AQA GCSE Biology · Topic 8
How disease spreads, viral, bacterial, fungal and protist pathogens, plus plant diseases and defences.
166 questions 7 subtopics Spec 4.3
Every subtopic below has its own questions, mark schemes and notes once you start.
Communicable diseases are caused by pathogens that pass from one organism to another. There are four kinds of pathogen, three main routes they travel by, and four broad ways of blocking them.
Three viral diseases are named in the specification: measles and HIV in humans, and tobacco mosaic virus in plants.
Two bacterial diseases are named: Salmonella food poisoning, taken in with contaminated food, and gonorrhoea, a sexually transmitted disease that is becoming harder to treat.
Rose black spot is the named fungal disease. Athlete's foot is the fungal example used when transmission by direct contact is being illustrated.
Malaria is the named protist disease. The protist is the pathogen and the mosquito is the vector that carries it between hosts.
Plants can be damaged by pathogens, by insect pests and by a shortage of mineral ions. Telling these apart from the visible signs is the skill being tested.
Plants defend themselves in three ways: physical barriers that keep pathogens out, chemicals that kill or deter attackers, and mechanical features that stop animals eating them.
Key definitions, in exam-ready wording.
The mistakes that come up again and again in this topic.
Bacteria make you ill by releasing toxins; viruses make you ill by damaging the cells they reproduce inside. Do not use one explanation for the other.
A vector is not a pathogen. The mosquito carries malaria, the protist causes it.
The definition of a vector needs the words 'without getting the disease itself' to score.
HIV is the virus; AIDS is the late stage of the infection. They are not interchangeable.
For TMV, always go through reduced photosynthesis before you mention reduced growth, the middle step carries a mark.
Measles is viral, so antibiotics would not treat it.
It is the bacteria that become resistant, not the patient. Never write that a person became immune to an antibiotic.
Salmonella symptoms are caused by toxins, say so rather than just naming the bacteria.
Vaccinating poultry protects people, by keeping the bacteria out of the food chain.
Antibiotics do not work on fungi. Rose black spot needs a fungicide.
Removing the diseased leaves is only half the treatment, they must be destroyed as well.
Rose black spot is a plant disease but it is fungal, not viral. Tobacco mosaic virus is the viral one.
The mosquito is the vector, not the cause. Saying malaria is caused by mosquitoes loses the mark.
The symptom is repeating episodes of fever, the word repeating matters.
Match the control measure to the transmission route before you suggest it.
Chlorophyll is the pigment; chlorosis is the condition caused by a lack of it. Read the question carefully.
Nitrate goes with stunted growth, magnesium with yellow leaves. Learn them as a pair.
Aphids are insects, not pathogens. An infestation is not an infection.
Mimicry and drooping leaves are mechanical defences, not chemical ones, nothing is being poisoned.
If a question asks for three physical defences, name the structures. Writing 'a barrier' three times scores once.
Plants do not have white blood cells or an immune system like ours; their defences are barriers, chemicals and mechanical tricks.
166 questions on Communicable disease and plant defences, marked against the scheme. Anything you get stuck on will be scheduled to come back sooner.