AQA Biology GCSE: Infection and Response

Study Notes

Pathogens (viruses, bacteria, protists, and fungi) are microorganisms that cause infectious diseases and can infect plants or animals through direct contact, water, or air.
Viruses:
- Very small
- Move into cells and use cell biochemistry to replicate
- Cause cell bursting and release of replicated viruses into the bloodstream
- Damage and destruction of cells make the individual feel ill
Bacteria:
- Small
- Multiply quickly through binary fission
- Produce toxins that can damage cells
Protists:
- Some are parasitic, using humans and animals as hosts, living on and inside them, causing damage
Immunization:
- Reduces the spread of pathogens by reducing the number of people susceptible to infection (herd immunity)
- Vaccines contain a dead or inactivated form of the pathogen, stimulating white blood cells to produce antibodies

Advantages:
- Eradicated many diseases (e.g., smallpox) and reduced the occurrence of many others (e.g., rubella)
- Epidemics can be prevented through herd immunity
Disadvantages:
- Not always effective in providing immunity
- Bad reactions (e.g., fevers) can occur in response to vaccines (although rare)

Antibiotics:
- Medicines that kill bacterial pathogens inside the body without damaging body cells
- Cannot kill viruses as they use body cells to reproduce
Painkillers:
- Treat symptoms of the disease, not the cause
- Examples: aspirin (originates from willow), digitalis (originates from foxgloves)

Penicillin:
- Discovered by Alexander Fleming
- Produced by Penicillium mould, which kills bacteria

Preclinical testing:
- Uses cells, tissues, and live animals to test safety and efficacy
Clinical testing:
- Uses healthy volunteers with a low dose to ensure no harmful side effects
- Tests on patients to find the most effective dose
- Compares results to a placebo group to observe the effect of the new drug

Common signs of plant diseases:
- Stunted growth (indicating nitrate deficiency)
- Spots on leaves (indicating black spot fungus on roses)
- Areas of decay (black spot fungus on roses, blights on potatoes)
- Abnormal growths (crown galls caused by bacterial infection)
- Malformed stems or leaves (due to aphid infestation)
- Discolouration (indicating magnesium deficiency, or tobacco mosaic virus)
- Pests on leaves (such as caterpillars)
Identification methods:
- Using a gardening manual or website
- Observing the infected plant in a laboratory
- Using monoclonal antibodies in testing to identify the pathogen

Physical defences:
- Tough, waxy cuticle stops entry into leaves
- Cellulose cell walls form a physical barrier into cells
- Dead cells around stems (such as bark) which stop pathogens entering
Chemical defences:
- Poisons (e.g., from foxgloves, tobacco plants, deadly nightshades, yew) deter herbivores
- Antibacterial compounds (e.g., mint plant, witch hazel) kill bacteria
Mechanical defences:
- Thorns and hairs make it difficult and painful for animals to eat them
- Leaves can droop or curl when touched, moving insects off
- Mimicry to trick animals