AQA GCSE Biology (8461) · Paper 1

🦠 Infection & Response

Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.

Communicable diseases

A pathogen is any microorganism that causes disease. There are four types to know: bacteria, viruses, fungi and protists, and they can attack plants as well as animals.

BacteriaBacteria make us ill by releasing toxins that damage tissues.

VirusesViruses invade cells and reproduce inside them, eventually bursting the cells open, and it is this cell damage that makes us ill.

Pathogens spread in several ways: through the air, in droplets from coughs, as with flu and measles; through water, such as dirty drinking water, as with cholera; and through direct contact, including touching, body fluids or contaminated surfaces, as with HIV and athlete's foot. The spread of disease can be slowed with good hygiene, clean water, isolating infected people, destroying vectors, and vaccination.

The six diseases on the spec

Measles (virus)Measles, a virus, is spread by droplets. It causes a fever and a red skin rash, and it can be fatal, which is why children are vaccinated against it.

HIV (virus)HIV, a virus, is spread by sexual contact or by contact with infected blood, such as through shared needles. It causes flu-like symptoms at first, then attacks the white blood cells of the immune system; in its late stage it causes AIDS. It is controlled with antiretroviral drugs.

Tobacco mosaic virusTobacco mosaic virus is a plant virus that produces a mosaic pattern of discoloured patches on leaves, which reduces photosynthesis and results in stunted growth.

Salmonella (bacterium)Salmonella, a bacterium, causes food poisoning from undercooked or contaminated food, with symptoms including fever, cramps, vomiting and diarrhoea caused by its toxins. Poultry in the UK is vaccinated against it.

Gonorrhoea (bacterium)Gonorrhoea, a bacterium, is a sexually transmitted disease causing a thick yellow-green discharge and pain when urinating. It is treated with antibiotics, although resistance is growing, and it can be prevented using barrier contraception.

Rose black spot (fungus)Rose black spot, a fungus, causes purple-black spots on leaves, which then drop off, leading to weak growth. It spreads by wind and water, and is treated with fungicides and by burning the infected leaves.

Malaria (protist)Malaria, caused by a protist, is spread by mosquito bites; the mosquito is the vector rather than the pathogen itself. It causes repeating fevers, and it is prevented by stopping mosquito bites, for example with nets, and by killing mosquitoes.

🎯 Examiner tip: A vector is an organism that carries a pathogen from one host to another without being the pathogen itself. Malaria's vector is the mosquito, while the pathogen is the protist that the mosquito injects.

The body's defences

The body has several non-specific barriers that try to keep every pathogen out.

The skin acts as a physical barrier; scabs seal cuts, and its oils kill microbes.
The nose has hairs and mucus that trap particles in the air that is breathed in.
The trachea and bronchi are lined with mucus that traps pathogens, and cilia, tiny hairs, sweep this mucus up and away.
The stomach produces hydrochloric acid, which kills most of the pathogens that are swallowed.

The immune system

If a pathogen gets into the body, white blood cells attack it in three ways: by phagocytosis, engulfing and digesting the pathogen; by making antibodies; and by making antitoxins, which neutralise bacterial toxins.

Every pathogen carries unique marker molecules on its surface, called antigens. Antibodies are proteins built to lock onto one specific antigen, tagging or clumping the pathogen so it can be destroyed. After the first infection, a memory of the correct antibody remains, so if the same pathogen is met again, antibodies are produced faster and in greater numbers, killing it before symptoms appear. This is called immunity, and it is why the same disease is rarely caught twice.

Vaccination

A vaccine injects a dead or inactivated version of the pathogen. This version cannot cause harm, but its antigens still trigger the full white blood cell response, so the body learns to make the correct antibody in advance. If enough of a population is vaccinated, the pathogen cannot find enough hosts to spread through, which protects even people who are not vaccinated. This is called herd immunity.

Drugs

Antibiotics, such as penicillin, kill bacteria, but they have no effect on viruses. Viruses hide and reproduce inside the body's own cells, so a drug that kills the virus tends to damage the cell as well, which is why antiviral drugs are difficult to make. Painkillers only relieve symptoms and have no effect on the pathogen itself.

Many drugs originally came from natural sources: digitalis, a heart drug, comes from foxgloves; aspirin comes from willow bark; and penicillin comes from mould, as discovered by Fleming. Modern drugs are designed in laboratories, but the starting point is often still a plant or microorganism.

Testing new drugs

Every new drug is tested for three things: its toxicity, whether it is safe; its efficacy, whether it works; and the correct dose. Testing follows a set sequence.

In preclinical testing, the drug is tested on cells, tissues and animals, and never on people.
In clinical trials, the drug is first given in very small doses to healthy volunteers to test its safety, then given to patients to find the optimum dose and to prove that it works.

PlaceboA placebo is a fake treatment containing no drug, used so that researchers can distinguish real effects of the drug from effects caused by patients' expectations.

Double-blind trialIn a double-blind trial, neither the patients nor the doctors know who has received the real drug, which removes bias from both sides.

🎯 Examiner tip: Six-mark questions on drug testing want the correct order, from cells, to animals, to healthy people, to patients, together with the reason each stage exists. The examiner is checking that students understand testing exists to protect people, ahead of profit.

Monoclonal antibodies (Triple) Separate science only

Monoclonal antibodies are identical copies of a single antibody, mass-produced to bind to one chosen antigen, and they can be targeted at almost any molecule.

They are made by first injecting a mouse with the chosen antigen. The mouse's white blood cells, or lymphocytes, then make the right antibody, but they will not divide in a dish, so one is fused with a fast-dividing tumour cell. The resulting fused hybridoma cell both divides continuously and makes the antibody. This cell is then cloned, and the antibodies are harvested.

In pregnancy tests, antibodies on the test strip bind to the pregnancy hormone HCG in urine.
In diagnosis and research, antibodies with fluorescent dyes attached can light up specific molecules or pathogens.
Monoclonal antibodies are used for measuring hormone and chemical levels in the blood.
In cancer treatment, they deliver drugs or radioactive substances directly to cancer cells, which carry distinctive antigens.

One drawback is that they caused more side effects than expected when first used in patients, so their use is not yet as widespread as had been hoped.

Plant disease (Triple) Separate science only

A sick plant can show several signs: stunted growth, spots on leaves, rot or decay, abnormal growths, discolouration, or visible pests. The disease can be identified using a gardening manual or website, laboratory testing of the pathogen, or testing kits that use monoclonal antibodies.

Nitrate deficiencyNitrate deficiency causes stunted growth, because nitrates are needed to build amino acids and proteins.

Magnesium deficiencyMagnesium deficiency causes leaves to turn yellow, a condition called chlorosis, which reduces photosynthesis, because magnesium is a central part of chlorophyll.

Plant defences

Physical defences include a waxy leaf cuticle, cellulose cell walls, and bark that peels away, taking any infection with it.
Chemical defences include antibacterial chemicals, such as those in mint and witch hazel, and poisons that deter herbivores, such as those in deadly nightshade and foxgloves.
Mechanical defences include thorns and hairs, leaves that droop or curl when touched, and mimicry, where a plant looks like something unappetising.

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