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AQA GCSE Biology 8461

3. Infection and Response

Communicable disease and plant defences

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.

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What communicable disease and plant defences covers

Every subtopic below has its own questions, mark schemes and notes.

4.3.1.1Pathogens and how disease spreads

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.

4.3.1.2Viral diseases

Three viral diseases are named in the specification: measles and HIV in humans, and tobacco mosaic virus in plants.

4.3.1.3Bacterial diseases

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.

4.3.1.4Fungal diseases

Rose black spot is the named fungal disease. Athlete's foot is the fungal example used when transmission by direct contact is being illustrated.

4.3.1.5Protist diseases

Malaria is the named protist disease. The protist is the pathogen and the mosquito is the vector that carries it between hosts.

4.3.3.1Plant diseases and deficiencies

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.

4.3.3.2Plant defence responses

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 terms

The definitions examiners expect, in the wording that earns the mark.

Pathogen — A microorganism that infects another organism and causes disease.
Communicable disease — A disease that can be passed from one organism to another.
Toxin — A poison released by bacteria that damages the host's cells and tissues.
Vector — An organism that carries a pathogen between hosts without getting the disease itself.
Parasite — An organism that lives on or inside another organism and causes it damage.
Hyphae — The thread-like structures that make up the body of many fungi.
Encephalitis — Inflammation of the brain, a possible complication of measles.
Antiretroviral drug — A drug that stops HIV replicating inside the body.
AIDS — Late stage HIV infection, when the immune system is too damaged to fight other infections.
Mosaic pattern — The patchy discolouration on leaves caused by tobacco mosaic virus.
Salmonella — A bacterium that causes food poisoning.
STD — A sexually transmitted disease, passed on by sexual contact.
Antibiotic resistance — When bacteria are no longer killed by an antibiotic that used to work against them.
Barrier method — Contraception such as a condom that physically blocks contact, and so also blocks pathogen transfer.
Fungicide — A chemical used to kill fungi, for example on an infected plant.
Spore — The reproductive structure produced by fungal hyphae, which spreads the fungus to new hosts.
Protist — A eukaryotic organism, usually single-celled, some of which are parasites that cause disease.
Malaria — A disease caused by a protist and carried between hosts by mosquitoes, causing recurrent fever.
Chlorosis — The yellowing of leaves caused by too little chlorophyll, usually from magnesium deficiency.
Stunted growth — Growth that is much smaller than normal, a sign of nitrate deficiency or of disease.
Infestation — Damage to a plant caused by insect pests such as aphids, rather than by a pathogen.
Waxy cuticle — The waterproof layer covering leaves and stems that acts as a barrier to pathogens.
Cellulose — The substance plant cell walls are made from.
Herbivore — An animal that eats plants.
Mimicry — A defence in which a plant looks like another organism or object to avoid being eaten.

Where marks get lost

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.

Other topics

Cell biology Transport in cells Cell division Organisation in animals Enzymes and the human digestive system Plant tissues, organs and systems Non-communicable disease Preventing and treating disease Monoclonal antibodies Photosynthesis Respiration Homeostasis and the nervous system Hormonal coordination DNA, genes and variation Reproduction Evolution and Classification Adaptations, interdependence and competition Organisation of an ecosystem Humans and biodiversity Working scientifically Practical skills

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