AQA A-Level Biology Paper 3, June 2025: Question 4

7 marks · Medium difficulty · Short Answer

Draw the structure of an amino acid, identify correct statements about protein structure, calculate antibody concentration over time, and evaluate an investigation into age-related immune responses.

Practise this question

Question

Question 04 consists of four parts. 04.1 asks to draw the general structure of an amino acid. 04.2 is a multiple-choice question asking to tick one box for the correct statement describing protein structure. 04.3 provides data: a single B cell produces 4000 plasma cells, each producing 2000 molecules of antibody Q per second; blood volume is 5.4 dm³; calculate the number of antibody molecules per cm³ of blood after 24 hours. 04.4 describes an investigation where young and old mice were infected with influenza and the mean number of B cells per gram of lung tissue was measured over 19 days, shown on a line graph in Figure 3. Students are asked to suggest three reasons why further investigations are needed.
Question text

04.1 Draw the general structure of an amino acid.

[1 mark]

04.2 Which statement describing protein structure is correct?

Tick ( ) one box.

[1 mark]

Proteins with a quaternary structure have one or more polypeptide chains.

Disulfide bridges are present in all proteins with a tertiary structure.

The secondary structure of a protein is maintained by ionic and hydrogen

bonds.

The primary structure of a protein is the sequence of amino acids joined by

peptide bonds. 13

04.3 Antibodies are proteins produced and secreted by plasma cells.

Once activated, a single B cell divides and produces 4000 plasma cells.

Each one of these plasma cells produces 2000 molecules of antibody Q per second.

A human male has a blood volume of 5.4 dm3

Calculate the total number of molecules of antibody Q per cm3 of blood 24 hours after

the production of these plasma cells.

[2 marks]

Answer = 14 molecules of antibody Q per cm3 of blood

04.4 Scientists investigated the effect of age on the immune response in healthy

female mice.

The scientists used the following method.

1. Infected a large number of young and old mice with the same strain of

influenza virus.

2. Removed the lungs from 6 young and 6 old mice on the day of infection (day 0).

3. Repeated step 2 on 8 different days after infection.

4. Determined the mean number of B cells present per gram of lung tissue for

young and old mice.

Figure 3 shows the scientists’ results.

Figure 3

The scientists decided that further investigations would be required to conclusively

determine the effect of age on the immune response in healthy female mice.

Using all the information provided, suggest three reasons for the scientists’ decision.

Do not refer to statistical tests, repeats or sample size in your answer.

[3 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 04. 04.1 awards 1 mark for the general amino acid structure showing a central carbon bonded to H, R, NH2 (or H2N), and COOH. 04.2 awards 1 mark for 'The primary structure of a protein is the sequence of amino acids joined by peptide bonds.' 04.3 awards 2 marks for 128 000 000 / 1.28 × 10⁸ / 1.3 × 10⁸ / 1 × 10⁸, with 1 mark for working showing total produced or incorrect magnitude. 04.4 awards up to 3 marks for points including: only measuring B cells / humoral response (no T cells), only looking at lungs, only tested one virus, day 9 results do not fit pattern / anomalous, lack of data between days 11–15 / 15–19 or beyond 19 days, or age range not specified.

Question Marking Guidance Mark Comments

Accept other correct

1 representations

04.1 (1 x

AO1)

The primary structure of a protein is the sequence 1

04.2 of amino acids joined by peptide bonds. (1 x

AO1)

Correct answer of 128 000 000/1.28 × 108/ Accept any correct

1.3 × 108 /1 × 108 = 2 marks;; rounding

Answer shows 128 (ignore position of decimal

point and preceding/subsequent zeroes, incorrect

order of magnitude) = 1 mark

OR

Evidence of 691 200 000 000/6.912 × 1011/

6.9 × 1011 (total number produced in 24 hours in

5.4 dm3) = 1 mark

OR 2

04.3 (2 x

Evidence of 4000 × 2000 × 60 × 60 × 24 (total AO2)

number produced in 24 hours in 5.4 dm3) = 1 mark

OR

Evidence of 5 333 333.333/5.3 × 106/5 × 106

(total number produced in one hour per cm3 of

blood) = 1 mark

OR

Evidence of 32 000/3.2 × 104 (number produced

from one plasma cell per cm3 of blood) = 1 mark;

1. Only measuring the number of B cells

OR

Only provides data for the humoral response

OR

No data provided for the T cell/cellular

response;

2. Accept not looked

2. Only looking at (immune response in) the at other organs

lungs;

3. Accept need to

3. Only (measured response to) influenza (virus)

investigate other

viruses/pathogens

OR 3. Accept ‘flu’ for

‘influenza’

Only (measured response to) one virus;

3 max

04.4 (3 x

4. Day 9 results do not fit the pattern

AO3)

OR

Day 9 results are anomalous;

5. Cannot predict if there would be fluctuations 5. Accept long term

between days 11–15/15–19 response/effects

unknown

OR 5. Accept

investigation/

More data needed between days 11–15/15–19 19 days is too

short

OR

Do not know what happens (to the number of B

cells) after 19 days;

6. Do not know age (range) of young and old

(mice);

How to answer it

Amino Acids, Proteins & Immune Response Investigation

📋 What this question tests
  • Biochemistry: Drawing the general monomer structure of an amino acid and defining the four levels of protein structure.
  • Mathematical Skills: Multistep unit conversions (dm³ to cm³, hours to seconds) and standard form calculations in an immunological context.
  • Experimental Evaluation (AO3): Critiquing experimental design, recognising limiting factors, and evaluating biological conclusions beyond statistical controls.

Question 04.1

General Structure of an Amino Acid (1 Mark)

✅ Mark Scheme Requirement

A central alpha-carbon atom bonded to four distinct chemical groups:

    R
    |
H₂N — C — COOH
    |
    H

Must show: an amine group (–NH₂ / H₂N–), a carboxyl group (–COOH), a hydrogen atom (–H), and a variable side chain (–R).

1 Mark: All four groups correctly attached to the central carbon atom.

🧠 Exam Technique & Connectivity

  • Ensure chemical bonds are drawn clearly touching the correct atoms. For example, connect the bond to the nitrogen in H₂N– or draw it out fully as H–N–H .
  • Do not lose marks by writing the carboxyl group with incorrect valency (e.g. bonding to oxygen improperly if displaying all bonds). Writing condensed functional groups ( –NH₂ , –COOH ) is the safest approach.

Question 04.2

Identifying Correct Protein Structure Statements (1 Mark)

✅ Correct Answer

Box 4: "The primary structure of a protein is the sequence of amino acids joined by peptide bonds."

1 Mark: Fourth box ticked only.

❌ Why the Distractors are False

  • Option 1: Quaternary structure requires two or more polypeptide chains (not "one or more").
  • Option 2: Disulfide bridges only occur if cysteine amino acids with sulfur-containing R-groups are present; they are not in all tertiary proteins.
  • Option 3: Secondary structure (α-helices and β-pleated sheets) is held solely by hydrogen bonds between peptide backbone groups, not ionic bonds.

Question 04.3

Antibody Production Calculation (2 Marks)

📐 Step-by-Step Calculation

  1. Convert total time to seconds:
    24 hours = 24 × 60 minutes = 1440 minutes
    1440 × 60 seconds = 86,400 seconds
  2. Calculate total antibodies produced in 24 hours:
    Total molecules = 4000 plasma cells × 2000 molecules/s × 86,400 s
    = 6.912 × 10¹¹ molecules (or 691,200,000,000)
  3. Convert male blood volume from dm³ to cm³:
    1 dm³ = 1000 cm³
    5.4 dm³ = 5.4 × 1000 = 5400 cm³
  4. Calculate antibody concentration per cm³:
    Molecules per cm³ = 6.912 × 10¹¹ ÷ 5400
    = 128,000,000 or 1.28 × 10⁸ molecules cm⁻³
2 Marks: Correct answer of 128 000 000 or 1.28 × 10⁸ (accepts 1.3 × 10⁸ or 1 × 10⁸).
1 Mark: Calculation showing digits "128" (incorrect power of ten) OR evidence of total molecules produced (6.912 × 10¹¹) OR molecules per cell per cm³ (32,000).

❌ Common Calculation Traps

  • Volume conversion error: Dividing by 5.4 instead of 5400 (forgetting to convert dm³ into cm³).
  • Time conversion error: Multiplying by 24 (hours) instead of converting hours to seconds (× 3600).

💡 Examiner Tip

Always show your intermediate figures! Even if your final rounding or order of magnitude is wrong, intermediate working such as 4000 × 2000 × 60 × 60 × 24 automatically secures 1 method mark.

Question 04.4

Evaluating the Mouse Immune Response Study (3 Marks)

🧠 Pay Attention to Question Constraints

The prompt explicitly instructs: "Do not refer to statistical tests, repeats or sample size in your answer." Any answers mentioning standard deviation, sample size of 6, or repeating the experiment will score 0 marks.

✅ Acceptable Points (Any 3 from below)

  • Limited branch of immune system: Only measured B cells / only provides data for the humoral response (no data on T cells / cellular immune response).
  • Organ specificity: Only investigated lung tissue; immune response in other organs/blood was not evaluated.
  • Pathogen specificity: Only used influenza virus / one strain; response might differ for other viruses, bacteria, or pathogens.
  • Anomalous trend: The Day 9 result is anomalous / does not fit the overall upward trend (young mice spike while old mice dip).
  • Missing data points/duration: Large gap without data between days 11–15 and 15–19 (unknown fluctuations); or the study ended at day 19, so long-term response is unknown.
  • Age undefined: The exact ages/age ranges of "young" and "old" mice are not defined.
3 Marks: Any 3 valid suggestions (1 mark per distinct point, max 3).

💡 Why Top Students Scored Full Marks

  • Biological breadth: They linked B cells specifically to humoral immunity and pointed out the absence of cellular immunity (T lymphocytes).
  • Data analysis: They inspected the graph closely to spot specific issues: the strange crossover at Day 9 and the large 4-day gaps later in the timeline where no measurements were taken.

Topics

Biology · Practical skills · 3.1 Biological molecules · 3.2 Cells · Data analysis · Uncertainty and evaluation

Question and mark scheme from the AQA A-Level Biology examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.