Module 4 Biology

• Communicable diseases

  Diseases that can spread between organisms
• Viruses

  • Acellular
  • Have genetic material (RNA or DNA) but no organelles or nucleus
  • Cannot replicate without a host cell
• Viral diseases

  • HIV/AIDS (affects humans, spread through sexual contact)
  • Influenza (affects animals including humans, spread through infected droplets)
  • Tobacco mosaic virus (affects plants, spread by direct contact)
• Protoctist diseases

  • Malaria (affects animals including humans, spread by a vector (mosquitos))
  • Potato (or tomato) late blight (affects potatoes or tomatoes, produces spores during reproduction that are spread in the air to other hosts)
• Fungal diseases

  • Black sigatoka (affects bananas, spread via spores)
  • Ringworm (affects cattle)
  • Athlete's foot (affects humans, spread by direct and indirect contact)
• Bacterial diseases

  • Tuberculosis (TB) (affects animals including humans, spread via infected droplets)
  • Bacterial meningitis (affects humans)
  • Ring rot (affects potatoes and tomatoes, spread through direct contact)
• Vector

  An organism which carries a pathogen without getting symptoms of the disease
• Transmission

  • Direct transmission (directly from one organism to another, e.g. through infected droplets, sexual contact, touch)
  • Indirect transmission (from one organism to another via an intermediate, e.g. through a vector, spores, through the air or food)
• Climate affects transmission

  Having the ideal temperature for a vector to survive and reproduce
• Living conditions affect transmission

  • Overcrowding makes transmission easier
  • Poor hygiene practices e.g. not washing hands
• Social factors affect transmission

  • Lack of sexual education and hygiene education
  • Lack of good healthcare facilities and poor drug or vaccine availability
• Measures to decrease transmission

  • Mask wearing and improving ventilation
  • Meeting less people
  • Destroying vectors
  • Quarantine of infected individuals
  • Increasing hygiene practices e.g. hand washing
  • Restrict travel
• Plant defences against pathogens

  • Cell walls act as a physical barrier
  • Waxy cuticles are a physical barrier
  • Production of toxic antimicrobial chemicals
  • Leaf drop to prevent pathogens from spreading
• Animal primary non-specific defences

  • Skin (physical barrier, produces antimicrobial chemicals)
  • Blood clotting
  • Wound repair
  • Inflammation
  • Expulsive reflexes
  • Mucous membranes
• Antigens

  Molecules found on a cell surface which identify the cell, can be on pathogens, abnormal cells, cells from other organisms, or toxins
• Phagocytosis

  1. Pathogens produce chemicals which attract phagocytes

  2. Phagocytes recognise the foreign antigens on pathogens

  3. Phagocytes engulf pathogens

  4. Pathogen is contained in a phagosome

  5. Phagosome fuses with a lysosome to form a phagolysosome

  6. Pathogen is digested by lysozymes from the lysosome

  7. Antigens from the pathogen are presented on the plasma membrane of the phagocyte
• Types of white blood cells

  • Neutrophils
  • Monocytes
  • Lymphocytes
• Opsonins

  Antibodies which bind to antigens of pathogens to increase recognition by phagocytes
• Agglutinins

  Antibodies which clump pathogens together so that many can be engulfed at once
• Cytokines

  Molecules released by T helper cells which attract phagocytes
• T lymphocyte activation

  1. T lymphocytes have receptors on their surface which are complementary to one specific antigen

  2. Binding of an antigen to receptors on a T helper cell activates the specific T helper cell (clonal selection)

  3. The T helper cell divides by mitosis to produce clones of itself (clonal expansion)
• B lymphocyte activation

  1. B lymphocytes have antibodies on their surface which are complementary to one specific antigen

  2. Binding activates the specific B lymphocyte (clonal selection)

  3. The B lymphocyte divides by mitosis and produces clones of itself (clonal expansion)

  4. The clones differentiate into plasma cells and B memory cells
• Types of T cells

  • T killer cells
  • T helper cells
  • T memory cells
  • T regulatory cells
• Antibodies

  • Proteins consisting of four polypeptide chains
  • Variable regions bind to a specific complementary antigen
  • Constant regions bind to plasma membranes of immune system cells
  • Hinge regions give flexibility
  • Disulfide bridges between the heavy chains and light chains form the quaternary structure
• Anti-toxins

  Antibodies that can bind to toxins to inactivate them and cause them to be engulfed by phagocytes
• Primary response

  The immune response to a new foreign antigen
• Secondary response

  The immune response to a known foreign antigen
• Autoimmune disease

  A disease in which the antigens on an individual's own cells are recognised as non-self (foreign) by the immune system, resulting in an abnormal immune response
• Examples of autoimmune diseases

  • Lupus
  • Rheumatoid arthritis
• Vaccination

  Vaccines contain a safe form of an antigen
• Passive immunity

  A baby receiving antibodies from the mother through the placenta or breastmilk
• Artificial passive immunity

  Injecting somebody with antibodies
• Active immunity

  Lymphocytes are activated and memory cells are produced
• Secondary response

  • A higher concentration of antibodies is produced more quickly
• Passive immunity

  Quicker than active immunity because the body does not need to produce its own antibodies, but the protection is short term because no memory cells are produced
• The immune response is influenced by genetics (e.g. alleles for immune cells antibodies) and environment (e.g. diet and exposure to pathogens)
• Vaccination

  Injecting a safe form of an antigen to trigger the primary immune response, resulting in antibodies and memory cells being produced
• Immunisation

  The process of developing immunity
• Sources of medicines

  • Microorganisms
  • Plants
• Medicinal drugs

  • Penicillin (an antibiotic) from a fungus
  • Compound in St. John's wort (a plant) acts as an antidepressant