AQA GCSE Biology 8461: Infection and response – Revision Notes

These are condensed recall notes for section 4.3 Infection and response of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. They cover 4.3.1.1 to 4.3.3.2. The topic is assessed on Paper 1 (topics 1 to 4), which is set at Foundation and Higher Tier. Content marked (HT only) in the specification is labelled Higher tier only here: all of 4.3.2 Monoclonal antibodies, and the lists of signs and identification methods in 4.3.3.1.

For full explanations, use the Infection and response study guide. When you are ready to test yourself, go to the Infection and response practice questions. The course hub is AQA GCSE Biology, and the printable checklist lists every specification point.

4.3.1.1 Pathogens and spread

• Pathogen: a microorganism that causes infectious disease. The four types are viruses, bacteria, protists and fungi.
• Pathogens can infect plants or animals.
• Three routes of spread: direct contact, water, air.
• Bacteria reproduce rapidly in the body and may release toxins that damage tissues and make you feel ill.
• Viruses live and reproduce inside cells, causing cell damage.

Reducing spread – the four standard controls

Control	How it cuts spread
Hygiene (hand washing, clean food preparation)	Removes pathogens before they reach a new host
Isolating infected individuals	Stops direct contact and droplet spread
Destroying or controlling vectors	Removes the organism that carries the pathogen
Vaccination	Makes people immune, so the pathogen has fewer hosts

The seven named diseases (4.3.1.2 to 4.3.1.5)

Disease	Pathogen	Host	Spread	Key symptoms or signs	Control or treatment
Measles	Virus	Human	Inhaled droplets from sneezes and coughs	Fever, red skin rash; can be fatal if complications arise	Most young children vaccinated
HIV	Virus	Human	Sexual contact; exchange of body fluids such as blood (shared needles)	Flu-like illness at first; attacks immune cells; late stage = AIDS	Antiretroviral drugs
Tobacco mosaic virus (TMV)	Virus	Many plants, e.g. tomatoes	Contact between plants, or on hands and tools	“Mosaic” discolouration of leaves	Remove and destroy infected plants; clean hands and tools
Salmonella food poisoning	Bacterium	Human	Ingested in food, or food prepared in unhygienic conditions	Fever, abdominal cramps, vomiting, diarrhoea (bacteria and their toxins)	UK poultry vaccinated
Gonorrhoea	Bacterium	Human	Sexual contact	Thick yellow or green discharge; pain on urinating	Antibiotics (many strains now resistant to penicillin); condoms
Rose black spot	Fungus	Roses	Water or wind	Purple or black leaf spots; leaves yellow and drop early	Fungicides; remove and destroy affected leaves
Malaria	Protist	Human	Mosquito (vector)	Recurrent episodes of fever; can be fatal	Stop mosquitoes breeding; mosquito nets

Why plant diseases reduce growth: TMV and black spot both reduce the area of healthy green leaf, so less photosynthesis takes place and less glucose is available for growth.

4.3.1.6 Human defences

Non-specific defences (they act against any pathogen):

Structure	How it defends
Skin	Physical barrier; covers the body so pathogens cannot get in
Nose	Hairs and mucus trap particles that may contain pathogens
Trachea and bronchi	Mucus traps pathogens; cilia move the mucus up and away from the lungs
Stomach	Produces hydrochloric acid, which kills pathogens in food and drink

Immune system: if a pathogen gets into the body, white blood cells try to destroy it in three ways.

1. Phagocytosis – the white blood cell engulfs and digests the pathogen.
2. Antibody production – antibodies bind to a specific pathogen and help destroy it.
3. Antitoxin production – antitoxins neutralise the toxins released by bacteria.

4.3.1.7 Vaccination – method in steps

1. Small quantities of a dead or inactive form of the pathogen are put into the body.
2. White blood cells are stimulated to produce antibodies against it.
3. If the same live pathogen enters later, white blood cells respond quickly and produce the correct antibodies.
4. The pathogen is destroyed before it can cause illness.

Population effect: if a large proportion of people are immune, the pathogen has few hosts to pass to, so spread falls and unvaccinated people are also protected. You do not need to learn vaccination schedules or the side effects of particular vaccines.

4.3.1.8 Antibiotics and painkillers

	Antibiotics (e.g. penicillin)	Painkillers and other medicines
What they do	Kill infective bacteria inside the body	Treat symptoms only
Kill pathogens?	Yes – bacteria only	No
Work on viruses?	No	Relieve symptoms, do not kill the virus

• Specific bacteria should be treated with specific antibiotics.
• Antibiotics have greatly reduced deaths from bacterial disease, but resistant strains are a serious concern.
• Drugs that kill viruses are hard to develop, because viruses live inside cells and the drug may damage body tissues.

4.3.1.9 Discovery and development of drugs

Traditional sources

Drug	Use	Source
Digitalis	Heart drug	Foxgloves
Aspirin	Painkiller	Willow
Penicillin	Antibiotic	Penicillium mould, discovered by Alexander Fleming

Most new drugs are now synthesised by chemists in the pharmaceutical industry, though the starting point may still be a chemical extracted from a plant.

Testing in order – new drugs are tested for toxicity, efficacy and dose.

1. Preclinical testing in a laboratory on cells, tissues and live animals.
2. Clinical trials on healthy volunteers and patients, starting with very low doses.
3. If safe, further trials find the optimum dose.
4. In double blind trials, some patients get a placebo; neither patients nor doctors know who has which.
5. Results are published only after peer review.

4.3.2 Monoclonal antibodies (Higher tier only)

Production – method in steps

1. Stimulate mouse lymphocytes to make a particular antibody.
2. Combine the lymphocytes with a particular kind of tumour cell to make a hybridoma cell.
3. The hybridoma cell can both divide and make the antibody.
4. Clone a single hybridoma cell to give many identical cells that all make the same antibody.
5. Collect and purify a large amount of the antibody.

Monoclonal antibodies come from a single clone of cells. Each is specific to one binding site on one protein antigen, so it can target a specific chemical or specific cells.

Uses

• Diagnosis, such as pregnancy tests.
• Laboratory measurement of hormones and other chemicals in blood, or detecting pathogens.
• Research: locating specific molecules in a cell or tissue by binding to them with a fluorescent dye.
• Treating disease: for cancer, the antibody carries a radioactive substance, a toxic drug or a chemical that stops cells growing and dividing to the cancer cells, without harming other cells.

You do not need to recall specific tests or treatments, but you must be able to explain one from information given to you. Monoclonal antibodies have caused more side effects than expected, so they are not yet as widely used as was hoped.

4.3.3 Plant disease

Detection (Higher tier only): stunted growth; spots on leaves; areas of decay (rot); growths; malformed stems or leaves; discolouration; the presence of pests.

Identification (Higher tier only): a gardening manual or website; taking infected plants to a laboratory to identify the pathogen; testing kits containing monoclonal antibodies.

Plant pathogens and pests – your named examples are limited to TMV (virus), black spot (fungus) and aphids (insects).

Ion deficiencies

Ion	Needed for	Deficiency sign
Nitrate	Protein synthesis, and therefore growth	Stunted growth
Magnesium	Making chlorophyll	Chlorosis (yellow leaves)

Plant defence responses

Type	Examples
Physical	Cellulose cell walls; tough waxy cuticle on leaves; layers of dead cells around stems (bark) that fall off
Chemical	Antibacterial chemicals; poisons to deter herbivores
Mechanical adaptations	Thorns and hairs; leaves that droop or curl when touched; mimicry to trick animals

Must-know distinctions

• Antibody vs antitoxin: an antibody binds to a pathogen; an antitoxin neutralises a toxin.
• Antibiotic vs painkiller: an antibiotic kills bacteria; a painkiller treats symptoms and kills nothing.
• Antibiotic vs vaccine: an antibiotic treats an existing bacterial infection; a vaccine prevents illness in advance.
• Pathogen vs vector: the malarial protist is the pathogen; the mosquito is the vector that carries it.
• Aphid vs pathogen: aphids are insect pests, not microorganisms.
• Placebo vs double blind: a placebo is the dummy treatment; “double blind” means neither patient nor doctor knows who received it.

Quick self-test

1. Name the four types of pathogen.
2. How is measles spread?
3. Which named disease is caused by a protist, and what is its vector?
4. Why does TMV reduce plant growth?
5. Give the three ways white blood cells defend the body.
6. Why do antibiotics not cure measles?
7. Which drug comes from willow?
8. Why are very low doses used at the start of a clinical trial?
9. In a study, 36 of 1200 vaccinated people and 150 of 1200 unvaccinated people caught a disease. Calculate the percentage infected in each group.
10. (Higher tier only) What two properties of a hybridoma cell make it useful?
11. A plant has yellow leaves. Which ion is it likely to be short of?
12. Give one chemical defence of plants.

Answers

1. Viruses, bacteria, protists, fungi.
2. By inhaling droplets from sneezes and coughs.
3. Malaria; the mosquito.
4. The mosaic discolouration reduces photosynthesis, so less glucose is made for growth.
5. Phagocytosis, antibody production, antitoxin production.
6. Measles is caused by a virus, and antibiotics cannot kill viruses.
7. Aspirin.
8. To check the drug is safe before larger doses are given.
9. Vaccinated: 36 ÷ 1200 × 100 = 3%. Unvaccinated: 150 ÷ 1200 × 100 = 12.5%.
10. It can divide and it makes the antibody.
11. Magnesium (chlorosis).
12. Antibacterial chemicals, or poisons that deter herbivores.

Where marks are usually lost

• Calling a vaccine a cure, or saying it “contains antibodies”. It contains dead or inactive pathogen and stimulates white blood cells to make antibodies.
• Saying antibodies “kill” toxins. Antitoxins neutralise toxins; antibodies target the pathogen.
• Writing “the body becomes immune” without saying white blood cells produce the correct antibodies quickly on re-infection.
• Mixing up the pathogen type for a named disease, especially calling malaria a virus or black spot a bacterium.
• Saying antibiotics can treat colds, flu or measles. They only kill bacteria.
• Describing resistance as the patient becoming resistant. It is the bacteria that become resistant.
• Leaving out the reason in plant-disease answers: “less photosynthesis, so less glucose for growth” is the link examiners look for.
• Giving the order of drug testing wrongly, or omitting “low dose first” and “optimum dose” in clinical trials.
• (Higher tier only) Describing monoclonal antibody production without the hybridoma cell or without cloning a single cell.
• Treating aphids as a pathogen, or learning extra named plant diseases the specification does not require.

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.3 Infection and response. Next, try the practice questions or one of the free 10-minute diagnostics.