AQA AS Level Biology Paper 1, November 2020: Question 3
7 marks · Medium difficulty · Short Answer
Describe phagocytosis, identify cells that stimulate an immune response, label an antibody diagram where the antigen-binding site forms, and state the role of disulfide bridges in quaternary antibody structure.
Practise this questionQuestion
Question text
03.1 Describe how a phagocyte destroys a pathogen present in the blood.
[3 marks]
03.2 Give two types of cell, other than pathogens, that can stimulate an immune response.
[2 marks]
03.3 Figure 2 shows the structure of an antibody.
Figure 2
Label Figure 2 with an X to show where an antigen-antibody complex forms.
[1 mark]
03.4 A disulfide bridge is labelled in Figure 2.
What is the role of the disulfide bridge in forming the quaternary structure of an
antibody?
[1 mark]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
1. Accept endocytosis
03.1 1. Engulfs; 3
2. Forming vesicle/phagosome and fuses with OR
lysosome;
3. Enzymes digest/hydrolyse; Description
1. Ignore ‘taken in’
3. Accept lysozymes
for ‘enzymes’
03.2 1. (Cells from) other organisms/transplants; 2 max Accept ‘own cells’ if
autoimmune response
2. Abnormal/cancer/tumour (cells);
suggested
3. (Cells) infected by virus; Accept APCs
Accept non-self
03.3 ‘X’ written at either or both ends of Y shape; 1
03.4 Joins two (different) polypeptides; 1 Accept holds/attaches
Accept ‘prevents
polypeptide chains
separating’
TOTAL 7
How to answer it
Phagocytosis and Antibody Structure Study Guide
What this question tests
This question assesses your core knowledge of the non-specific immune response (phagocytosis), cellular recognition triggers (non-self vs self markers), and protein structure (quaternary structure and disulfide bridges in antibodies). Success requires precise biological terminology, particularly when describing intracellular vesicle pathways and protein folding.
Question 03.1: Phagocytosis
Describe how a phagocyte destroys a pathogen present in the blood. [3 marks]
✅ Correct Answer / Mark Scheme
- Mark 1: Phagocyte engulfs the pathogen (accepts endocytosis).
- Mark 2: Forms a vesicle/phagosome which then fuses with a lysosome.
- Mark 3: Enzymes (or lysozymes) digest and hydrolyse the pathogen.
💡 Key Knowledge
Phagocytes (such as macrophages and neutrophils) circulate in the blood and detect non-self antigens. The process relies on membrane fluidity to engulf large particles and intracellular trafficking to combine digestive enzymes with the trapped pathogen.
🧠 Exam Technique
This is a sequenced process question. Make sure you state the steps in chronological order: engulfment → vesicle/phagosome formation → lysosome fusion → enzymatic hydrolysis. Missing out the intermediary vesicle step is a very common way to drop a mark.
❌ Common Errors
- Saying the pathogen is "taken in" instead of explicitly using engulfs or endocytosis .
- Forgetting to mention lysosomes or confusing them with general cell organelles.
- Stating that enzymes "kill" the pathogen rather than digest or hydrolyse it.
Question 03.2: Immune Response Triggers
Give two types of cell, other than pathogens, that can stimulate an immune response. [2 marks]
✅ Correct Answer (Any two of):
- Cells from other organisms / transplanted cells / tissues.
- Abnormal / cancer / tumour cells.
- Cells infected by a virus.
💡 Key Knowledge
The immune system responds to non-self or altered self markers (antigens) on cell surface membranes. This prevents colonisation by foreign tissue and eliminates mutated or virally hijacked host cells.
🧠 Exam Technique
Read the stem carefully: it explicitly says other than pathogens. Listing bacteria, fungi, or viruses will score zero. Be specific and concise with cell categories.
❌ Common Errors
- Naming microorganisms or pathogens (e.g., bacteria, viruses) despite the negative constraint in the question.
- Vague answers like "bad cells" instead of scientifically accepted terms like tumor/cancer cells or cells infected by a virus .
Question 03.3: Antibody-Antigen Complex Location
Label Figure 2 with an X to show where an antigen-antibody complex forms. [1 mark]
✅ Correct Answer
X written clearly at either or both of the tips (ends) of the variable regions of the Y-shaped antibody structure.
🧠 Exam Technique
Make sure your letter X is precisely placed on the tips of the upper arms of the Y shape. If your cross is placed too far down near the constant region or the hinge, examiners will penalise it.
Question 03.4: Role of Disulfide Bridges
What is the role of the disulfide bridge in forming the quaternary structure of an antibody? [1 mark]
✅ Correct Answer
It joins two (different) polypeptides (chains).
💡 Key Knowledge
Antibodies are composed of four polypeptide chains (two heavy chains and two light chains). Quaternary structure describes proteins made of more than one polypeptide chain. Strong covalent disulfide bonds hold these separate chains firmly together.
🧠 Exam Technique
Key examiners' note: Accept answers like "holds", "attaches", or "prevents polypeptide chains separating", but you must refer to polypeptides or polypeptide chains rather than just "amino acids" or "proteins".
❌ Common Errors
- Stating that disulfide bridges hold amino acids together (that is the role of peptide bonds in primary structure).
- Failing to mention multiple polypeptide chains when discussing quaternary structure.
Topics
Biology · 3.2 Cells
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.