Chapter 5

• Viruses


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


  • Inhalation of droplets produced by infected people when sneezing and coughing
  • Sexual contact
  • Exchange of body fluids (e.g. blood when drug users share needles)
  • Direct contact of plants with infected plant material
  • Animal and plant vectors
  • Soil
• Viral diseases


  • Measles
  • HIV (human immunodeficiency virus)
  • TMV (tobacco mosaic virus - plants)
• Measles


  Fever, red skin rash, complications can be fatal - young children are vaccinated to immunise them against measles
• HIV (human immunodeficiency virus)


  Flu-like symptoms at first, virus attacks the body's immune cells, can lead to AIDS - where the immune system is so damaged that it cannot fight off infections or cancers
• TMV (tobacco mosaic virus - plants)


  Mosaic pattern of discolouration on the leaves - where chlorophyll is destroyed, reduces plant's ability to photosynthesise, affecting growth
• Bacteria


  Reproduce rapidly inside organisms and may produce toxins that damage tissues and cause illness
• Spread of bacteria


  • Bacteria in or on food that is being ingested
  • Direct sexual contact - gonorrhoea is a sexually transmitted disease (STD)
• Bacterial diseases


  • Salmonella
  • Gonorrhoea
• Salmonella


  Salmonella bacteria and the toxins they produce cause fever, abdominal cramps, vomiting, diarrhoea
• Gonorrhoea


  Thick yellow or green discharge from the vagina or penis, pain when urinating, treatment with antibiotics (many antibiotic-resistant strains have appeared)
• Communicable diseases can be spread from one organism to another
• Spread of communicable diseases


  • Air
  • Water
  • Direct contact
• Fungi


  Spread by water and wind
• Rose black spot


  Purple or black spots on leaves, which turn yellow and drop early, reduces plant's ability to photosynthesise, affecting growth
• Prevention and treatment of rose black spot


  Fungicides, affected leaves removed and destroyed
• Protists


  Malaria - caused by a protist pathogen that is spread from person to person by mosquito bites, and causes recurrent fevers
• Aphids


  Insects that suck sap from the stems of plants, resulting in reduced rate of growth, wilting, discolouration of leaves
• Controlling aphids


  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 defences of plants


  • Cellulose cell walls
  • Tough waxy cuticle on leaves
  • Bark on trees - a layer of dead cells that can fall off
• Chemical defences of plants


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


  • 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
• Methods of controlling the spread of communicable disease


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