OCR A Level Biology A: Pathogens, transmission and the first lines of defence (H420) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover OCR A Level Biology A (H420) statements 4.1.1(a) to 4.1.1(e), from the OCR specification Version 4.1 (April 2026), for first assessment in 2025. Read the study guide first, or use the revision notes for a quick recap. Hub: OCR A Level Biology. Checklist: OCR A Level Biology checklist.

Questions

1. State the type of pathogen that causes each of the following diseases.

(a) Tuberculosis. [1] (b) Black sigatoka of bananas. [1] (c) Malaria. [1] (d) Tobacco mosaic disease. [1]

2. Malaria is common in warm, wet regions. Explain how the pathogen is transmitted and why the climate affects how often this happens. [4]

3. A potato leaf infected with late blight has a lesion 6 mm across on day 0 and 21 mm across on day 5.

(a) Calculate the mean rate of increase in diameter, in mm per day. [2] (b) Calculate the percentage increase in diameter over the five days. [2] (c) Suggest two reasons why late blight spreads faster through a crop in wet, windy weather. [2]

4. Plants respond to infection with chemicals and with changes to their cells.

(a) Explain how deposition of callose limits the spread of a pathogen. [3] (b) Describe how production of a named chemical helps a plant to defend itself. [2]

5. Describe how each of the following protects the body against pathogens.

(a) The mucous membranes lining the trachea. [3] (b) Expulsive reflexes. [2]

6. A person cuts a finger.

(a) Describe how a blood clot forms. [4] (b) Describe two ways in which the clot and the following wound repair help to prevent infection. [2]

7. Explain how inflammation helps to remove pathogens from damaged tissue. [4]

8. A photomicrograph of a blood smear has a scale bar 30 mm long labelled 10 µm. A neutrophil in the image is 33 mm across.

(a) Calculate the magnification of the photomicrograph. [2] (b) Calculate the actual diameter of the neutrophil, in µm. [2] (c) State two features that would allow you to identify the cell as a neutrophil. [2]

9. A student mixed macrophages with bacteria. In one dish the bacteria were coated with opsonins. In the other they were not. After 20 minutes the student examined 50 macrophages in each dish. In the dish with opsonins, 38 macrophages had engulfed bacteria. In the dish without opsonins, 17 had.

(a) Calculate the percentage of macrophages that had engulfed bacteria in each dish. [2] (b) Calculate the percentage increase in engulfing macrophages when opsonins were present. Give your answer to 3 significant figures. [2] (c) Explain how the results show the role of opsonins in phagocytosis. [3]

10. Describe and explain how a neutrophil and a macrophage deal with bacteria that have entered a wound, and how this links to the specific immune response. [8]

11. A technician examines a stained blood smear from a patient with a suspected bacterial infection. In 250 white cells counted, 160 were neutrophils.

(a) Calculate the percentage of the white cells that were neutrophils. [1] (b) The patient’s neutrophil count was 5.0 (×10⁹ per dm³) before the infection and 11.5 (×10⁹ per dm³) during it. Calculate the percentage increase. [2] (c) Suggest why the neutrophil count rises during a bacterial infection. [2] (d) The technician is asked to draw two cells from the smear for a record. State three rules for making a good biological drawing. [3]

12. Explain how living conditions or social factors increase the spread of the following.

(a) Tuberculosis in a town with overcrowded, poorly ventilated housing. [3] (b) Athlete’s foot among users of a public swimming pool. [3]

Answers

1. (a) A bacterium. [1] (b) A fungus. [1] (c) A protoctist. [1] (d) A virus. [1] [4] Examiner insight: A one-word answer is enough for a “state” question, but “microbe” or “germ” does not name a type, so it is unlikely to answer the question.

2. The pathogen is carried by a vector, the female mosquito. [1] The mosquito picks up the pathogen when it feeds on the blood of an infected person. [1] It passes the pathogen to a new person when it feeds again. [1] Warm, wet climates give standing water where mosquitoes breed, and warmth speeds mosquito development, so there are more vectors and more transmission. [1] [4] Examiner insight: Say that the mosquito carries the pathogen. A statement that the mosquito “causes” malaria describes the wrong role and is likely to lose the credit.

3. (a) Increase in diameter = 21 - 6 = 15 mm. [1] Rate = 15 / 5 = 3.0 mm per day. [1] (b) Percentage increase = 15 / 6 × 100. [1] = 250 %. [1] (c) Rain splashes spores from infected leaves onto healthy ones. [1] Wind carries spores over longer distances; wet leaves help the spores to germinate. [1] [6] Examiner insight: Show the subtraction and the division. A correct final number with a missing unit on the rate in part (a) can lose the last credit. In part (c) each reason must link one weather feature to the spores.

4. (a) Callose is deposited in the plasmodesmata and in the sieve plates. [1] This blocks the channels between cells, so the pathogen cannot pass from cell to cell. [1] Blocking sieve plates stops it travelling in the phloem, and callose also strengthens the cell wall. [1] (b) A named chemical, such as chitinase, is made by the plant. [1] Chitinase digests the chitin in fungal cell walls, so the fungus is damaged or killed. [1] [5] Examiner insight: “Callose blocks the pathogen” is too vague. Name where it is deposited and what movement is stopped. Any suitable named plant chemical can be used in part (b), but the action must match it.

5. (a) Goblet cells secrete mucus. [1] Mucus traps microorganisms and dust. [1] Cilia on the lining cells beat and sweep the mucus towards the pharynx, where it is swallowed. [1] (b) Coughing and sneezing force mucus and trapped pathogens out of the airways. [1] Vomiting or diarrhoea removes pathogens from the gut. [1] [5] Examiner insight: Mucus alone is not enough for part (a). The sweeping action of the cilia is the second half of the mechanism and is what separates a full answer from a partial one.

6. (a) Platelets release substances at the damaged vessel. [1] This starts a cascade of reactions. [1] Soluble fibrinogen is converted to insoluble fibrin. [1] Fibrin forms a network that traps platelets and red blood cells to make a clot. [1] (b) The clot seals the wound so pathogens cannot enter. [1] The scab protects the new tissue while cells at the wound edge divide and repair the damage. [1] [6] Examiner insight: Mixing up fibrinogen and fibrin is a common slip, so write the soluble to insoluble direction clearly. Names of individual clotting factors are not needed.

7. Damaged cells and mast cells release histamine and cytokines. [1] Histamine causes vasodilation, so more blood flows to the area. [1] Capillary walls become more permeable, so fluid and phagocytes leave the blood and enter the tissue. [1] Phagocytes then engulf and destroy the pathogens. [1] [4] Examiner insight: “The area swells” is a description, not an explanation. Link each change (vasodilation, permeability) to its effect on getting phagocytes to the site.

8. (a) 30 mm = 30 000 µm. [1] Magnification = 30 000 / 10 = ×3000. [1] (b) Actual size = image size / magnification = 33 000 / 3000. [1] = 11 µm. [1] (c) Nucleus with several lobes. [1] Pale cytoplasm with granules. [1] [6] Examiner insight: Convert to a single unit before dividing. A magnification has no unit, whereas an actual size needs one. Features must be ones visible in a stained smear, not general statements about function.

9. (a) With opsonins: 38 / 50 × 100 = 76 %. [1] Without: 17 / 50 × 100 = 34 %. [1] (b) Increase = (76 - 34) / 34 × 100, or (38 - 17) / 17 × 100. [1] = 123.5 %, which is 124 % to 3 significant figures. [1] (c) More macrophages engulfed bacteria when opsonins were present. [1] Opsonins coat the bacteria and make it easier for receptors on the macrophage to bind to them. [1] Binding triggers engulfment, so more phagocytosis occurs; the dish without opsonins acts as a control. [1] [7] Examiner insight: In a percentage change, the original value is the one without opsonins. Using 76 as the denominator gives a different, wrong answer. For part (c), quote figures from the data as well as stating the mechanism.

10. Infected or damaged cells release cytokines, which attract neutrophils by chemotaxis. [1] Neutrophils squeeze out of the capillary into the tissue. [1] Receptors on the phagocyte bind to the bacterium, helped by opsonins coating it. [1] The membrane engulfs the bacterium by endocytosis, forming a phagosome. [1] Lysosomes fuse with the phagosome. [1] Enzymes released from the lysosomes digest the bacterium. [1] A macrophage, as an antigen-presenting cell, places antigens from the bacterium on its surface membrane. [1] This allows lymphocytes to recognise the antigens and start the specific immune response. [1] [8] Examiner insight: An extended answer gains credit for a logical order and correct terms. List the events in sequence and use the words phagosome, lysosome and antigen-presenting cell rather than “it eats the bacterium”.

11. (a) 160 / 250 × 100 = 64 %. [1] (b) Increase = (11.5 - 5.0) / 5.0 × 100. [1] = 130 %. [1] (c) Bacteria at the infection site release substances and damaged cells release cytokines, which signal that neutrophils are needed. [1] More neutrophils are released from the bone marrow, because they are the first phagocytes to reach infected tissue. [1] (d) Use a sharp pencil and clear continuous lines. [1] Do not shade or colour. [1] Label with ruled lines that do not cross, and add a title, magnification and scale. [1] [8] Examiner insight: Part (d) asks for three rules, so each must be a separate point. Avoid repeating the same rule in different words, which is only worth one point.

12. (a) Overcrowding puts people close together, so droplets from coughs and sneezes are easily inhaled. [1] Poor ventilation means droplets stay in the air for longer. [1] Many people sharing a small space gives more chances for an infected person to pass the bacterium to others. [1] (b) Spores are shed onto warm, damp floors and benches. [1] They survive there until picked up on bare feet. [1] Shared floors, towels and changing rooms give repeated contact between infected and uninfected people. [1] [6] Examiner insight: A “social factors” question wants the factor linked to the route of spread. Naming overcrowding is not enough without saying how it increases contact with the pathogen.

Where marks are usually lost

• Writing “germ” or “microbe” when a named type is required.
• Describing the mosquito as the cause of malaria rather than the vector.
• Missing the unit, or mixing mm and µm, in magnification and rate questions.
• Using the new value rather than the original value as the denominator for percentage change.
• Stating a weather or social factor without linking it to how the pathogen spreads.
• Describing callose without saying where it is deposited or what it blocks.
• Writing fibrin and fibrinogen the wrong way round.
• Describing phagocytosis without a phagosome, lysosomes or enzymes.
• Drawing with shading, overlapping label lines, or no scale.

Next steps

• Recap with the revision notes and the study guide.
• Course hub: OCR A Level Biology and the checklist.
• Take one of the free 10-minute diagnostics to find other weak spots.
• Book a free trial class.

Official syllabus

OCR A Level in Biology A (H420), specification Version 4.1 (April 2026), for first assessment in 2025, published by OCR. Statements 4.1.1(a) to 4.1.1(e).