AQA AS Level Biology Paper 2, June 2025: Question 1

8 marks · Medium difficulty · Short Answer

Identify blood vessels in mammalian circulation, explain differences in haemoglobin tertiary structure, and explain mountain goat adaptation using an oxyhaemoglobin dissociation curve.

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Question

Question text

01 Figure 1 shows the arrangement of some of the blood vessels in a mammalian

circulatory system.

Figure 1

01.1 Name the blood vessels labelled B and E in Figure 1.

[2 marks]

01.2 Give the letters of all the blood vessels shown in Figure 1 that transport

deoxygenated blood.

[1 mark]

01.3 Many animals are adapted to their environments by having different types of

haemoglobin with different tertiary structures.

Using your knowledge of the structure of proteins, explain how different types of

haemoglobin can have different tertiary structures.

[3 marks]

Figure 2 shows the oxyhaemoglobin dissociation curve for two species of goat.

Figure 2

01.4 The mountain goat lives at high altitudes where the pO2 is lower than where the

domestic goat lives.

Use Figure 2 to explain how the mountain goat is adapted to living at high altitudes.

[2 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

B – Pulmonary vein; 2

01.1 (2 x

E – Renal artery; AO1)

A, C, F; All three needed for

1 one mark

01.2 (1 x Accept in any order

AO2) Reject if there are

additional letter(s)

1. (Polypeptides have) different sequences of amino 1. Ignore different

acids/different primary structures; amino acids

2. (So different) secondary structure determined by 2. Reject

01.3 (3 x

(position of) hydrogen bonds; ionic/disulfide bonds

AO1)

3. Different hydrogen/ionic/disulfide bonds (in 2 and 3. Reject

tertiary structure); peptide bonds

1. Accept low(er) pO2

1. Loads/associates with oxygen more readily at

for lower partial

low(er) partial pressures

pressures

1. Accept low(er) KPa

01.4 (2 x

Has a higher affinity for oxygen at lower partial

AO2)

pressures;

2. (Supplies tissues with sufficient) oxygen for

respiration;

How to answer it

Mammalian Circulation, Protein Structure & Haemoglobin Adaptations

WHAT THIS QUESTION TESTS

This question examines core concepts across Unit 3 (Organisms exchange substances with their environment) and Unit 1 (Biological molecules):

Gross circulation anatomy: Naming systemic and pulmonary vessels associated with major organs (heart, lungs, kidneys, liver, gut).
Blood oxygenation states: Tracing the route of deoxygenated blood through the double circulatory system.
Hierarchical protein structure: Linking amino acid sequences (primary structure) to folding patterns (secondary structure) and tertiary bonding.
Oxygen dissociation curves: Explaining environmental adaptations (high altitude) via haemoglobin affinity and aerobic cellular respiration requirements.

QUESTION 01.1 • 2 MARKS

Identifying Blood Vessels B and E

Anatomy of the Mammalian Circulatory System

✅ Correct Answers

B: Pulmonary vein
E: Renal artery

Mark Scheme: 1 mark for each vessel correctly identified (2 × AO1).

💡 Key Knowledge

Direction rule: Arteries carry blood Away from the heart; Veins carry blood towards the heart.
Organ prefixes:
- Lungs = Pulmonary
- Kidneys = Renal
- Liver = Hepatic
Vessel B carries blood directly from the lungs back to the left atrium of the heart → Pulmonary vein.
Vessel E branches from the aorta (D) towards the kidneys → Renal artery.

❌ Common Errors

Swapping artery and vein (e.g., calling B the pulmonary artery).
Mixing up organ terms (e.g., calling E the hepatic artery or mesenteric artery).
Incomplete names such as just "Renal" or "Vein" (neither receives credit).

QUESTION 01.2 • 1 MARK

Blood Vessels Transporting Deoxygenated Blood

Pathways of the Double Circulatory System

✅ Correct Answer

A, C, F

Mark Scheme: All 3 letters required for 1 mark (1 × AO2). Accept in any order. Reject if any extra letters are included.

🧠 Exam Technique: Systematic Tracing

A (Pulmonary artery): Carries deoxygenated blood pumped from the right ventricle up to the lungs.
C (Vena cava): Collects systemic deoxygenated blood from the body and returns it to the right atrium.
F (Hepatic vein): Carries deoxygenated blood away from the liver back towards the vena cava.
Why D and B are wrong: D is the aorta (oxygenated, going to body organs) and B is the pulmonary vein (freshly oxygenated blood from the lungs).

QUESTION 01.3 • 3 MARKS

Explaining Differences in Haemoglobin Tertiary Structures

Protein Structure: Primary, Secondary, and Tertiary Levels

✅ Marking Points Breakdown

Different primary structures / different sequences of amino acids;
Different secondary structure determined by (position of) hydrogen bonds;
Different hydrogen, ionic, and disulfide bonds forming the tertiary structure;

Mark Allocation: 3 marks (3 × AO1). Each step must logically link how one level of protein folding determines the next.

🧠 How Top Candidates Formulate the Answer

Structure your response chronologically through the protein folding levels:

Step 1 (Primary): Haemoglobin variants have a different sequence of amino acids in their polypeptide chains.
Step 2 (Secondary): This alters the locations where hydrogen bonds form between NH and C=O groups, creating different secondary structures (α-helices / β-pleated sheets).
Step 3 (Tertiary): Consequently, hydrogen, ionic, and disulfide bonds form in different locations between variable R-groups, creating a uniquely folded 3D shape.

❌ Crucial Mark Scheme Penalties

Do NOT just say "different amino acids": The mark scheme explicitly states: "Ignore different amino acids". You must specify the sequence of amino acids or state primary structure.
No ionic or disulfide bonds in secondary structure: Secondary folding is stabilized only by hydrogen bonds. Stating ionic/disulfide bonds for MP2 is explicitly rejected!
Never mention peptide bonds in tertiary structure: Peptide bonds link adjacent amino acids in the primary chain. They are rejected if cited as holding the tertiary structure together.

QUESTION 01.4 • 2 MARKS

Adaptation of the Mountain Goat to High Altitudes

Interpreting the Oxyhaemoglobin Dissociation Curve

Graph Analysis (Figure 2): The dissociation curve for the mountain goat is shifted distinctly to the left of the domestic goat. At any given low partial pressure of oxygen (pO₂), the mountain goat's haemoglobin has a higher percentage saturation.

✅ Model Answer

Mark 1: Mountain goat haemoglobin has a higher affinity for oxygen / loads (associates with) oxygen more readily at lower partial pressures (pO₂);
Mark 2: This provides/supplies tissues with sufficient oxygen for (aerobic) respiration;

Mark Scheme: 2 marks (2 × AO2). Accept low/lower pO₂ or kPa.

💡 The "Left-Shift" Principle

Shift to the Left:
Higher affinity → loads O₂ easily at low environmental pO₂ (e.g., high-altitude animals, human foetus, lugworms).
Shift to the Right (Bohr effect):
Lower affinity → unloads O₂ more readily to respiring tissues with high metabolic rates (e.g., small mammals, exercising muscles).
Complete the story: Never stop at "absorbs more oxygen". Always explain the biological outcome: maintains ATP synthesis / aerobic cellular respiration.

❌ Common Errors

Vague terms like "takes in oxygen better" instead of scientific terms like associates / loads / higher affinity.
Forgetting to specify "at lower partial pressures / lower pO₂".
Saying "oxygen is needed for energy" instead of respiration (energy cannot be created; respiration produces ATP).

Topics

Biology · 3.1 Biological molecules · 3.3 Organisms exchange substances with their environment

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.