Biological Molecules: Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — 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

---

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 adds equal volumes of Benedict’s solution to equal volumes of two food solutions, X and Y, and heats both tubes in the same water bath for the same length of time. Solution X turns brick-red; solution Y turns green.

(a) Name one sugar that this test detects. [1] (b) State what the two colours suggest about solutions X and Y. [1] (c) Explain why the student kept the volumes, the water-bath temperature and the heating time the same for both tubes. [2]

---

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) Glucose or maltose (a reducing sugar) [1]. (b) Both contain reducing sugar, but X contains more (a higher concentration) than Y, because the colour moves further along the sequence blue → green → yellow → brick-red as the amount of reducing sugar increases [1]. (c) The colour reached also depends on the volumes used, the temperature and how long the tubes are heated [1], so keeping these the same means any difference in colour is caused only by the amount of reducing sugar in each solution, making the comparison fair [1].

---

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).
• Treating the Benedict’s test as only positive or negative: green, yellow and brick-red show increasing amounts of reducing sugar, so colours can be compared only when volumes, temperature and heating time are kept the same.

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.