module 4 biodiversity, evolution and disease

4.1.1 a) the different types of pathogen that can cause communicable diseases in plants and animals (part 1)

bacteria: prokaryotic organisms

• some are pathogenic, others are not

• M. tuberculosis causes TB in humans

  • the bacteria infects the lung, causing a chronic cough and bloody mucus

  • it is a disease often associated with poor hygiene and sanitation

  • M. bovine in cows can also transport to humans and cause TB

• ring rot in potatoes

  • the bacteria infect the vascular tissue and prevent the transport of water, causing the plant to wilt and die

  • the infection spreads into potato tubers where the vascular tissue is arranged in a ring, producing the characteristic black ring of rot

4.1.1 a) the different types of pathogen that can cause communicable diseases in plants and animals (part 2)

viruses: do not have cellular structure - unable to respire, produce ATP, replicate genetic material or synthesise protein

• infect host cells and hijack the machinery to replicate their own genetic material and proteins

• TMV, tobacco mosaic virus infects several plant species - causes a distinct yellowing of the leaves which forms a mosaic pattern

• three different influenza viruses infect humans to cause the flu

  • influenza A, B and C infect the cells that line the airways

  • they cause high temperature, body aches and fatigue

  • influenza A is the virus that causes the most cases of flu globally

    • has a capsid that surrounds 8 single stranded molecules of DNA

• HIV, infects specific cells of the immune system

  • an enveloped retrovirus, contains reverse transcriptase

  • the viral enzyme, reverse transcriptase, produces single-stranded DNA from its viral RNA

  • DNA polymerase synthesises double-stranded DNA from this single strand DNA

  • the double-stranded DNA is inserted into the host DNA and can remain inactive for many years

  • once activated, the DNA provirus is used to synthesise new viruses

4.1.1 a) the different types of pathogen that can cause communicable diseases in plants and animals (part 3)

protoctista: unicellular eukaryotes

• plasmodium falciparum is a protist that causes severe forms of malaria in humans

  • the parasite is spread by mosquitos

  • infected individuals experience fever, chills + fatigue

• P. infestans causes potato blight

  • pathogen contains fungal characteristics, transmitted via spores

  • small, dark brown spots on the leaves which quickly increase in size + number

  • the protist destroys potato and tomato crops, leaving them completely inedible

4.1.1 a) the different types of pathogen that can cause communicable diseases in plants and animals (part 4)

fungi: eukaryotic cells with cell walls + vacuoles

• instead of being made of separate cells, their bodies consist of filaments called hyphae - which form a network and spread through host/soil

• black sigatoka: fungal disease in bananas

  • spreads through leaves of plant, reducing its ability to photosynthesise

  • lack of photosynthesis causes parts of the leaf to die, producing black streaks - causing death of whole leaf

• athlete’s foot: exists on the surface of the skin

4.1.1 b) the means of transmission of animal and plant communicable pathogens (part 1)

disease transmission: transfer of pathogens from infected host to uninfected host

in order for a population of pathogens to survive, they must be able to successfully transfer from host to host - would otherwise go extinct

4.1.1 b) the means of transmission of animal and plant communicable pathogens (part 2)

vectors: an organism that transfers a pathogen from an infected to uninfected individual

• vector itself is usually not harmed in the process

• many disease vectors are insects - reproduce in large numbers, increasing the likelihood of pathogen transmission

spores: small reproductive structures, released into the environment, dispersed via wind/water

• once they reach a food source (host) they begin growing

• depending on the organism, the spored may be produced via mitosis or meiosis, making them haploid or diploid

4.1.1 b) the means of transmission of animal and plant communicable pathogens (part 3)

factors affecting transmission:

• proximity between potential hosts (either direct contact or droplet infection)

  • high population densities means more likely to be high infection rates)

• environment for vectors: ie high rainfall, temperature and humidity for mosquitos

• degree of poverty in an area - particularly for waterborne diseases (typhoid, cholera and polio)

4.1.1 c) plant defences against pathogens (part 1)

passive defences: always present, physical barriers or chemical defences

active defences: activated when pathogens invade - hypersensitivity, formation of physical barriers

• cell signalling coordinates active defense mechanisms

4.1.1 c) plant defences against pathogens (part 2)

chemical defences: prevent pathogens from growing on the surface of the plant by creating acidic conditions

• toxic compounds such as catechol created

• sticky resin found in the bark - traps the pathogens so they cannot spread

• compounds that encourage the growth of competing microorganisms

  • microorganisms such as yeast found on the leaf surface are completely harmful to plants, but strong competitors against harmful pathogens

• enzyme inhibitors such as tannins

• receptor molecules:

  • detect the presence of pathogens _ trigger other defence mechanisms

• hypersensitivity: rapid death of tissue surrounding the infection site - deprives the pathogen of host tissue, nutrients and energy

4.1.1 c) plant defences against pathogens (part 3)

cellulose deposition: release of callose and lignin due to an invasion of pathogens

• molecules deposited between cell surface membrane and cell wall

• callose is a polysaccharide that forms a matrix shape - anti microbial compounds that kill pathogens (hydrogen peroxide and phenols) can be deposited in this shape

  • callose helps to reduce the size of the channels that connect neighbouring plant cells (plasmodesmata)

  • the cytoplasm of nearby cells grow into the xylem to create a wall made of callose ingrowths - known as tyloses

  • the sieve pores are filled with callose which prevents phloem sap from being transported

4.1.1 d) the primary non-specific defences against pathogens in animals

physical: body tissues act as barriers, preventing the entry of pathogens - skin, mucous membrane of alimentary canal

cellular: cells detect and signal the presence of pathogens - protective substances secreted and pathogens ingested + digested

chemical: secreted substances generate an inhospitable environment for growth of pathogens - substances can trap pathogens, causing them to burst, or preventing them from entering cells and reproducing

commensal organisms: harmless bacteria and fungi present on and in the body compete with pathogens for nutrients

4.1.1 e) i) the structure and mode of action of phagocytes (part 1)

phagocytes: carry out phagocytosis - process of recognising and engulfing a pathogen

• white blood cells produced in the bone marrow responsible for removing deal cells and invasive microorganisms

• distributed around the body in the blood, carrying out a non-specific immune response

• neutrophils - short lived cells released in large numbers during infections - have a lobed nucleus

  • the cell membrane engulfs the pathogen once its receptor proteins recognise the pathogen’s antigens, killing and digesting it after which the neutrophil dies - pus is secreted

• macrophages - large, longer lived than neutrophils, travel through the blood as monocytes (develop into macrophages once they leave the blood)

  • initiate specific immune responses, carry out phagocytosis but cut up the pathgen so they display the antigens

• dendritic cells - large phagocytic cells with large extensions for larger SA to interact with pathogens + lymphocytes

  • once they ingest foreign material, they transport it to the lymph nodes

macrophages and dendritic cells are both antigen presenting cells - can be recognised by lymphocytes

4.1.1 e) i) the structure and mode of action of phagocytes (part 2)

cytokines: cell signalling compounds (small protein molecules) that stimulate inflammation and an immune response

• interleukins (a group of cytokines) - IL1 and IL6 promote inflammation, IL1: targets the brain, causing drowsiness and fever

opsonins: antibody proteins which bind to the surface antigens of pathogens, allowing phagocytes (such as neutrophils and macrophages) to engulf the pathogen via endocytosis

phagosomes: vacuole formed within the phagocyte as a result of endocytosis

lysozymes: antimicrobial enzymes that break down the cell wall of bacteria

• found in body fluid such as blood, tears, sweat and breast milk

4.1.1 f) the structure, different roles and modes of action of B and T lymphocytes in the specific immune response (part 1)

lymphocytes: type of white blood cell with a large nucleus that fills most of the cell - produced in the bone marrow before birth

4.1.1 f) the structure, different roles and modes of action of B and T lymphocytes in the specific immune response (part 2)

T-lymphocytes:

• during maturation T-cells gain specific cell surface receptors (TCRs)

• in order to produce an immune response, T cells need to be activated and increase in number

  • antigen presentation: macrophages become antigen presenting cells (APCs) - undergo phagocytosis and present pathogen antigens on their own cell surface membrane

  • clonal selection: T cells with complementary TCRs to the specific pathogenic antigen bind to the APC - these are the clones that have been selected for replication, binding to the complementary antigens activates the T celll

• now many types of T cell in the blood

  • T helper cells: release interleukins

  • T killer cells: attach to foreign antigens and kill infected body cell along with pathogen

  • T memory cell: remain in blood, meaning clonal selection is faster in a repeat infection

4.1.1 f) the structure, different roles and modes of action of B and T lymphocytes in the specific immune response (part 3)

B-lymphocytes:

• clonal selection and activation:

  • selection: B cells with complementary antibody receptors bind to antigens on APCs - APCs can be phagocytes, infected cells or the pathogens themselves

  • activation: this binding, as well as interleukins released by T helper cells activates the B cells

• clonal expansion:

  • activated B cells divide by mitosis to produce clones

  • some B-lymphocytes differentiate into plasma cells - secrete lots of antibody molecules into the blood, lymph or linings of lungs and gut

  • other B-lymphocytes become memory cells that remain circulating in the blood for a long time