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

These revision notes condense 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. For full explanations read the study guide, then test yourself with the practice questions. Hub: OCR A Level Biology. Checklist: OCR A Level Biology checklist. Quick check of where you stand: free diagnostics.

4.1.1(a) Pathogen types: learn as pairs

Type	Plant disease	Animal disease
Bacterium	Ring rot (potato)	Tuberculosis (TB)
Virus	Tobacco Mosaic Virus	HIV/AIDS, influenza
Protoctist	Potato and tomato late blight	Malaria
Fungus	Black sigatoka (banana)	Athlete’s foot

• Virus: not a cell, reproduces only inside host cells.
• Bacterium: prokaryote. Protoctist and fungus: eukaryotes.
• Binomial names and symptom detail are not required.

4.1.1(b) Transmission

Route	Example	Detail to state
Vector	Malaria; many plant viruses	Female mosquito carries the pathogen between hosts; aphids carry plant viruses
Spores	Late blight, black sigatoka, athlete’s foot	Light; moved by wind, rain splash, floors, towels
Droplets	TB, influenza	Coughs and sneezes; inhaled
Body fluids	HIV	Blood, semen, mother to child, shared needles
Contact and tools	Tobacco Mosaic Virus, ring rot	Sap on hands and tools; infected seed potatoes

Conditions to link in your answer:

• Climate: warm, wet = mosquito breeding; cool, damp = late blight.
• Social: overcrowding, poor ventilation, poor sanitation, limited healthcare, travel, trade, monoculture.

Method for a transmission question. (1) Name the route. (2) Say what carries the pathogen. (3) Link to the condition given in the question. (4) Say why that raises the chance of infection.

4.1.1(c) Plant defences

Type	Examples	How it helps
Chemicals	Terpenoids, phenols, alkaloids, defensins, chitinase	Kill or inhibit pathogens; chitinase digests fungal chitin
Callose	In plasmodesmata and sieve plates, and between wall and membrane	Blocks cell-to-cell and phloem movement; strengthens the wall
Lignin and thicker walls	Around the site of attack	Physical barrier
Hypersensitive response	Cells at the site die	Pathogen has no living cells to feed on
Stomatal closure	Pores shut	Fewer bacteria enter
Signalling	Salicylic acid	Distant cells switch on defences

4.1.1(d) Non-specific defences in animals

Defence	Key points
Skin	Barrier, sebum, harmless microorganisms compete; no structure detail needed
Mucous membranes	Goblet cells make mucus that traps microorganisms; cilia sweep it to the throat; lysozyme in mucus and tears
Expulsive reflexes	Cough, sneeze, vomit, diarrhoea
Clotting	Platelets release substances; cascade; fibrinogen to fibrin; fibrin mesh traps platelets and cells
Wound repair	Scab; cell division at the edges; new tissue
Inflammation	Histamine and cytokines; vasodilation; leakier capillaries; phagocytes arrive

Clotting in order: damaged vessel, platelets release substances, cascade, soluble fibrinogen becomes insoluble fibrin, fibrin network traps platelets and red cells, clot. Names of clotting factors are not required.

4.1.1(e)(i) Phagocytes

	Neutrophil	Macrophage
Nucleus	Lobed (2 to 5)	Large, kidney-shaped
Lifespan	Short	Long
Role	First responder	Antigen-presenting cell

Phagocytosis in six steps.

1. Cytokines released by infected cells attract and activate phagocytes.
2. Receptors bind the pathogen; opsonins coat it and make binding easier.
3. Membrane engulfs the pathogen by endocytosis.
4. Pathogen sits in a phagosome.
5. Lysosomes fuse with the phagosome and release enzymes.
6. Pathogen is digested; an antigen-presenting cell displays its antigens on its surface.

4.1.1(e)(ii) Blood smears and drawing

Cell	Identify by
Red blood cell	Small, pink, no nucleus
Neutrophil	Lobed nucleus, pale granular cytoplasm
Lymphocyte	Big round nucleus, thin rim of cytoplasm
Monocyte	Largest; kidney-shaped nucleus
Platelet	Tiny fragment

Drawing checklist. Sharp pencil. Continuous lines. No shading. Fill the space. Ruled label lines that do not cross. Title, magnification, scale bar.

Magnification. Magnification = image size / actual size. Put both lengths in one unit before dividing (1 mm = 1000 µm). Actual size = image size / magnification. Image size = magnification × actual size.

• A cell drawn 60 mm across with actual diameter 12 µm: 60 000 / 12 = ×5000.
• Percentage change = (new - old) / old × 100.

Must-know distinctions

Pair	Difference
Pathogen and vector	The pathogen causes the disease; the vector only carries it
Fibrinogen and fibrin	Fibrinogen is soluble and made first; fibrin is insoluble and forms the mesh
Phagosome and lysosome	The phagosome holds the pathogen; the lysosome supplies enzymes
Neutrophil and macrophage	Neutrophil is short-lived and acts first; the macrophage lives longer and presents antigens
Chemical defence and callose	Chemicals attack the pathogen; callose limits its spread
Magnification and scale bar	Magnification is a ratio with no unit; the scale bar shows a real length

Worked reminders

Rate of spread. A lesion grows from 4 mm to 14 mm in 3 days. Rate = (14 - 4) / 3 = 3.3 mm per day. Always give the unit as mm per day.

Differential count. Out of 200 white cells, 124 are neutrophils. Percentage = 124 / 200 × 100 = 62 %. Show the division, then the answer.

Scale bar. A bar 25 mm long is labelled 5 µm. Magnification = 25 000 / 5 = ×5000. A cell that is 60 mm across in the image is then 60 000 / 5000 = 12 µm across.

Model answer shapes

Explain how mucous membranes protect. (1) Goblet cells secrete mucus. (2) Mucus traps pathogens and dust. (3) Cilia beat and move the mucus towards the pharynx. (4) It is swallowed and the pathogens meet acid and enzymes in the stomach.

Describe how a clot forms. (1) Platelets release substances at the damaged vessel. (2) A cascade of reactions follows. (3) Soluble fibrinogen is converted to insoluble fibrin. (4) Fibrin threads form a network that traps platelets and red cells. (5) The clot dries to a scab.

Explain why inflammation helps. (1) Histamine and cytokines are released. (2) Vasodilation brings more blood. (3) Capillaries become more permeable, so fluid and phagocytes pass into the tissue. (4) Phagocytes destroy the pathogen.

Describe how a phagocyte destroys a bacterium. Use the six steps above, in order, using the words chemotaxis, receptors, endocytosis, phagosome, lysosome and enzymes.

Command words in this unit

• Name or State: one correct term, no explanation.
• Describe: say what happens, in order.
• Explain: give the reason as well as the event.
• Suggest: apply what you know to an unfamiliar situation and justify it.
• Calculate: show the working, give the unit and keep sensible significant figures.

Last-week checklist

• Say the pathogen table aloud, disease first, type second.
• Write one transmission sentence for each disease on the list, naming the carrier and one condition.
• Sketch a neutrophil and a lymphocyte side by side, label them and add a scale bar.
• Practise one magnification calculation in both directions: finding magnification, and finding actual size.
• Rewrite the six phagocytosis steps from memory, using the key terms.

Quick self-test

1. Name the type of pathogen that causes black sigatoka.
2. Give two ways that late blight spores are carried between plants.
3. State what a vector is.
4. Name the polysaccharide deposited in plasmodesmata after infection and say what it does.
5. Which protein forms the network in a blood clot?
6. Describe what cilia do in the airways.
7. A photomicrograph is at ×6000 and a cell measures 72 mm in it. Calculate the actual diameter in µm.
8. A cell has an actual diameter of 9 µm. It is drawn at ×2500. Calculate the drawn diameter in mm.
9. Neutrophils rise from 6.5 to 15.6 (×10⁹ per dm³). Calculate the percentage increase.
10. Name the structure formed when a phagocyte surrounds a pathogen, and the organelle that fuses with it.
11. State one role of cytokines in phagocytosis.
12. Which blood cell has a kidney-shaped nucleus and is the largest white cell in a smear?

Answers

1. A fungus.
2. Wind and rain splash (infected tubers carried between fields are another route).
3. An organism that carries a pathogen from one host to another.
4. Callose. It blocks plasmodesmata so the pathogen cannot pass from cell to cell, and it strengthens the wall.
5. Fibrin.
6. They beat to sweep mucus, with trapped microorganisms, towards the throat.
7. 72 mm = 72 000 µm. 72 000 / 6000 = 12 µm.
8. 9 × 2500 = 22 500 µm = 22.5 mm.
9. (15.6 - 6.5) / 6.5 × 100 = 140 %.
10. A phagosome; a lysosome.
11. They attract phagocytes to the infection, or activate them.
12. The monocyte.

Where marks are usually lost

• Naming the wrong type for a listed disease, especially late blight (protoctist, not fungus) and malaria.
• Writing “viruses are living cells” or “bacteria are viruses”.
• Saying the mosquito “causes” malaria rather than carrying the pathogen.
• Giving “bad weather” without saying which weather feature helps which stage of spread.
• Describing callose without saying where it is deposited or what it blocks.
• Reversing fibrinogen and fibrin.
• Leaving out units, or mixing mm and µm, in magnification questions.
• Drawing with shading, overlapping label lines or no title and scale.
• Stopping phagocytosis at engulfment without the phagosome, lysosomes and enzyme digestion.

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).