B3

• Pathogens are microorganisms such as viruses and bacteria that cause infectious diseases in animals and plants. They depend on their host to provide the conditions and nutrients that they need to grow and reproduce. They frequently produce toxins that damage tissues and make us feel ill
• Diseases caused by viruses, bacteria, protists and fungi are spread in animals and plants
• The spread of diseases can be reduced or prevented
• Bacteria and viruses may reproduce rapidly inside the body
• Bacteria may produce poisons (toxins) that damage tissues and make us feel ill
• Viruses live and reproduce inside cells, causing cell damage
• Measles is a viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise. The measles virus is spread by inhalation of droplets from sneezes and coughs
• HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body's immune cells. Late stage HIV infection, or AIDS, occurs when the body's immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles
• Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive 'mosaic' pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis
• Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete
• Gonorrhoea is a sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain on urinating. It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared. Gonorrhoea is spread by sexual contact
• Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth of the plant as photosynthesis is reduced. It is spread in the environment by water or wind
• The pathogens that cause malaria are protists. The malarial protist has a life cycle that includes the mosquito. Malaria causes recurrent episodes of fever and can be fatal. The spread of malaria is controlled by preventing the vectors, mosquitos, from breeding and by using mosquito nets to avoid being bitten
• Non-specific defence systems of the human body against pathogens


  • Skin
  • Nose
  • Trachea and bronchi
  • Stomach
• If a pathogen enters the body the immune system tries to destroy the pathogen
• How white blood cells help to defend against pathogens


  • Phagocytosis
  • Antibody production
  • Antitoxin production
• Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies. If the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection
• Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. It is important that specific bacteria should be treated by specific antibiotics
• The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. However, the emergence of strains resistant to antibiotics is of great concern
• Antibiotics cannot kill viral pathogens
• Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens
• It is difficult to develop drugs that kill viruses without also damaging the body's tissues
• Traditionally drugs were extracted from plants and microorganisms
• Most new drugs are synthesised by chemists in the pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant
• New medical drugs have to be tested and trialled before being used to check that they are safe and effective

  1. Preclinical testing is done in a laboratory using cells, tissues and live animals

  2. Clinical trials use healthy volunteers and patients

  3. Very low doses of the drug are given at the start of the clinical trial

  4. If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug

  5. In double blind trials, some patients are given a placebo
• Monoclonal antibodies are produced from a single clone of cells. The antibodies are specific to one binding site on one protein antigen and so are able to target a specific chemical or specific cells in the body
• Producing monoclonal antibodies


  1. Stimulating mouse lymphocytes to make a particular antibody

  2. Combining the lymphocytes with a particular kind of tumour cell to make a hybridoma cell

  3. Cloning single hybridoma cells to produce many identical cells that all produce the same antibody

  4. Collecting and purifying a large amount of the antibody
• Some ways in which monoclonal antibodies can be used


  • For diagnosis such as in pregnancy tests
  • In laboratories to measure the levels of hormones and other chemicals in blood, or to detect pathogens
  • In research to locate or identify specific molecules in a cell or tissue by binding to them with a fluorescent dye
  • To treat some diseases: for cancer the monoclonal antibody can be bound to a radioactive substance, a toxic drug or a chemical which stops cells growing and dividing. It delivers the substance to the cancer cells without harming other cells in the body
• Monoclonal antibodies create more side effects than expected. They are not yet as widely used as everyone hoped when they were first developed
• Plant diseases can be detected by

  • Stunted growth
  • Spots on leaves
  • Areas of decay (rot)
  • Growths
  • Malformed stems or leaves
  • Discolouration
  • The presence of pests
• Identifying plant diseases


  1. Reference to a gardening manual or website

  2. Taking infected plants to a laboratory to identify the pathogen

  3. Using testing kits that contain monoclonal antibodies
• Plants can be infected by a range of viral, bacterial and fungal pathogens as well as by insects
• Plants can be damaged by a range of ion deficiency conditions: stunted growth caused by nitrate deficiency, chlorosis caused by magnesium deficiency
• Knowledge of ions is limited to nitrate ions needed for protein synthesis and therefore growth, and magnesium ions needed to make chlorophyll
• Pathogens that can infect plants

  • Viral
  • Bacterial
  • Fungal
  • Insects
• Tobacco mosaic virus


  Viral disease
• Black spot


  Fungal disease
• Ion deficiency conditions that can damage plants

  • Stunted growth caused by nitrate deficiency
  • Chlorosis caused by magnesium deficiency
• Nitrate ions


  Needed for protein synthesis and growth
• Magnesium ions


  Needed to make chlorophyll