of α-glucose and β-glucose
how the structure of triglycerides is related to their functions in living organisms
how to define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide
the structure and functions of phospholipids in living organisms
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