Biological Molecules: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Biological Molecules revision notes

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Section A

1. Name the chemical elements present in carbohydrates, lipids and proteins. [3]

2. Name the building blocks (monomers) of starch, protein and a lipid. [3]

Section B

3. Describe how you would test a food sample for starch, reducing sugar, protein and fat, giving the positive result for each. [8]

4. A student tests a sample of milk.

(a) Explain why the emulsion test gives a white cloudy layer. [2] (b) Suggest why the biuret test does not require heating but Benedict’s does. [2]

5. Explain, in terms of structure, why:

(a) starch is a good storage molecule in plants [3] (b) proteins have a huge variety of shapes and functions [3]

6. State the roles of water in living organisms, giving four points. [4]

7. DNA is a large molecule found in the nucleus.

(a) Name the elements present in DNA that are not found in a carbohydrate. [2] (b) Name the monomer (repeating unit) of DNA. [1] (c) State the two complementary base-pairing rules. [2]

8. A student is given a colourless solution and must determine whether it contains starch, protein, or both.

(a) Describe the test and the positive result that would show whether starch is present. [2] (b) Describe the test and the positive result that would show whether protein is present. [2]

9. A student compares two food samples, X and Y. Sample X gives a positive Benedict’s test result without any prior treatment. Sample Y only gives a positive Benedict’s test result after first being boiled with dilute hydrochloric acid and then neutralised.

(a) State what type of sugar is present in sample X. [1] (b) Explain what the treatment applied to sample Y suggests about the sugar it contains. [3]

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Answers

1. Carbohydrates and lipids — carbon, hydrogen and oxygen [1] [1]. Proteins — carbon, hydrogen, oxygen and nitrogen (and often sulfur) [1].

2. Starch — glucose [1]. Protein — amino acids [1]. Lipid — glycerol and fatty acids [1].

3. Starch — add iodine solution; goes from orange-brown to blue-black [1] [1]. Reducing sugar — add Benedict’s solution and heat in a water bath; blue to brick-red precipitate [1] [1]. Protein — add biuret reagent; blue to purple/lilac [1] [1]. Fat — dissolve in ethanol then add water; a white emulsion forms [1] [1].

4. (a) Lipid is soluble in ethanol but not in water [1], so when water is added it comes out of solution as tiny droplets suspended in the water, scattering light [1]. (b) Biuret detects peptide bonds directly by forming a coloured complex at room temperature [1], whereas Benedict’s requires heat energy to reduce the copper(II) ions to copper(I) oxide [1].

5. (a) It is a large, insoluble molecule [1], so it does not move out of the cell and has no osmotic effect on water potential [1]; it is compact and coiled, storing many glucose units in a small space, and is readily hydrolysed back to glucose [1]. (b) There are 20 different amino acids [1] which can be arranged in any order and any length [1]; the sequence determines how the chain folds into a specific 3-D shape, and shape determines function [1].

6. Any four: it is a solvent in which reactions take place and substances are transported [1]; it is a reactant, for example in photosynthesis and hydrolysis [1]; it transports substances in blood and xylem [1]; it helps regulate temperature through sweating and its high heat capacity [1]; it provides support through turgor in plant cells [1].

7. (a) Nitrogen and phosphorus [1] [1]. (b) Nucleotide [1]. (c) Adenine pairs with thymine (A–T) [1]; cytosine pairs with guanine (C–G) [1].

8. (a) Add a few drops of iodine solution directly to the sample at room temperature [1]; a colour change from orange-brown to blue-black shows starch is present, and no colour change shows it is absent [1]. (b) Add biuret reagent (or sodium hydroxide solution followed by a few drops of copper sulfate solution) at room temperature, without heating [1]; a colour change from blue to purple/lilac shows protein is present, and no colour change shows it is absent [1].

9. (a) A reducing sugar (e.g. glucose) [1]. (b) Sample Y contains a non-reducing sugar (e.g. sucrose) [1]; boiling with dilute hydrochloric acid hydrolyses the non-reducing sugar into its constituent reducing sugars [1], and neutralising the acid is necessary before adding Benedict’s solution, since the test does not give a reliable result in acidic conditions [1].

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Where marks are usually lost

• Forgetting nitrogen in proteins.
• Not stating both the starting and final colour in a food test.
• Saying starch is insoluble without linking that to osmosis.
• Giving only the solvent role of water.
• Forgetting phosphorus alongside nitrogen when naming the elements unique to DNA compared with a carbohydrate.
• Naming DNA’s monomer as “a base” rather than a full nucleotide (which includes a sugar, a phosphate group and a base).
• Forgetting to hydrolyse (boil with dilute HCl, then neutralise) before testing a suspected non-reducing sugar with Benedict’s solution.

The four food tests, side by side

Test for	Reagent	Heat needed?	Positive result
Starch	Iodine solution	No	Orange-brown → blue-black
Reducing sugar	Benedict’s	Yes	Blue → brick-red precipitate
Protein	Biuret	No	Blue → purple/lilac
Fat	Ethanol, then water	No	White/cloudy emulsion

Only the reducing-sugar test with Benedict’s solution requires heating — every other test in this set is carried out at room temperature, and stating the method precisely (including whether heat is used) is often worth a mark by itself. For the full explanation of each molecule’s structure, including DNA’s double helix, see the Biological Molecules revision notes.