AQA A-level Biology: Monomers, Polymers and Carbohydrates (7402) – 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 from your board or school.

These questions cover sections 3.1.1 (Monomers and polymers) and 3.1.2 (Carbohydrates) of the AQA AS and A-level Biology specification, version 1.5, for exams from June 2016 onwards. This is AS content, also examined within A-level. Where a question needs them, use relative molecular masses of glucose 180, sucrose and lactose 342, and water 18.

Read the study guide first, or recall with the revision notes. Course pages: AQA A-level Biology hub and printable checklist.

Questions

1. (a) Define the term polymer. [1] (b) Name three monomers found in living organisms. [3]

2. Disaccharides are formed by condensation of two monosaccharides. (a) Name the two monosaccharides joined in maltose, in sucrose and in lactose. [3] (b) State which of these three disaccharides gives a negative result when heated directly with Benedict’s solution. [1]

3. (a) Describe what happens, in terms of the molecules involved, when a glycosidic bond forms between two α-glucose molecules. [3] (b) A 51.3 g sample of lactose is completely hydrolysed. Calculate the mass of water used and the total mass of sugars formed. [3]

4. (a) Compare the structure of β-glucose with that of α-glucose. [2] (b) Name the polysaccharide made from β-glucose. [1]

5. A plant cell is placed in distilled water. Explain how the structure of cellulose prevents the cell wall from breaking as water enters the cell. [5]

6. Starch and glycogen are both storage polysaccharides. (a) State two ways in which they are similar in structure. [2] (b) State two ways in which glycogen differs from starch. [2] (c) Explain how the structure of glycogen suits it to its function in a muscle cell. [2]

7. Four samples, P to S, are each one of maltose solution, sucrose solution, starch suspension or distilled water. The results were:

Sample	Heated with Benedict’s	Boiled with acid, neutralised, heated with Benedict’s	Iodine solution
P	Brick-red	Not done	Yellow-brown
Q	Blue	Blue	Blue-black
R	Blue	Brick-red	Yellow-brown
S	Blue	Blue	Yellow-brown

(a) Identify each sample. [4] (b) Explain why sample R gave a different result after acid hydrolysis. [2] (c) Explain why it was necessary to test R with Benedict’s solution before hydrolysing it. [1]

8. A student makes standards from an 8 g dm⁻³ glucose stock. After a Benedict’s test the absorbance of the remaining solution was measured.

Glucose (g dm⁻³)	0	2	4	6	8
Absorbance	0.90	0.72	0.55	0.35	0.18

(a) Work out how much stock solution and how much distilled water are mixed to give 20 cm³ of the 6 g dm⁻³ standard. [2] (b) An unknown gives an absorbance of 0.62. Estimate its concentration. [2] (c) Explain why absorbance falls as glucose concentration rises. [2]

9. A mixture produced by hydrolysing a disaccharide was run on paper chromatography with standards. The solvent front moved 9.0 cm. Spots moved: galactose 2.7 cm, glucose 3.6 cm, fructose 5.4 cm. The mixture gave spots at 2.7 cm and 3.6 cm. (a) Calculate the R_f value of the galactose standard. [1] (b) Identify the sugars in the mixture and justify your answer. [2] (c) Name the disaccharide that was hydrolysed and explain your choice. [2]

10. A glycogen fragment contains 60 glucose units, including several branch points. (a) State the number of glycosidic bonds in the fragment. [1] (b) Calculate its relative molecular mass. [2] (c) Calculate the mass of water needed to hydrolyse 9738 g of the fragment completely. [1]

11. Starch and cellulose are both polymers of glucose, yet a plant stores energy as one and builds its walls from the other. Explain how differences in structure make each suited to its role. [6]

12. A student wants to find out whether a plant extract contains sucrose, with no other sugar present. Describe a method, including the results that would show sucrose is present. [7]

Answers

1. (a) A molecule made from a large number of monomers joined together [1]. (b) Any three of: monosaccharide (for example glucose) [1], amino acid [1], nucleotide [1] [4] Examiner insight: Naming “sugars” or “proteins” does not answer part (b); give monomers, not polymers.

2. (a) Maltose: glucose and glucose [1]. Sucrose: glucose and fructose [1]. Lactose: glucose and galactose [1]. (b) Sucrose [1] [4] Examiner insight: A common slip is to list all disaccharides as non-reducing; maltose and lactose give a direct positive.

3. (a) A condensation reaction [1] in which an OH group on each molecule reacts, and one molecule of water is released [1]. The two glucose molecules are joined through an oxygen atom by a glycosidic bond [1] [3] (b) Moles = 51.3 ÷ 342 = 0.150 mol [1]. One water per bond: 0.150 × 18 = 2.7 g [1]. Sugars formed: 51.3 + 2.7 = 54.0 g [1] [3] Examiner insight: Use conservation of mass for the second figure (sugars formed equal lactose plus the water used), and show each step so your reasoning is visible.

4. (a) The OH group on carbon 1 [1] is above the ring in β-glucose but below the ring in α-glucose [1]. (b) Cellulose [1] [3] Examiner insight: Naming carbon 1 as well as the direction is what makes this answer precise.

5. Cellulose is made of β-glucose joined with alternate units rotated 180° [1], giving straight, unbranched chains [1]. Hydrogen bonds form between neighbouring chains [1] so they bundle into microfibrils [1]. Many hydrogen bonds give high tensile strength, so the wall resists the pressure from water entering by osmosis and does not burst [1] [5] Examiner insight: Strong answers state that the bonds between chains are hydrogen bonds; writing “glycosidic bonds between chains” is a different, incorrect idea.

6. (a) Both are made of α-glucose [1]. Both are branched, or both are compact and insoluble [1] (any two correct similarities). (b) Glycogen is more highly branched [1] and has shorter chains [1]. (c) More branches give more free ends [1] so enzymes can hydrolyse glucose off quickly for respiration during activity [1] [6] Examiner insight: “More branched” scores better when you finish the chain of reasoning: more ends, faster hydrolysis, faster release of glucose.

7. (a) P is maltose [1]. Q is starch [1]. R is sucrose [1]. S is distilled water [1]. (b) Acid hydrolysed the glycosidic bond in sucrose [1], releasing glucose and fructose, which are reducing sugars [1]. (c) To show there was no reducing sugar before hydrolysis, so the later result is due to a non-reducing sugar [1] [7] Examiner insight: Linking the later colour to the products of hydrolysis, not just to “acid”, is the reasoning that earns credit in part (b).

8. (a) Volume of stock = 6 ÷ 8 × 20 = 15 cm³ [1] and water = 5 cm³ [1]. (b) The value lies between the 2 and 4 standards [1]: 2 + (0.72 − 0.62) ÷ (0.72 − 0.55) × 2 = 3.2 g dm⁻³ [1]. (c) More glucose reduces more copper(II) ions in Benedict’s solution [1], so less blue copper(II) remains and the solution absorbs less [1] [6] Examiner insight: State units with your estimate, and show the reading on the curve or the interpolation, not only the number.

9. (a) 2.7 ÷ 9.0 = 0.30 [1]. (b) The spots match the R_f values of galactose (0.30) and glucose (0.40) [1], and there is no spot at the fructose position (0.60) [1]. (c) Lactose [1], because it is the disaccharide made from glucose and galactose [1] [5] Examiner insight: R_f has no units, and the distance is measured from the origin line, not from the bottom of the paper.

10. (a) 60 − 1 = 59 [1]. (b) 60 × 180 = 10 800 and 59 × 18 = 1062 [1]; 10 800 − 1062 = 9738 [1]. (c) Complete hydrolysis uses 59 × 18 = 1062 g of water per 9738 g [1] [4] Examiner insight: A branch point is just another glycosidic bond, so branching does not change the n − 1 rule.

11. Starch is made of α-glucose [1] and is helical or branched, so it is compact [1] and insoluble, so it leaves the water potential of the cell unchanged [1]. Its branched ends can be hydrolysed to release glucose [1]. Cellulose is made of β-glucose [1] in straight chains joined by hydrogen bonds into strong microfibrils, giving tensile strength for a cell wall [1] [6] Examiner insight: Pair each structural feature with its consequence; a list of features alone seldom answers an “explain” question.

12. Heat one portion of the extract with Benedict’s solution [1]; the mixture stays blue, which rules out a reducing sugar [1]. Heat a second portion with dilute hydrochloric acid [1], then neutralise it, for example with sodium hydrogencarbonate [1]. Now heat it with Benedict’s solution again [1]. A brick-red precipitate appears [1], which means hydrolysis released reducing sugars from a non-reducing sugar [1] [7] Examiner insight: Mention a negative control of water to show the reagents are not responsible, and give start and end colours for each stage.

Where marks are usually lost

• Giving “sugars” or “proteins” where monomers are asked for.
• Saying hydrolysis releases water.
• Writing that sucrose, maltose or lactose are all non-reducing.
• Not naming carbon 1 when comparing α- and β-glucose.
• Writing that glycosidic bonds hold cellulose chains together.
• Leaving out neutralisation in the non-reducing sugar test.
• Forgetting the n − 1 rule when counting bonds or water molecules.
• Giving R_f with units or measuring from the wrong baseline.
• Stating features of glycogen or starch without linking them to function.

Next steps

• Return to the study guide wherever an answer fell short.
• Recall with the revision notes.
• Find gaps with the free 10-minute diagnostics.
• Check progress against the printable checklist and the course hub.
• Book a free trial class.

Official syllabus

AQA, AS and A-level Biology specification (7401 and 7402), version 1.5, for exams from June 2016 onwards. Sections 3.1.1 and 3.1.2.