Infection and response

• Viruses


  Live and reproduce rapidly inside an organism's cells, can damage or destroy the cells
• Bacteria


  Reproduce rapidly inside organisms and may produce toxins that damage tissues and cause illness
• Communicable diseases


  Can be spread from one organism to another
• Plant diseases can also be directly caused by insects
• Aphids are insects that suck sap from the stems of plants, resulting in reduced rate of growth, wilting, and discolouration of leaves
• Ladybirds can be used to control aphid infestations as ladybird larvae eat aphids
• Signs that a plant is diseased

  • Stunted growth
  • Spots on leaves
  • Areas of rot or decay
  • Growths
  • Malformed stems or leaves
  • Discolouration
  • Pest infestation
• Ways of identifying plant diseases

  • Gardening manuals and websites
  • Laboratory testing of infected plants
  • Testing kits containing monoclonal antibodies
• Physical barriers


  • Cellulose cell walls - provide a barrier to infection
  • Tough waxy cuticle on leaves
  • Bark on trees - a layer of dead cells that can fall off
• Chemical barriers


  • Many plants produce antibacterial chemicals
  • Poison production stops animals eating plants
• Mechanical adaptations


  • Thorns and hairs stop animals eating plants
  • Leaves that droop or curl when touched to scare herbivores or dislodge insects
  • Some plants mimic the appearance of unhealthy or poisonous plants to deter insects or herbivores
• Ways to control the spread of communicable disease

  • Good hygiene
  • Isolating infected individuals
  • Controlling vectors
  • Vaccination
• Key terms


  • Aphid
  • Bacterium
  • Communicable disease
  • Fungicide
  • Fungus
  • Isolation
  • Mimic
  • Pathogen
  • Protist
  • Sexually transmitted disease (STD)
  • Toxin
  • Vaccination
  • Vector
  • Virus