B12 - Communicable diseases

Question 1

What are communicable diseases?

Answer 1

• diseases that are caused by pathogens (infective microorganisms)
• vectors carry pathogens between organisms

Question 2

What are the types of pathogens?

Answer 2

• bacteria
• viruses
• fungi
• protoctista

Question 3

What is bacteria?

Answer 3

• prokaryotes
• do not have membrane-bound organelles or a nucleus
  ‎
• can be classified by:
  
  • shape = rod, spherical, comma, spiralled, corkscrew
  • cell walls = react differently with Gram staining (affects how bacteria reacts to diff. antibiotics)
  • +ve = purple-blue
  • -ve = red

Question 4

What are viruses?

Answer 4

• non-living infectious agents
• much smaller than bacterium
• structure:
  
  • DNA/RNA is surrounded by protein
    ‎
• invade living cells and genetic material of virus takes over biochem of host cell to produce more viruses
• bacteriophages = attack bacteria
  
  • use bacteria to replicate/destroy them
  • they can be used to treat some diseases

Question 5

What is protoctista/protista?

Answer 5

• group of eukaryotic organisms
• single-celled/colonies
• small % act as pathogens
  
  • pathogenic protists = need vector to transfer them to their host
  • e.g. malaria

Question 6

What is fungi?

Answer 6

• not a major problem in animals
• eukaryotic organisms (multicellular)
• cannot photosynthesise and digest food extracellularly
  
  • most are saprophytes (feed on dead/decaying matter)
    ‎
• some are parasitic and can cause plants to stop photosynthesising (leaves affected)
• produce tiny fungal spores which can spread over large distances

Question 7

How can pathogens cause damage?

Answer 7

• viruses:
  
  • take over cell metabolism
  • genetic material inserted into host DNA
  • uses cell to make new viruses
• protoctista:
  
  • take over cells and break them open
  • digest and use cell contents to reproduce
• fungi:
  
  • digest living cells and destroy them
  • some produce toxins that affect host cells
• bacteria:
  
  • produce toxins that damage/poison host cells causing disease
  • some break down cell membranes, inactivate enzymes, interfere with genetic material

Question 8

What are the different plant diseases?

Answer 8

• ring rot (b)
• tobacco mosaic virus (v)
• potato blight (p)
• black sigatoka (f)

Question 9

What is ring rot?

Answer 9

• bacterial disease of potatoes, tomatoes, aubergines
• caused by Gram +ve bacterium
• damages leaves, tubers, fruit

Question 10

What is Tobacco Mosaic Virus (TMV)?

Answer 10

• caused by virus that infects tobacco plants
• damages leaves, flowers and fruit
• stunting growth and reduces yields

Question 11

What is potato blight?

Answer 11

• caused by protoctist oomycete
• hyphae penetrate host cells and destroy leaves, tubers, fruit

Question 12

What is black sigatoka?

Answer 12

• banana disease caused by fungus which attacks/destroys leaves
• hyphae penetrate/digest cells
• fungicide can control spread

Question 13

What are the different animal diseases?

Answer 13

• tuberculosis
• bacterial meningitis
• HIV/AIDS
• influenza
• malaria
• ring worm
• athlete’s foot

Question 14

What is tuberculosis (TB)?

Answer 14

• bacterial disease that destroys lung tissue and suppresses immune system
• TB is curable (antibiotics) and preventable (vaccination)

Question 15

What is bacterial meningitis?

Answer 15

• bacterial infection of the meninges of the brain
• can spread to the rest of the body causing septicaemia (blood poisoning)
• antibiotics can cure it

Question 16

What is HIV/AIDS?

Answer 16

• caused by virus which targets T-helper cells
• gradually destroys immune system over time (open to other infections)
• spread through bodily fluids
• no vaccine or cure

Question 17

What is influenza?

Answer 17

• viral infection of ciliated epithelial cells in gas exchange system
• leaves airways open to secondary infection
• affects mammals and birds

Question 18

What is malaria?

Answer 18

• caused by protoctista Plasmodium and is spread by mosquito bites
• reproduce in female mosquito and is spread when female takes blood meals
• invades red blood cells, liver and the brain

Question 19

What is ring worm?

Answer 19

• fungal disease affecting mammals
• causes grey-white, crusty infectious areas of skin
• not damaging but may be itchy

Question 20

What is Athlete’s foot?

Answer 20

• human fungal disease
• grows on and digests warm, moist skin between toes
• causes cracking and scaling
• antifungal creams are an effective cure

Question 21

How can pathogens directly be spread?

Answer 21

• direct contact:
  
  • kissing, or contact with bodily fluids
  • skin-to-skin contact
  • microorganisms from faeces
• inoculation:
  
  • break in the skin
  • e.g. animal bite
• ingestion;
  
  • taking in contaminated food/drink
  • transferring pathogens from hands to mouth

Question 22

How can pathogens be indirectly spread?

Answer 22

• fomites:
  
  • bedding, socks, cosmetics (inanimate objects)
• inhalation:
  
  • minute droplets of saliva/mucus
  • when healthy individuals breathe them in, they become infected
• vectors:
  
  • transmit communicable pathogens from one host to another
  • include: mosquitoes, water, rat fleas

Question 23

What are the factors affecting communicable diseases in animals?

Answer 23

• over crowded living/working conditions
• poor nutrition
• compromised immune system
• poor disposal of waste
• climate change (can introduce new vectors/diseases)
• culture/infrastructure (traditional medical practises)
• socioeconomic factors (lack or trained workers)

Question 24

How are pathogens transmitted between plants?

Answer 24

• direct contact between a healthy/infected plant
• soil contamination:
  
  • infected plants may leave pathogens/reproductive spores in the soil
• vectors:
  
  • wind
  • water
  • animals
  • humans

Question 25

What are the factors affecting communicable diseases in plants?

Answer 25

• planting varieties of crops that are susceptible to disease
• over-crowding
• poor mineral nutrition
• damp, warm conditions
• climate change (e.g. increased rainfall promotes spread of diseases)

Question 26

How do plants recognise an attack?

Answer 26

• receptors in cells respond to molecules from pathogens/chemicals produced
• stimulates release of signalling molecules

Question 27

What are the physical defences of plants?

Answer 27

• callose is synthesised/deposited between cell walls/membrane
  
  • prevents pathogens from entering
• lignin is added making the mechanical barrier
• callose blocks sieve plates and plasmodesmata
  
  • seals them off from healthy cells and prevent spread of pathogens

Question 28

What are the chemical defences of plants?

Answer 28

• insect repellents
• insecticides
• antibacterial compounds (antibiotics)
• antifungal compounds
• anti-oomycetes
• general toxins

Question 29

What are the non-specific defences (preventing pathogens) of animals?

Answer 29

• skin:
  
  • covers body and prevents entry of pathogens
  • produces sebum that inhibits pathogen growth
• mucous membranes:
  
  • secretes mucus which traps microorganisms
• lysozymes (urine/tears)/stomach acid

Question 30

How does blood clotting/wound repair prevent pathogens enetring?

Answer 30

• when platelets come into contact with collagen, they secrete:
  
  • thromboplastin = enzyme that triggers the formation of a blood clot
  • serotonin = makes smooth muscle in blood vessels contract
• clot dries and forms a scab preventing pathogens
• collagen is deposited to give new tissue strength

Question 31

What is the inflammatory response?

Answer 31

• localised response that leads to pain, heat, redness, swelling
• mast cells are activated and release histones/cytokines
  
  • histamines = dilate blood vessels (increase in temp. prevents pathogens reproducing)
  • also make blood vessels more ‘leaky’ to force out blood plasma (tissue fluid)
    ‎
  • cytokines = attract wbcs (phagocytes)
  • they dispose of pathogens by phagocytosis

Question 32

What are the non-specific defences (removing pathogens) in animals?

Answer 32

• fevers:
  
  • normal body temp. = 37 degrees C
  • cytokines stimulates hypothalamus to increase temp.
  • most pathogens reproduce best at 37 degrees C/under (higher temps. inhibit this)
  • specific immune system works better at higher temps.
    ‎
• phagocytosis:
  
  • specialised wbcs (phagocytes) engulf and destroy pathogens
  • types of phagocytes = neutrophils and macrophages

Question 33

What are the stages of phagocytosis?

Answer 33

• pathogens produce chemicals to attract phagocytes
• phagocytes recognise them as ‘non-human’ (non-self)
• phagocyte engulfs the pathogen and encloses it in a vacuole (phagosome)
• it then combines with a lysosome to form a phagolysosome
• enzymes from the lysosome digest and destroy the pathogen
  ‎
• neutrophil = 10 mins to engulf
• macrophages have a more complex process:
  
  • after digesting, they combine the pathogen antigens with special glycoproteins (MHC = major histocompatibility complex)
  • it then moves the antigens to the macrophage and becomes an antigen-presenting cell
  • the antigens stimulate others cells in the specific immune response

Question 34

What are useful chemicals in non-specific animal defences?

Answer 34

• cytokines:
  
  • act as cell-signalling molecules
  • inform other phagocytes about pathogen invasion
  • increase body temp./stimulate SIR
    ‎
• opsonins:
  
  • bind to pathogens and ‘tag’ them to be more recognisable
  • phagocytes have receptors on cell membranes that bind to common opsonins
  • antibodies such as immunoglobulin G (IgG) and immunoglobulin (IgM) have the strongest effect

Question 35

What is the specific immune system?

Answer 35

• active/acquired immunity
• much slower than non-specific responses (can take up to 14 days)
  
  • immune memory cells helps to speed up reaction

Question 36

What are antibodies?

Answer 36

• Y-shaped glycoproteins called immunoglobulins
• they bind to specific antigens on a pathogen/toxin
  ‎
• they have two identical long polypeptide chains (heavy) and two shorter ones (light)
• disulfide bridges hold them together
  ‎
• antibodies bind to antigens with lock-and-key mechanism
• variable region = binding site (top of Y shape)
• constant region = the rest of the shape
  ‎
• antibody + antigen = antigen-antibody complex
• the hinge region allows for flexibility

Question 37

How do antibodies defend the body?

Answer 37

• antibody of a-a complex acts as opsonin so it is easily engulfed/digested
• most pathogens cannot effectively invade host cells once part of a-a complex
• antibodies act as agglutinins which cause a-a complexes to clump together
  
  • makes it easier for phagocytes to engulf multiple at a time
• antibodies can act as anti-toxins and bind to them making them harmless

Question 38

What are the types of T lymphocytes?

Answer 38

• T helper:
  
  • CD4 receptors allow them to bind to antigens on APCs
  • produce interleukins (type of cytokine) to stimulate the production of other T cells
  • attracts/stimulates macrophages to ingest pathogens with a-a complexes
    ‎
• T killer:
  
  • destroy pathogen with antigen
  • produce perforin which kills pathogen by making membrane permeable
    ‎
• T memory:
  
  • part of immunological memory
  • divide rapidly when they meet antigen second time (form clones of T killer cells)
    ‎
• T regulator:
  
  • suppress immune system (interleukins)
  • stop immune response to ensure body recognises self antigens and does not set up autoimmune response

Question 39

What are the types of B lymphocytes?

Answer 39

• plasma:
  
  • produce antibodies to a particular antigen
  • lives for only a few days
    ‎
• B effector:
  
  • divide to form plasma cell clones
    ‎
• B memory:
  
  • provide immunological memory
  • remember a specific antigen and enable body to make rapid response

Question 40

What is cell-mediated immunity?

Answer 40

• T lymphocytes respond to changed cells
• important against viruses and early cancers
  ‎
• macrophages engulf/digest pathogens in phagocytosis
  
  • process antigens to form APCs
• receptors on T helper cells fit antigens (activate and form interleukins)
  
  • stimulates production of more T cells
• cloned T cells may:
  
  • form T memory cells
  • produce interleukins (stim. phagocytosis)
  • produce interleukins (stim. B cells to divide)
  • stim. development of clone T killer cells specific for presented antigen and destroy infected cells

Question 41

What is humoral immunity?

Answer 41

• body responds to antigens outside cells/APCs
• produces antibodies soluble in blood/tissue fluid
  ‎
• activated T helper cells bind to B cell APC (clonal selection)
• interleukins produced by T helper cells activate B cells
• those B cells divide by mitosis and produce plasma cell clones/B memory cells (clonal expansion)
• cloned plasma cells produce antibodies
  
  • bind to antigens
  • act as opsonins/agglutinins
  • ** primary immune response **
• cloned B cells can develop into B memory cells
  
  • divide rapidly to form plasma cell clones
  • wipes out pathogens before symptoms are shown
  • ** secondary immune response **

Question 42

What is the action of opsonins, agglutinins and anti-toxins?

Answer 42

• opsonins:
  
  • chemicals that bind to pathogens and tag them so they are recognised more easily by phagocytes
• agglutinins:
  
  • chemicals that cause pathogens to clump together so they are easier for phagocytes to engulf/digest
• anti-toxins:
  
  • chemicals that bind to toxins produced by pathogens so they no longer have an effect