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

This guide follows the OCR GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), for first assessment in 2018. It covers statements B6.3f to B6.3i in topic B6.3, Monitoring and maintaining health: named infections in humans and plants, sexually transmitted infections (STIs), plant defences and ways of detecting plant disease. Statement B6.3i is printed in bold in the specification, so it is Higher Tier only. The rest is assessed in both tiers. Other statements in B6.3 have their own pages, so spread of disease, white blood cells, vaccination and drug development are not repeated here.

Pages in this set: this study guide, the revision notes and the practice questions. Course pages: OCR GCSE Biology hub, printable checklist and the free diagnostics. For cell walls and plant cell structure, see the cell level revision notes.

What you must be able to do

Statement	What you must be able to do	Tier
B6.3f	Describe at least one common human infection (one viral, one fungal, one bacterial example), one plant disease, and STIs including HIV/AIDS	Both
B6.3g	Describe physical plant defences: leaf cuticle, cell wall	Both
B6.3h	Describe chemical plant defences: antimicrobial substances	Both
B6.3i	Describe ways plant disease is detected and identified in the lab (DNA or antigen from the pathogen) and in the field (observation and microscopy)	Higher Tier only

The specification names three plant diseases to include: tobacco mosaic virus (TMV), barley powdery mildew caused by the fungus Erysiphe graminis, and crown gall disease caused by the bacterium Agrobacterium tumefaciens. Learn all three.

B6.3f: common human infections

A pathogen is a microorganism that causes disease. For each infection you need the pathogen type, how it spreads, the main effects and how it is controlled. The examples below are one choice for each type; any correct example earns credit.

Viral: influenza (“flu”). The influenza virus infects the cells lining the airways. It spreads in tiny droplets when an infected person coughs or sneezes, and by hands and surfaces carrying the virus. Effects include fever, aching muscles, headache, cough and tiredness. Antibiotics do not work on viruses, so treatment eases symptoms while the immune system clears the virus. Vaccination before winter reduces risk, and covering coughs and washing hands reduce spread.

Bacterial: Salmonella food poisoning. Salmonella bacteria live in the guts of some animals and contaminate raw poultry and eggs. People are infected by eating undercooked food or food that raw meat has touched. The bacteria and the toxins they release cause fever, abdominal cramps, vomiting and diarrhoea. Thorough cooking, separate chopping boards and washing hands after handling raw meat prevent it.

Fungal: athlete’s foot. A fungus grows in the skin between the toes. The skin becomes itchy, red, flaky and cracked. Fungal spores pass on by direct contact or from damp floors in changing rooms and showers. Antifungal creams or powders treat it, and drying feet carefully and not sharing towels help prevent it.

Sexually transmitted infections

An STI is an infection passed from person to person during sexual contact. Some can also pass in other ways, as HIV does.

HIV and AIDS (viral). The human immunodeficiency virus (HIV) infects and destroys certain white blood cells (T helper lymphocytes), which coordinate the immune response. HIV spreads through body fluids: unprotected sex, sharing needles, from an infected mother to her baby (in the womb, at birth or in breast milk) and, rarely now, through infected blood. People may have no symptoms for years. As white blood cells fall in number, the immune system cannot fight other infections or some cancers. This late stage is acquired immune deficiency syndrome (AIDS). Antiretroviral drugs slow the virus and keep it at very low levels but do not remove it from the body. Prevention: condoms, never sharing needles, screening donated blood, testing, and antiretroviral treatment in pregnancy.

Chlamydia (bacterial). Spread by unprotected sex. Many people have no symptoms, so they pass it on without knowing. It can cause pain on urinating and discharge, and untreated infection can damage the reproductive system and cause infertility. Antibiotics cure it. Condoms and screening of sexually active young people reduce its spread.

Gonorrhoea (bacterial). Spread by unprotected sex. It causes a thick yellow or green discharge and pain on urinating. It is treated with antibiotics, but some strains have become resistant, so prevention with condoms matters.

Worked example 1: STI screening

A clinic screens 1,850 people for chlamydia and 111 test positive. Calculate the percentage who tested positive.

111 ÷ 1,850 = 0.06, so 0.06 × 100 = 6.0%.

Worked example 2 shows why a sample result is only an estimate.

The three named plant diseases

Plants have no antibodies or white blood cells, but pathogens damage them just as seriously. Disease lowers photosynthesis, so there is less glucose for growth and crop yields fall.

	Tobacco mosaic virus (TMV)	Barley powdery mildew	Crown gall disease
Pathogen	Virus	Fungus, Erysiphe graminis	Bacterium, Agrobacterium tumefaciens
Signs	Mosaic of pale and dark green or yellow patches on leaves; stunted growth	White powdery fungal growth on leaves and stems	Rounded, tumour-like swellings (galls) near the base of the stem and on roots
Spread	Contact with infected sap on hands, tools and leaves	Spores blown by wind	Soil and water; bacteria enter through wounds
Effect	Chlorophyll destroyed in patches, so less photosynthesis	Mould covers the leaf, blocking light; fungus takes nutrients from leaf cells	Galls disturb water and mineral transport; plant grows poorly
Control	Remove and destroy infected plants; wash hands and tools; resistant varieties; no cure	Fungicides; resistant varieties; spacing plants; removing infected material	Buy healthy stock; avoid wounding plants; clean tools; destroy infected plants

For crown gall, the bacterium transfers some of its own DNA into plant cells, which makes them divide and form the gall. You do not need more detail than this.

Worked example 2: estimating from a sample

A farmer wants to know how many plants in a field of 6,500 barley plants have powdery mildew. She checks 40 plants at random places and finds 9 with white powder.

1. Proportion infected in the sample: 9 ÷ 40 = 0.225, which is 22.5%.
2. Estimate for the field: 0.225 × 6,500 = 1,462.5.
3. Give an answer to a sensible accuracy: about 1,460 plants.

This is an estimate. A bigger sample, or several samples averaged, would be more reliable, and the 40 plants must be chosen randomly, not just from the field edge where wind carries spores first.

B6.3g: physical plant defences

Physical defences stop pathogens getting in. They work all the time, whether or not the plant is infected.

• Leaf cuticle. A waxy layer on the outer surface of the leaf. It is waterproof and forms a continuous barrier that most pathogens cannot cross. Water runs off it, so fewer spores and bacteria stay on the surface.
• Cell wall. A tough layer of cellulose around every plant cell. A pathogen that gets past the cuticle must still break through the cell wall to reach the cell contents, and the wall is hard to digest or penetrate.

Pathogens can still get in through wounds, which is why crown gall is linked to damaged tissue.

B6.3h: chemical plant defences

Many plants make antimicrobial substances, chemicals that kill or stop the growth of bacteria and fungi. They can be made all the time or in larger amounts after infection. Garlic makes allicin when its cells are damaged, and oats make saponins in their roots that act against fungi. Some plants also make poisonous chemicals that discourage animals from eating them. Chemical defences work even where a pathogen has crossed the physical barrier, so the two types of defence support each other.

Worked example 3: judging a control method

Mean leaf area covered by mildew was 34% on untreated plots and 8% on plots sprayed with a fungicide. Calculate the percentage reduction.

1. Reduction: 34 − 8 = 26 percentage points.
2. As a fraction of the starting value: 26 ÷ 34 = 0.7647.
3. 76.5% reduction (3 significant figures).

B6.3i: detecting and identifying plant disease (Higher Tier only)

Correct identification matters because the treatment depends on the pathogen: a fungicide will not work on a virus.

In the field: observation and microscopy. You look at the symptoms: mosaic patterns, white powder or galls. You compare them with identification guides or photographs. You can also look at how the disease is spread across the crop and check whether pests are present. A hand lens or a microscope is used to view fungal structures or spores from a sample. Field diagnosis is quick and cheap, but different pathogens, and even mineral deficiencies, can cause similar-looking damage, so it may not be certain.

In the lab: DNA or antigen from the pathogen.

• Antigen detection. Antibodies (often monoclonal) that bind to a specific antigen from the pathogen are mixed with a plant sample. A colour change or visible line shows the antigen is present. Test kits used in the field work in this way.
• DNA testing. Each pathogen has its own DNA sequence. A small sample of its DNA is copied many times (by the polymerase chain reaction, PCR), and the product is compared with the DNA of the known pathogen, for example by looking for a band of the expected size on a gel.

Worked example 4: reading a DNA test

A lab tests four leaf samples for TMV DNA. A known infected sample (positive control) gives a band at 410 base pairs. A known healthy sample (negative control) gives no band.

Sample	Band seen
A	410 base pairs
B	none
C	410 base pairs
D	250 base pairs only

Samples A and C match the positive control, so they contain TMV DNA. Sample B has no band, like the healthy control, so no TMV DNA was found. Sample D has a band at a different size, so it is not TMV DNA. The controls show the test worked.

Worked example 5: patterns in data

Ten-plant groups were given different numbers of wounds, and the mean number of galls counted: 0 wounds, 0 galls; 2 wounds, 3 galls; 4 wounds, 6 galls; 6 wounds, 10 galls; 8 wounds, 13 galls. The scatter diagram shows a strong positive correlation: more wounds go with more galls. It fits the biology because the bacteria enter through wounds. A correlation alone does not prove the cause, but it supports a hypothesis you can test further.

Common errors

• Writing “bacteria” for TMV or “virus” for powdery mildew. Match each pathogen type to its disease.
• Saying HIV and AIDS are the same. HIV is the virus; AIDS is the later stage after the immune system fails.
• Saying antiretroviral drugs cure HIV.
• Saying the cuticle or cell wall “kills” pathogens. They act as barriers.
• Describing detection of plant disease using human white blood cells or antibodies made by the plant. Plants do not make antibodies; the antibodies in a test come from the laboratory.
• Giving field and lab detection methods the wrong way round.

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, with B6.3i in bold (Higher Tier only).