4.1.1 Diseases

• Communicable disease is caused by pathogens (eg bacteria, viruses, protoctista and fungi). They cause harm through directly damaging tissue or releasing toxins.
• Prokaryotic cells (bacteria) are classified by their shape or cell wall structure:

  • Rod (bacilli)
  • Spherical (cocci)
  • Comma (vibrio)
  • Spiralled (spirilla)
  • Corkscrew (spirochaetes)

  The two types of cell wall determine whether a bacteria is Gram-positive or Gram-negative.
• 1. HIV is transported in the blood until it attaches the CD4 protein on the helper T cells.
  2. Protein capsule then fuses with the helper T membrane so RNA and enzymes from the HIV enters.
  3. Enzyme reverse transcriptase copies viral RNA into DNA copy and moves to helper T cell nucleus (hence the name retrovirus).
  4. mRNA is transcribed and helper T cell starts to create viral proteins to make new viral molecules.
  5. Helper T cells destroyed by virus. Host unable to produce adequate immune response to other pathogens and is left vulnerable to infections and cancer. Destruction of immune system = death.
• Bacterium pathogens - Tuberculosis:

  • Pathogens - M.tuberculosis and M.bovis.
  • Transmission - Direct, airborne droplets.
  • What? - Infects lungs causing a chronic cough and bloody mucus.
  • Extra? M.bovis commonly affects cattle but can transmit to humans.
• Bacterium pathogens - Bacterial meningitis:

  • Pathogen - Neisseria meningitidis.
  • Transmission - Direct, droplets in the air and exchange of fluids.
  • What? - Pathogen passes through the meninges (tissue around the brain and spinal cord keeping bacteria out) causing acute inflammation. This causes symptoms of headaches, fevers, neck stiffness and a characteristic rash.
• Bacterium pathogens - Ring rot:

  • Pathogen - Clavibacter michiganesis.
  • Host - Potatoes and tomatoes.
  • Transmission - Direct, contact with infected tubers. Cultivation helps the spread of the bacteria as it remains on the machinery.
  • What? - Bacteria infects the vascular tissue and block it so that less water reaches the leaves and they wilt. It continues to spread through the vascular tissue and stem into the developing tubers. Because the vascular tissue in tubers are arranged as rings, it makes the disease appear as a black ring surrounding the plant.
• Viruses do not have cellular structure, meaning they cannot respire, produce ATP, replicate genetic materials or synthesise proteins. They infect host cells and hijack their machinery to replicate their own genetic materials and proteins, creating viral DNA and proteins.
• Tobacco mosaic was the first ever discovered virus and was found to cause a distinct yellowing pattern of the leaves, producing a mosaic pattern. The pathogen causing it was one of the more simpler viruses with a rod of protein surrounding a coil of single-stranded RNA (which codes for 4 proteins).
• Virus - HIV / AIDS:

  • Pathogen - Human immunodeficiency virus.
  • Transmission - Direct, exchange of bodily fluids.
  • What? - Infects specific cells of the immune system. It is a retrovirus (can make DNA from RNA because of its reverse transcriptase).
• Virus - Influenza:

  • Pathogen - Influenza A, B and C.
  • Transmission - Direct, droplets in the air.
  • What? - Most common is type A, a capsid that surrounds 8 single-stranded molecules of RNA which codes for 11 genes. Infects the cells linking the airways of the gas exchange system. As they leave their host, they are enveloped in a phospholipid bilayer derived from the cell surface membrane. This contains two glycoproteins (haemagglutinin and neuraminidase) which are involved in infecting new host cells and are coded by the genes in the viral RNA.
• Protoctista - Malaria:

  • Pathogen - Plasmodium falciparum or P.malariae.
  • Transmission - Indirect, female Anopheles mosquito.
  • What? - Parasite goes through some of the stages in its life cycle in humans it infects. The infected individuals may experience fever, chills and fatigue.
• Fungi are eukaryotic organisms that have a structure like plants, with cell walls and large central vacuoles. Instead of being made up of separate cells, it is composed of many filaments known as hyphae that form an extensive network throughout the soil, or (in the case of parasitic fungi) over the surface / within the body of their hosts
• Parasitic fungi - Potato blight:

  • Pathogen - P.infestans (CLASSIFIED AS PROTOCTIST BUT SHARE FEATURES WITH FUNGI).
  • Transmission - Direct, spores.
  • What? - Has hyphae (like mould fungi) but also has features not associated with fungi - walls of hyphae are made of cellulose and not chitin. The main substance storage is starch, not glycogen, and it has motile spores that have flagella. These swim through water in the soil and on the surface of plants. The first signs of potato blight are small, dark brown marks on the leaves which increase in size and number. Plants are left completely inedible.
• Fungi - Cattle ringworm:

  • Pathogen - Trichophyton verrucosum.
  • Transmission - Direct, contact with infected cattle.
• Fungi - Athletes foot:

  • Pathogen - Epidermophyton floccosum.
  • Transmission - Both, direct skin to skin contact, or indirect contact with towels used by infected people.
• Fungi - Black sigatoka:

  • Pathogen - Mycosphaerella fijienesis.
  • Transmission - Direct, spores dispersed through air.
  • What? - Fungal disease in bananas that spreads through the leaves, reducing the ability to photosynthesise. The lack of photosynthesis causes parts of the leaf to die which created black streaks. This continues until the whole leaf dies.
• Direct transmission of disease:

  1. Direct contact - Touching, kissing, contact with cuts in skin and sexual contact.
  2. Inoculation - Animal bites, sharing needles and cuts in the skin.
  3. Ingestion - Drinking and eating contaminated water and foods.
• Indirect transmission of disease:

  1. Vectors - Usually animals which pass on the pathogen to humans.
  2. Droplets - Pathogens transmitted in droplets of water (eg - saliva and mucus expelled when sneezing).
  3. Fomites - Dirty bedding, socks and cosmetics are inanimate objects that can carry and transmit pathogens.
• Certain living conditions may make disease transmission more likely:

  • Hot climates - Increased heat provides more kinetic energy for chemical reactions and reproduction.
  • Social factors (poverty / developing countries) - Could result in poorer sewage infrastructure, a lack of fresh water and food, poorer sanitation and overcrowded living quarters. Medicines and vaccines being less readily available to prevent the spread.
• Factors influencing transmission of communicable diseases:

  • Infected individuals - Pathogen is present and so can infect new hosts. An endemic is when a disease is always present in a population.
  • Resistance - Inherited genes coding for a defence mechanism to a specific disease (eg - people who are heterozygous for the sickle cell allele are resistant to malaria). This means that when they are exposed to the pathogen for the first time, it does not develop inside their bodies and does not cause any symptoms.
• Factors affecting direct transmission:

  • Proximity - Diseases spread by droplets in the air depend on individuals being in close proximity to one another. For example, there are often higher rates of infection in schools than in the general population as pupils are all in close proximity.
  • Monoculture growths - Crops grown in high densities ensure the use of absorption of light energy. Under these conditions, leaves infected with TMV can easily spread to uninfected leaves as they touch.
• Factors affecting indirect transmission:

  • Vectors - Mosquitoes and Aphids are heavily influenced on the climate and by weather. Anopheles mosquitoes need small areas of water to breed leading to the transmission of malaria being higher in wetter and rainier climates. The development of the malarial parasite within mosquitoes is temperature dependant. If the temperature drops below 20 degrees, the parasites cannot complete their life cycle stages in the mosquitoes.
• Factors affecting transmission of human diseases:

  • Digestion - Many of the human diseases are diseases of poverty. For example, water-bore diseases (eg - cholera, typhoid and polio) spread when human faecal waste contaminate drinking water. This can happen in areas of poor housing where there is little to no sanitation.
  • Migration - Rapid transmission of disease occurs when they are bought into new populations with little to no natural resistance and no immunity. For example, smallpox from Europe travelled to America through Spanish conquistadores, which had a very big effect on death rates.
• Plants do not have blood or immune systems, so they use alternative mechanisms in response to pathogens and diseases. These are chemical and physical defences.
• Plant responses:

  Passive defence mechanisms are those which are present at all times. Some of these are physical barriers while others are chemical barriers.
• Physical defence mechanisms:

  • Waxy cuticle over the leaf epidermis.
  • Bark.
  • Cellulose cell walls.
  • Casparian strip in the endodermis in the root.
  • Stomata that open and close to stop pathogens from entering.
• Chemical defence mechanisms:

  • Secrete compounds that influence the growth of microorganisms that compete with the pathogens.
  • Secretion of compounds that are toxic to pathogens.
  • Secretion of inhibitors of enzymes (eg - cellulose used by pathogens to break down cell walls to gain entry to cells).
  • Receptor molecules on cell surface membranes that detect pathogens and activate plant defences.
  • Sticky resins in bark that prevent the spread of pathogens.
• Plant defences - Waxy cuticles:

  Reduces the chances of plant pathogens gaining entry or spreading very far inside a plant. Viruses and bacteria cannot enter through the cuticles unless there is a wound on the surface of the leaf or stem. Such wounds are made by herbivores such as caterpillars. Fungi, on the other hand, infect deep into the roots but usually are unable to grow any further because of the impenetrable Casparian strip.
• Plant responses - Chemicals:

  Plants secrete substances on the surface of leaves and roots making the environment too acidic for the growth of the pathogens. The leaves also secrete nutrients to support a community of microorganisms such as yeasts, which are harmless to the plant but compete successfully with any pathogens which present themselves on the surface of the leaves. Plants also create substances that are directly toxic to pathogens, such as catechol, and enzyme inhibitors, such as tannins.
• Plant responses - Active defence mechanisms:

  Hypersensitivity occurs in the presence of pathogens, where the tissue surrounding the site of infection is immediately killed off. This is highly effective as many pathogens require living host tissue to survive, but if the cells are dead there are no nutrients for the pathogen to grow. Plants also respond by creating physical barriers, for example when bacteria and fungi attempt to penetrate cell walls, it often stimulates the cell to produce compounds such as callose and lignin to thicken and reinforce them to make them harder to break.
• Plant responses - Cell signalling:

  Bacteria and fungal pathogens secrete cellulases to digest a pathway into cells. The breakdown products of cellulose hydrolysis act as signals that are detected by receptors on the surfaces of cells. Once detected, the breakdown products stimulate the production of phytoalexins, which are defence mechanisms that...

  • Disrupt the cell surface membrane of bacteria.
  • Stimulate the secretion of chitinases that break down the cell walls of the hyphae of fungal pathogens.
  • Disrupt metabolism and reproduction of the pathogen.
• Systematic acquired mechanisms are long-term responses to infections. An example of this is signalling molecules like Salicylic acid, which travel through plants to activate defence mechanisms in uninfected areas, giving them protection against many pathogens for some time after the original infection. Ethylene is another cell signalling compound, that when secreted by plants under attack from pathogens, it vaporises to stimulate other leaves of the same plant and surrounding plants.
• Plant responses - Callose:

  Callose is a polysaccharide between the cell surface membrane and cell wall that forms a matrix in which antimicrobial compounds can be deposited, such as hydrogen peroxides and phenols. It also reduces the diameter of the plasmodesmata which reduces the spread of viruses from cell to cell.
• Animal responses - Primary line of defences (non-specific):

  • Skin is a physical barrier and contains skin flora (healthy microorganisms), which outcompete pathogens for space and resources.
  • Mucous membranes.
  • Lysozomes are hydrolytic enzymes.
  • Expulsive refexes.
  • Inflammation in areas of damaged cells, triggering histamines and cytokines. The histamines cause the blood vessels to dilate and allow more blood to flow to the area, increasing the temperature of the blood which can kill pathogens. They also make the walls more permeable so white blood cells can be delivered to the site of damage.
• Phagocytes, like macrophages and neutrophils, travel in the blood and squeeze out of capillaries to engulf and digest pathogens. This is known as phagocytosis and is a non-specific response.
• The process of phagocytosis:

  1. Damaged cells and pathogens release cell-signalling chemicals (cytokines) that attract the phagocytes to the site of infection.
  2. An opsonin protein attaches to the pathogens to mark them and make them easier for the neutrophils and macrophages to engulf.
  3. Phagocytes have receptors which attach onto the chemicals on the surface of the pathogens.
  4. Phagocyte then engulfs the pathogen into a vesicle to create a phagosome.
• The process of phagocytosis - CONTINUES:

  5. Within the phagocytes are lysosomes which contain hydrolytic enzymes. The lysosomes fuse with the phagosome to expose the pathogen, and then it hydrolyses the pathogen any any soluble useful molecules are absorbed into the cytoplasm of the phagocyte.

  6. The phagocytes will then present the antigen of the digested pathogen on their surface - they are then called antigen-presenting cells.
• The second line of defence are the lymphocytes, which are a specific response to antigens. There at two types; B lymphocytes (B cells) and T lymphocytes (T cells). These are created in the bone marrow stem cells, but B cells mature in the bone marrow, whereas T cells mature in the thymus.
• Cell-mediated responses - T cells:

  Receptors on the T cells bind to antigens on the antigen-presenting cell (APC) which cause the T cell to divide rapidly by mitosis (clonal expansion).
• Antigen presenting cells are cells that present non-self antigens on their surface. These include:

  • Infected body cells presenting viral antigens on their surface.
  • A macrophage which has engulfed and destroyed a pathogen, presenting the antigens on their surface.
  • Cells of a transplanted organ will have a different shaped antigen on their surface compared to their self-cell antigens.
  • Cancer cells will have abnormal shaped self-antigens.