What is a communicable disease?
A disease caused by a pathogen that can be transmitted, directly or indirectly, from one individual in a population to another.
What are the types of pathogens?
Bacteria, viruses, protocista, fungi
Protocista
- Unicellular eukaryote
- Size 1-150 um
- Protists that cause disease are parasitic so they use animals/humans as a host.
Fungi
- Eukaryotic unicellular or multicellular organism
- Has own distinct kingdom
- Cell size 2-30 um in diameter
- Grow as hyphae which can be several cm long
Bacteria
- Unicellular, prokaryotic
- Size 0.5-5 um in length
Virus
- An infective agent containing only genetic material, protein coat (capsid) only able to multiply within the living cells of a host.
Plant diseases
- Ring rot: a bacterial disease of potatoes, tomatoes and aubergines caused by a gram positive bacterium. It can destroy up to 80% of the crop and there is no cure, once bacterial rot infects a field it cannot be used to grow again for atleast 2 years.
- Tobacco mosaic virus: damages leaves, flowers and fruit which stunts growth and reduces yields. Resistant crops strains are available but there is no cure.
- Potato blight: caused by fungus like protocist. The hyphae penetrate host cells which destroys leaves, tubers and fruit causing million pounds worth of crop damage each year. There is non cure but resistant strains, careful management and chemical treatment can reduce infection risk.
- Black sigatoka: a banana disease caused by fungus which attacks and destroys leaves. The hyphae penetrate and digest the cells leaving the leaves black. If plants are infected it can cause a 50% reduction in the yield. Resistant strains are being developed and fungicide can be used.
Animal diseases
- Tuberculosis: a bacterual disease. TB damages and destroys lung tissue and suppresses the immune system so the body is less able to fight off other diseases. Poeple with HIV/aids are more likely to develope TB. TB is curable by antibiotics and preventable by improved healthcare.
- Bacterial meningitis: a bacterial infection of the meninges of the brain which can spread into the rest of the body. It maninly affects very young children and teenagers between 15 and 19, the main symptom of both is a blotchy rash. Antibiotics will cure it if it is delivered early enough.
- HIV/AIDS: a virus which targets T helper cells in the immune system. It gradually destroys the immune system so people are more suseptible to other diseases.
- Influenza: a viral infection of the cilitated epithelial cells in the gas exchange system, it kills them leaving airways open to secondary infection. There are 3 main strains A, B and C strain. Flu viruses mutate regulary so there is no cure however vulnerable people are vaccinated yearly.
- Malaria: caused by procotista Plasmodium and spread by the bites ofAnopheles mosquitoes. They reproduce inside the female mosquito. There is no vaccine against malaria and limited cures but preventative measures can be effective.
- Ring worm: a fungal disease affecting mammals. It causes grey-white, crusty, infectious, circular areas of skin. antifungal creams can cure it.
- Athletes foot: human fungal disease. Ring worm grows and digests on the warm moist skin between the toes. It causes cracking and scaling, antifungal creams can be used.
Factors affecting transmission of communicable diseases in humans/animals
- Overcrowding
- Poor nutrition
- Cultural factors
- Lack of sewege system / waste disposal
- No / incomplete immunisation
- Shortage of trained medical professionals / underfunded healthcare systems
- Lack of public awareness / public education
- Climate change
Factors affecting transmission of communicable diseases in plants
- Overcrowding (monoculture / agriculture)
- Poor mineral nutrition
- Climate (damp and warm conditions)
- Climate change (migration of vectors to new areas)
Physical and chemical plants barriers
Physical barriers:
- Waxy cuticle
Cellulose cell wall (can be lignified)
Closing stomata
Casparian strip in endodermis of root tissue
Bark
Chemical barriers:
Secretion of toxins:
Eg in mint, garlic, cinnamon, tea tree oil, aloe vera
Secretion of enzyme inhibitors:
Eg cellulase inhibitors
Sticky resin in Bark:
May contain antibacterial compounds
Secretion of compound that promotes growth of microorganism in competition with pathogen
Receptors on cell surface that detect pathogen and activate plant defences
Active plant defence mechanisms
- Hypersensitivity = immediate death of tissues surrounding the site of infection.
Physical plant defences
- When plants are attacked they produce high levels of the polysaccharide called callose, which contains B 1,3 linkages and B 1,6 linkages between the glucose monomers.
Callose:
- Deposited between cell membrane and cell wall. (in plasmodesmata and in sieve plate pores of phloem tube elements)
- Blocks movement of pathogen through the plant.
Plant chemical defences
- Insect repellents e.g. pine resin, Citronella.
- Insecticides
- Antibacterial compounds including antibiotics.
- Antifungal compounds
- Anti oomycetes e.g. Glucanases which break down glucans
- General toxins
Defence against pathogens in animals - what are the 2 lines of defence?
- Innate immunity:
- Non specific rapid response to a range of pathogens.
- Primary defences (stop pathogens from entering the system)
- Secondary systems (killing the pathogen when its entered the system) - Acquired immunity:
- Specific immune response targeted at one specific animal and developed over the animals lifetime. Acquired by exposure to the antigen.
Primary non specific defences (innate immunity)
- 2 types of surfaces cover the body. Cutaneous memrane = skin, outer layer, dead. Mucous membranes = epithelial tissue which secretes mucas and lines body cavities and tubular organs.
- Expulsive reflexes = coughing and sneezing which is prompted by pathogens irritating the lining of airways. Vomiting.
- Clotting mechanism = stops bleeding and prevents entry of pathogens.
- Inflammatory response.
Inflammatory response
- Triggered by entry of pathogens.
- Mast cells release histamine
- histamine causes vasodilation
- This increases blood flow to area
- capillary walls become more permeable
- easier for phagocytes to leave the blood.
- Mast cells release cytokines
- Cytokines attract phagocytes
- Increases temperature in area
Non specific defences getting rid of pathogens
- Fevers: normal body temp is around 37 degrees celsius, when pathogen enters the body the hypothalmus cuases temperature to go up.
- Phagocytes: engulf and destroy pathogens. There are 2 main type of phagocytes (neutrophils and macrophages).
Stages of phagocytosis
- Pathogens produces chemicals which attract phagocytes.
- Phagocytes recognise non human proteins proteins on the pathogen.
- The phagocyte engulfs the pathogen and encloses it in a vacum called a phagosome.
- The phagosome combines with a lysosome to form a phagolysosome
- Enzymes from the lysosome digest and destroy the pathogen.
Cytokines and oposins
- Phagocytes hwich have engulfed a pathogen produce chemicals called cytokines which act as cell signalling molecules.
- This informs other phagocytes that the body is under attack and stimulates them to move to the site of infection.
- Oposins are chemicals which aid phagocytosis
- They bind to microbes making them more susceptibe to phagocytosis.
Antibodies
- antibodies are Y shaped glycoproteins called immunoglobins which bind to a specific antigen on the pathogen or otxin that has triggered the immune response.
- Antibodies are made of 2 identical long polypeptide chains called heavy chains and 2 much shorter identical chains called the light chains. The chains are held together by disulphide bridges and there are disulphide bridges within the polypeptide chains holding them in shape.
- Antibodies bind to antigens with the protein based lock and key mechanism. The binding site is an area of 110 amino acids on both the heavy and the light chains, known as the variable region.
- When antibody binds to antigen it forms a antigen-antibody complex
How antibodies defend the body
- The antibody of the antigen-antibody complex acts as an opsonin so the complex is easily engulfed and digested by phagocytes.
- Most pathogens can no longer effectively invade host cells once they are apart of the antigen-antibody complex.
- Antibodies act as agglutinins causing pathogens carrying aa complexes to clump together.
- antibodies can act as antitoxins.
T lymphocytes
- T lymphocytes mature in the Thymus gland.
The main types of T lymphocytes:
- T helper cells: these have CD4 receptors on their cell surface membranes which bind to the surface antigens on APCs. They produce interluekins, which are a type of cytokine. The interluekins made by the T helper cells stimulate the activity of B cells, this increases antibody production, stimulates production of other T helper cells and stimulates microphages to ingest pathogens.
- T killer cells: these destroy the pathogen carrying the antigen. They produce the chemical called perforin, which kills the pathogen by making holes in the cell membrane.
- T memory cells: these live for a long time and are part of the immunological memory. If they meet an antigen a second time they divide rapidly to form a huge number of clones of T killer cells that destroy the pathogen.
B lymphocytes
- Mature in the bone marrow
The main types of B lymphocytes:
- Plasma cells: these produce antibodies to a particular antigen and release them into the circulation.
- B effector cells: these divide to form the plasma cell clones.
- B memory cells: these live for a very long time and provide the immunological memory. They are programmed to remember a specific antigen and enable the body to make a specific repsonse when a pathogen carrying that antigen is encountered again.
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Biological molecules (water) 2.1.246
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Biological molecules 2.1.2 Carbohydrates56
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Biological molecules 2.1.2 Lipids23
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Biological molecules proteins 2.1.242
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Membranes 2.1.538
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Enzymes 2.1.436
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2.1.6 cell division etc56
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2.1.3 Nucleotides and nucleic acids20
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3.1.1 Exchange surfaces26
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3.1.2 transport in animals28
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4.1.1 disease31
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4.2.2 classification and evolution27
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4.2.1 Biodiversity18
