AQA AS Level Biology Paper 1, November 2020: Question 1

8 marks · Medium difficulty · Short Answer

Identify biological molecules from given structural diagrams, calculate the molecular formula of a trisaccharide, and describe the biochemical test for non-reducing sugars.

Practise this question

Question

Figure 1 shows four chemical structures labeled A, B, C, and D. A and D are sugar ring structures, B is an amino acid structure with an R group, and C is an ATP molecule showing adenine, ribose, and three phosphate groups. Below Figure 1, three questions test knowledge of these molecules, including completing Table 1 matching letters to statements, calculating the chemical formula of raffinose, and describing a biochemical test for a non-reducing sugar.
Question text

01 Figure 1 shows the structure of molecules found in organisms.

Figure 1

01.1 Complete Table 1 by putting the correct letter, A, B, C or D, in the box next to each

statement. Each letter may be used once, more than once, or not at all.

[4 marks]

Table 1

Letter Statement

is a monomer in an enzyme’s

active site

is a monomer in cellulose

is produced during

photosynthesis and respiration

forms a polymer that gives a

positive result with a biuret test3

01.2 Raffinose is a trisaccharide of three monosaccharides: galactose, glucose and

fructose. The chemical formulae of these monosaccharides are:

• galactose = C6H12O6

• glucose = C6H12O6

• fructose = C6H12O6

Give the number of carbon atoms, hydrogen atoms and oxygen atoms in a molecule

of raffinose.

*02* [1 mark]

Number of carbon atoms

Number of hydrogen atoms

Number of oxygen atoms

01.3 A biochemical test for reducing sugar produces a negative result with raffinose

solution.

Describe a biochemical test to show that raffinose solution contains a non-reducing

sugar.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme shows answers for questions 01.1, 01.2, and 01.3. Question 01.1 requires the letter sequence B, D, C, B for 4 marks. Question 01.2 requires C = 18, H = 32, O = 16 for 1 mark. Question 01.3 requires 3 marks for heating with acid and neutralising, heating with Benedict's solution, and observing a red precipitate or colour.

Question Marking Guidance Mark Comments

01.1 4

Letter Statement

is a monomer in an enzyme’s

B;

active site

D;

is a monomer in cellulose Must be in correct

order

C; is produced during

photosynthesis and respiration

B; forms a polymer that gives a

positive result with a biuret test

01.2 Accept only these

C = 18, H = 32, O = 16; 1

answers

Accept boil/water bath

01.3 1. Heat with acid and neutralise; 3

for heat

2. Heat with Benedict's (solution);

3. Red precipitate/colour; Accept named alkali

for neutralise

1. Accept named

examples, eg HCl,

NaHCO3

3. Accept other

colours eg orange/

brown/green

TOTAL 8

How to answer it

Biological Molecules Structure & Testing Study Guide

What this question tests

This question assesses your ability to recognize biochemical monomers and polymers (amino acids, alpha- and beta-glucose, nucleotides), calculate molecular formulae following condensation polymerization principles, and describe core practical biochemical tests for reducing and non-reducing sugars.

Question 01.1 [4 marks]

Identifying Biological Monomers from Structures

✅ Correct Answers

  • is a monomer in an enzyme's active site: B
  • is a monomer in cellulose: D
  • is produced during photosynthesis and respiration: C
  • forms a polymer that gives a positive result with a biuret test: B

💡 Key Knowledge

  • A & D: Monosaccharides (hexoses). Note the position of the -OH group on carbon-1: A is alpha-glucose, D is beta-glucose.
  • B: An amino acid (general structure with central carbon, amine group, carboxyl group, hydrogen, and variable R group).
  • C: ATP (Adenine + ribose sugar + 3 phosphate groups).

🧠 Exam Technique

Read each statement carefully and link it back to core topics: enzymes are proteins made of amino acids (B); cellulose is made of beta-glucose (D); ATP (C) is synthesised in respiration and photosynthesis; proteins test positive with Biuret reagent.

❌ Common Errors

Mixing up alpha-glucose (A) and beta-glucose (D) isomers. Remember: "B"eta points Both ways or look at the -OH group pointing upwards on carbon-1 for beta-glucose.

Mark breakdown: 1 mark per correct table entry, provided letters are in the exact specified order.
Question 01.2 [1 mark]

Calculating Molecular Formulae of a Trisaccharide

📐 Step-by-Step Calculation

  1. Identify monomer formulae: Three monosaccharides combine: galactose ( C₆H₁₂O₆ ), glucose ( C₆H₁₂O₆ ), and fructose ( C₆H₁₂O₆ ).
  2. Sum individual atoms before bonding:
    Carbon = 6 × 3 = 18
    Hydrogen = 12 × 3 = 36
    Oxygen = 6 × 3 = 18
  3. Account for condensation reactions: Forming a trisaccharide from three monosaccharides requires 2 glycosidic bonds. Each condensation bond releases one water molecule ( H₂O ).
    Water lost: 2 × H₂O = 4 hydrogens, 2 oxygens lost.
  4. Calculate final formula:
    Carbon = 18
    Hydrogen = 36 - 4 = 32
    Oxygen = 18 - 2 = 16

✅ Final Answers

Number of carbon atoms: 18
Number of hydrogen atoms: 32
Number of oxygen atoms: 16

❌ Common Errors

Forgetting to subtract water molecules. Students often multiply the monosaccharide formula by 3 directly without accounting for the water molecules lost during condensation synthesis.

Mark breakdown: 1 mark for all three correct values ( C = 18, H = 32, O = 16 ). Strict marking; no partial credit.
Question 01.3 [3 marks]

Biochemical Test for Non-Reducing Sugars

✅ Correct Answer / Marking Points

  1. Hydrolysis: Add dilute acid (e.g., HCl) and heat (e.g., using a water bath), then neutralise with an alkali (e.g., sodium hydrogen carbonate / NaHCO₃ ).
  2. Benedict's Test: Add Benedict's solution and heat again (boiling water bath).
  3. Observation: Observe a colour change to a red precipitate (or orange/brown/green).

💡 Key Knowledge

Non-reducing sugars (like sucrose or raffinose) do not possess a free aldehyde or ketone group capable of reducing copper(II) ions directly. Hydrolysing the glycosidic bonds using acid breaks down the oligosaccharide into its constituent monosaccharides, exposing reducing groups.

🧠 Exam Technique

When describing biochemical protocols, write sequentially as a clear set of steps. Always name the reagents used (e.g., "hydrochloric acid" and "Benedict's solution") and explicitly state the required condition ("heat" or "boil") along with the final colour change.

❌ Common Errors

Forgetting the neutralisation step after adding acid; Benedict's reagent requires alkaline conditions to function properly and will not work in an acidic solution. Another error is omitting the initial control test or failing to mention heating.

Mark breakdown: 3 marks awarded for (1) acid hydrolysis + neutralisation, (2) adding Benedict's and heating, and (3) stating the final red/orange/brown/green precipitate observation.

Topics

Biology · 3.1 Biological molecules

Question and mark scheme from the AQA AS Level Biology examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.