OCR GCSE Biology A: Named infections, STIs and plant disease defence and detection (J247) – Revision Notes

These notes condense statements B6.3f to B6.3i of the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. B6.3i is in bold in the specification, so it is Higher Tier only; B6.3f to B6.3h are for both tiers. Full explanations are in the study guide. When you have revised, try the practice questions, check coverage on the printable checklist, see the OCR GCSE Biology hub and take a free diagnostic.

Key terms

Term	Meaning
Pathogen	An organism or virus that causes an infectious disease
STI	Infection passed on by sexual contact (some also pass by other routes)
HIV	The virus that infects and destroys white blood cells called T helper lymphocytes
AIDS	The late stage of HIV infection, when the immune system can no longer fight other infections
Antiretroviral	Drug that slows HIV and keeps it at low levels; not a cure
Cuticle	Waxy, waterproof layer on the leaf surface
Cell wall	Cellulose layer around plant cells
Antimicrobial substance	Chemical made by a plant that kills or stops growth of microbes
Antigen	A molecule on a pathogen that an antibody can bind to
Positive / negative control	Known infected / known healthy sample used to show a test works

B6.3f: named human infections

Infection	Pathogen	Spread	Main effects	Control
Influenza	Virus	Droplets from coughs and sneezes; hands and surfaces	Fever, aching muscles, cough, tiredness	Vaccination, hygiene; antibiotics do not work
Salmonella food poisoning	Bacterium	Undercooked poultry or eggs; cross-contamination	Fever, cramps, vomiting, diarrhoea	Cook thoroughly, separate raw meat, wash hands
Athlete’s foot	Fungus	Contact; damp floors and shared towels	Itchy, red, flaky, cracked skin between toes	Antifungal cream or powder; dry feet
HIV/AIDS	Virus	Unprotected sex, shared needles, mother to baby, infected blood	Symptom-free for years, then weak immunity and repeated infections	Condoms, no needle sharing, blood screening, antiretrovirals
Chlamydia	Bacterium	Unprotected sex	Often no symptoms; can cause infertility	Antibiotics, condoms, screening
Gonorrhoea	Bacterium	Unprotected sex	Yellow or green discharge, painful urination	Antibiotics (some strains resistant), condoms

Reminders:

• One viral, one fungal and one bacterial human infection is the minimum. Know at least one of each, with its spread and control.
• HIV is the virus; AIDS is the late stage. HIV attacks white blood cells, so fewer cells are left to protect the body.
• Chlamydia is often silent, so screening and telling partners matters.
• Antibiotics only work on bacteria. They are no use against influenza or HIV, and nothing treats athlete’s foot except antifungals.

B6.3f: named plant diseases

	TMV	Barley powdery mildew	Crown gall
Pathogen	Virus	Fungus (Erysiphe graminis)	Bacterium (Agrobacterium tumefaciens)
Signs	Mosaic of light and dark green or yellow patches	White powdery growth	Tumour-like galls at stem base and roots
Spread	Contact with infected sap, tools, hands	Wind-blown spores	Soil, water; enters wounds
Why it harms	Less chlorophyll, so less photosynthesis	Blocks light; fungus feeds on leaf cells	Galls disrupt transport; bacterium inserts own DNA into plant cells
Control	Destroy infected plants, clean hands and tools, resistant varieties	Fungicide, resistant varieties, spacing, remove debris	Healthy stock, avoid wounds, clean tools

Link every plant disease to the effect on growth: less photosynthesis or damaged transport means less glucose, poorer growth and lower yield.

B6.3g and B6.3h: plant defences

Type	Example	How it helps
Physical	Leaf cuticle	Waxy, waterproof barrier; water runs off, so fewer microbes stay on the leaf
Physical	Cell wall	Tough cellulose layer pathogens must get through to reach cell contents
Chemical	Antimicrobial substances (for example allicin in garlic, saponins in oats)	Kill or slow bacteria and fungi, including those that cross the physical barriers

Must-know distinctions:

• Physical defences are barriers; chemical defences are substances that attack microbes.
• Plants do not have white blood cells or antibodies. Do not use animal immune responses to answer plant questions.
• Wounds bypass physical barriers, which is how crown gall bacteria enter.

B6.3i: detection and identification (Higher Tier only)

Where	Method	What it shows
Field	Observation of symptoms and comparison with identification guides	Likely disease from mosaic, powder or galls
Field	Microscopy of a sample	Spores or fungal structures; shows pathogen type
Lab	Antigen detection with antibodies (including monoclonal antibodies)	Colour change or line shows the pathogen antigen is present
Lab	DNA testing (DNA copied by PCR, then compared with known pathogen DNA)	Band of the expected size matches the pathogen

Method in steps for a DNA test result:

1. Look at the negative control: it should show no band. If it does, the test failed.
2. Look at the positive control: it should show the expected band. If not, the test failed.
3. Compare each sample band with the positive control.
4. State the conclusion: same size band means pathogen DNA present; no band means none found; a different size means a different DNA.

Field versus lab: the field is quick and cheap but symptoms can look similar for different causes. Lab tests are specific and can confirm the pathogen, but take longer and need equipment.

Worked reminders

• Percentage = part ÷ whole × 100. If 18 of 60 sampled plants are infected, that is 18 ÷ 60 × 100 = 30%. For a field of 2,400 plants the estimate is 0.30 × 2,400 = 720.
• Percentage reduction = (old − new) ÷ old × 100. From 50% leaf cover to 20%: 30 ÷ 50 × 100 = 60% reduction.
• A random sample avoids bias; a larger sample gives a more reliable estimate.
• A scatter diagram showing both variables rising together is a positive correlation. Correlation does not prove one causes the other.

Answer frames

• Describe a named disease: pathogen type, then how it spreads, then symptoms or signs, then how it is controlled. Four short points score better than one long one.
• Explain why a plant grows poorly: name the damage (chlorophyll lost, light blocked or transport disrupted), say photosynthesis falls, then say less glucose is made for growth.
• Describe a plant defence: name it (cuticle, cell wall or antimicrobial substance), say what it is made of or does, then say how that stops infection.
• Suggest how to reduce spread: link each measure to the route of spread. Condoms block sexual spread; clean tools block contact spread; resistant varieties lower the chance of infection in the first place.
• Use data: quote at least two figures from the table or graph, and say what they show before you explain why.

Quick checks before the exam

1. Can you give a viral, a fungal and a bacterial human infection with spread and control?
2. Can you give the pathogen, signs, spread and control for all three named plant diseases?
3. Can you name both physical defences and one chemical defence?
4. (Higher Tier only) Can you describe one field method and two lab methods, and read a DNA test result using the controls?

Quick self-test

1. Name the pathogen type that causes tobacco mosaic disease.
2. Name the fungus that causes barley powdery mildew.
3. Why do galls form in crown gall disease?
4. State two ways HIV is spread.
5. What is the difference between HIV and AIDS?
6. Give one reason chlamydia spreads easily.
7. Explain how the waxy cuticle protects a leaf.
8. What is an antimicrobial substance?
9. Of 250 people screened, 14 test positive for an STI. Calculate the percentage positive.
10. (Higher Tier only) Why is a positive control used in a DNA test for a plant pathogen?
11. (Higher Tier only) Give one limitation of diagnosing a plant disease in the field by observation alone.

Answers

1. A virus (tobacco mosaic virus, TMV).
2. Erysiphe graminis.
3. The bacterium Agrobacterium tumefaciens inserts some of its DNA into plant cells, making them divide and form swellings.
4. Any two: unprotected sex, sharing needles, mother to baby, infected blood.
5. HIV is the virus; AIDS is the later stage when the immune system has failed and other infections take hold.
6. Many people have no symptoms, so they do not know they are infected and pass it on.
7. It is a waterproof barrier that most pathogens cannot cross, and water runs off it, so fewer microbes stay on the surface.
8. A chemical, made by the plant, that kills or stops the growth of bacteria and fungi.
9. 14 ÷ 250 × 100 = 5.6%.
10. It shows what a positive result looks like, so sample bands can be compared with it and it confirms the test is working.
11. Different pathogens, or non-infectious causes, can produce similar-looking symptoms, so the cause may be misidentified.

Where marks are usually lost

• Giving the wrong pathogen type for TMV, powdery mildew or crown gall.
• Confusing the virus (HIV) with the syndrome (AIDS), for example saying AIDS is the pathogen.
• Saying antibiotics treat viral infections such as influenza or HIV.
• Describing HIV as curable by antiretrovirals instead of controlled.
• Saying the cuticle or cell wall kills microbes, instead of acting as a barrier.
• Writing “less photosynthesis” without saying why: chlorophyll destroyed, light blocked or transport damaged.
• Using “antibodies in the plant” or “white blood cells in the plant”.
• Mixing up field and lab methods in detection answers.
• Not showing the sum in percentage questions, or forgetting the units or the percent sign.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), first assessment 2018, published by OCR. Statements B6.3f to B6.3i; B6.3i is bold (Higher Tier only).