AQA AS Level Biology Paper 1, June 2025: Question 1

6 marks · Medium difficulty · Short Answer

Identify amino acid functional groups, bond type, dipeptide components, and calculate the number of amino acids in an antibody heavy chain using a given ratio.

Practise this question

Question

Question 01 consists of four parts: 01.1 shows Figure 1, the chemical structure of alanine with NH2, COOH, H, and CH3 attached to a central carbon, asking to name the NH2 and COOH groups. 01.2 shows Figure 2, a dipeptide formed from alanine and a second amino acid with R group CH2OH, asking for the bond type and molecule type. 01.3 provides Table 1 listing R groups of six amino acids (Alanine, Aspartic acid, Glutamine, Glycine, Serine, Valine) to identify the second amino acid. 01.4 shows a schematic antibody Y-shape (Figure 3) labeling a heavy chain, stating total amino acids is 1322 and heavy to light chain ratio is 2.13:1, asking to calculate amino acids in one heavy chain rounded to the nearest whole number.
Question text

01.1 Figure 1 shows the structure of the amino acid alanine.

Figure 1

Name the NH2 and COOH groups in the amino acid.

[1 mark]

NH2

COOH

Figure 2 shows the molecule formed from alanine and another amino acid.

Figure 2

01.2 Name the type of bond and the type of molecule formed when the two amino acids in

Figure 2 are joined.

[2 marks]

Type of bond

Type of molecule 3

01.3 Table 1 shows the R groups of six different amino acids.

Table 1

Amino acid R group Amino acid R group

Alanine CH3 Glycine H

Aspartic acid CH2COOH Serine CH2OH

Glutamine (CH2)2CONH2 Valine CH(CH3)2

Use the information in Table 1 to identify the amino acid joined to alanine in

*02Figure 2*.

[1 mark]

01.4 Figure 3 represents the structure of an antibody.

Figure 3

The total number of amino acids in a particular antibody molecule is 1322

The ratio of amino acids in the heavy chains to the amino acids in the light chains is

2.13 : 1

Calculate how many amino acids there are in one heavy chain.

Give your answer to the nearest whole number.

[2 marks]

amino acids

Mark scheme

Show the mark scheme Mark scheme for Question 01: 01.1 awards 1 mark for NH2 = amine (accept amino) and COOH = carboxyl (accept carboxylic acid/group). 01.2 awards 2 marks for 1. peptide and 2. dipeptide. 01.3 awards 1 mark for Serine. 01.4 awards 2 marks for the correct answer 450, or 1 mark for intermediate calculation of 449.8 (not rounded) or 900 (calculated for two heavy chains).

Question Marking Guidance Mark Comments

NH2 = amine Accept ‘amino’

and 1

01.1 (1 x

COOH = carboxyl; AO1) Accept ‘carboxylic

acid/group’

Both required for 1 mark.

1. (Type of bond) peptide; 2 Answers must be on

01.2 (2 x the correct lines

2. (Type of molecule) dipeptide; AO1)

Serine; 1

01.3 (1 x

AO2)

Correct answer of 450 = 2 marks;; Reject answer of

899.6 (two errors)

449.8 = 1 mark (correct calculation for one heavy

chain but not rounded to nearest whole number) 2

01.4 (2 x

OR AO2)

900 = 1 mark (incorrectly calculated for two heavy

chains but rounded to nearest whole number);

How to answer it

Amino Acids, Peptide Formation & Antibody Structure

What this question tests

This question evaluates core A-Level foundation knowledge on protein biochemistry and quaternary protein structure:

  • Identification of functional groups in a generalised amino acid monomer (amine and carboxyl groups).
  • Naming the specific covalent bond and resultant condensation product formed between two amino acids.
  • Pattern matching and identifying variable R-groups from structural formulae.
  • Applying mathematical ratio calculations to quaternary protein structures (specifically, an antibody consisting of 2 heavy and 2 light polypeptide chains).
Question 01.1 • 1 Mark (AO1)

Functional Groups of an Amino Acid

Identifying the NH₂ and COOH functional groups in alanine

✅ Correct Answer

  • NH₂: amine (accept 'amino')
  • COOH: carboxyl (accept 'carboxylic acid' or 'carboxylic group')
Both functional groups must be identified correctly to earn 1 mark.

💡 Key Knowledge

Every amino acid shares an identical backbone attached to a central alpha-carbon (C):

  • A basic amine group (-NH₂)
  • An acidic carboxyl group (-COOH)
  • A single hydrogen atom (-H)
  • A variable side-chain: the R group (which in alanine is a methyl group, -CH₃)

🧠 Exam Technique

Spell biochemical functional groups accurately. Ensure you do not confuse the organic chemistry functional group name with related terms like "ammonia" or "amide".

❌ Common Errors

  • Writing ammonia or ammonium instead of amine.
  • Writing carbon, carbonyl, or hydroxide instead of carboxyl.
  • Getting only one correct: no partial credit (0.5 marks) exists here.
Question 01.2 • 2 Marks (AO1)

Condensation Product & Bond Formation

Naming the bond and the molecule formed between two amino acids

✅ Correct Answer

  • Type of bond: peptide [1 mark]
  • Type of molecule: dipeptide [1 mark]

💡 Key Knowledge

A condensation reaction between the carboxyl group of one amino acid and the amine group of another results in the loss of a water molecule (H₂O) and the formation of a peptide bond ( -CO-NH- ). Joining two amino acids produces a dipeptide.

🧠 Exam Technique

Pay attention to the prompt labels: answers must be on the designated answer lines. Do not write "peptide" under molecule and "dipeptide" under bond, or both marks are forfeited.

❌ Common Errors

  • Writing polypeptide or protein instead of dipeptide (the question specifies that only two amino acids are joined).
  • Writing hydrogen bond or glycosidic bond for the bond type.
Question 01.3 • 1 Mark (AO2)

Identifying Amino Acids from R Groups

Using structural comparison with Table 1

✅ Correct Answer

Serine [1 mark]

💡 Key Knowledge

Figure 2 shows the dipeptide with the backbone:
H₂N-CH(CH₃)-CO-NH-CH(CH₂OH)-COOH

  • First amino acid alpha-carbon carries -CH₃ (Alanine).
  • Second amino acid alpha-carbon carries -CH₂OH .
  • Matching -CH₂OH against Table 1 gives Serine.

🧠 Exam Technique

Locate the peptide link ( -CO-NH- ) first. Look immediately to the right at the adjacent alpha-carbon (C) and isolate the group branching downwards.

❌ Common Errors

  • Misreading -CH₂OH as -CH₂COOH (Aspartic acid).
  • Writing the formula of the R-group instead of the name of the amino acid requested by the prompt.
Question 01.4 • 2 Marks (AO2)

Quantitative Quaternary Structure: Antibody Ratio Calculation

Calculating the number of amino acids in ONE heavy chain

✅ Correct Answer

450 [2 marks]

Compensatory partial credit (1 mark):
  • 449.8 (correct calculation for one heavy chain, but forgot to round to nearest whole number).
  • 900 (correctly rounded, but calculated for both heavy chains combined, failing to divide by 2).

📐 Step-by-Step Calculation

1 Find total ratio parts:
Ratio of (all heavy chains) : (all light chains) = 2.13 : 1
Total parts = 2.13 + 1 = 3.13

2 Calculate amino acids in BOTH heavy chains:
Amino acids in heavy chains = (2.13 / 3.13) × 1322 = 899.642...

3 Divide by 2 for ONE heavy chain:
An antibody contains 2 heavy chains (shown clearly in Figure 3).
Amino acids in one heavy chain = 899.642... ÷ 2 = 449.821...

4 Round to nearest whole number:
449.821... rounds to 450.

❌ Common Errors & Examiner Traps

  • Stopping at 900: Forgetting that an antibody has two heavy chains. The question asks for one heavy chain.
  • Leaving answer as 449.8: Overlooking the instruction: "Give your answer to the nearest whole number."
  • Answer of 899.6: Fails to divide by 2 AND fails to round (Mark scheme explicitly notes: "Reject answer of 899.6 (two errors) = 0 marks").
  • Incorrect ratio division: Dividing 1322 by 2.13 instead of using part-to-whole (dividing by 3.13).

🧠 Exam Technique: Checking Biological Sense

Always look at the structural diagram provided. Figure 3 illustrates an antibody's characteristic Y-shape consisting of 4 polypeptide chains: 2 identical heavy chains in the centre and 2 shorter light chains on the outside. Always underline words like "one", "both", and rounding requirements.

Topics

Biology · 3.1 Biological molecules · 3.2 Cells

Question and mark scheme from the AQA AS Level Biology examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.