OCR a-level biology - communicable diseases, prevention and the immune system

how does Mycobacterium tuberculosis cause disease?

1. triggers inflammatory response by infecting phagocytes in lungs

2. infected phagocyte are sealed in a waxy-coated tubercles so bacteria remain dominant. First infection has no symptoms

3. if another factor weakens the immune system, bacteria become active and destroy lung tissue.

How does HIV result in the symptoms of AIDS?

1.Attachment proteins bind to complementary CD4 receptor on Th cells

2. HIV particles replicate inside Th cells, killing or damaging them

3. Aids develops when there are too few Th cells for the immune system to function

4. individuals cannot destroy other pathogens and suffer from secondary diseases/infections. may cause death.

name 4 groups of pathogen that causes communicable diseases

Bacteria, fungi, protoctista, viruses

How does the tobacco mosaic virus cause disease

- Affects plants. mainly transmitted via infected sap.

- Contains ssRNA, which is directly transcribed by host cell to assemble new virions.

- Virions enter other cells via plasmodesmata then enter xylem and phloem

- Causes stunted growth and mottled leaves

How does influenza virus cause disease?

Transmitted via: droplet infection, contact with mucus containing virus, zoonotic infection, contact with fomites.

Injects viral RNA into ciliated epithelial cells of throat & lungs. Viral RNA hijacks cell biochemistry to produce new virions. Cell lysis releases virions.

5-7 days of headache, coughing, sneezing, sore throat,

vomiting, fever, muscular/joint pain.

what causes malaria

Female Anopheles mosquito acts as vector for Plasmodium spp. protoctista when it transfers saliva to another organism during feeding.

Parasite reproduces asexually in red blood cells in liver, causing lysis

what causes potato / tomato late blight?

The protoctista Phytophthora infests behaves similarly top a fungus. Mainly transmitted via spores.

what causes ring rot of potatoes

Sepedonicus subspecies of the bacterium Clavibacter michiganensis. Mainly transmitted by planting infected seeds/ contact with fomites. Planto-to-plant transmission is rare

what causes bacterial meningitis?

Often meningococcal bacteria. Affects meninges (protective membranes around brain).

Transmitted by droplet infection & direct contact with saliva e.g. kissing. Usually spread by carriers of the bacteria who are not ill & occasionally by individuals with meningitis.

Describe 3 fungal infection

- black Sigatoka in banana plants

- ringworm in cattle

- athletes foot in humans

How are communicable pathogens transmitted directly?

- inhalation (droplet infection)

- skin-to-skin contact or exchange of fluids

- penetrate skin actively using enzymes or passively through wounds, hair follicles or sweat glands

how are communicable pathogens transmitted indirectly?

- consumption of contaminated food and drink

- via a vector e.g mosquitoes transmit plasmodium parasite

- spores

How do living conditions affect disease transmission?

- overcrowding increases direct transmission

- climate determines which organisms can survive e.g malaria is more prevalent in tropical countries, where mosquitoes (the vector) can breed

- social factors influence how quickly people are treated, which can increase/ decrease direct transmission

Name 4 physical barriers to pathogen entry in plants.

- cellulose cell walls

- lignified layer

- waxy upper cuticle

old vascular tissue is blocked ton prevent pathogens from spreading inside the plant

Describe 2 mechanical responses to infection in plants

- Guard cells close stomata

- the thick polysaccharide callose is produced and deposited between the cell wall and plasma membrane to increase entry distance/limit spread

What is necrosis?

- injury activates intracellular enzymes in plants that kill cells near the site of infection to prevent pathogen from spreading

- necrosis of woody tissue is known as canker

Describe the chemical defenses plants use against pathogens

- Terpenoids (essential oils)

- phenols

- alkaloids

- defensins (cysteine-rich proteins) inhibit transport channels

- Hydrolytic enzymes

5 barriers to infection in animals

- Skin is tough keratin layer

- blood clotting prevents pathogens from entering through skin lesions

- hydrochloric acid in stomach kills bacteria

- harmless bacteria in gut and on skin surface increase interspecific competition with pathogens

- mucous membrane trap pathogens and may secrete antimicrobial enzymes

what are expulsive reflexes

Body attempts to force foreign substances out:

- irritation of mucous membranes in nostrils causes sneezing

- irritation of ciliated epithelium in respiratory tract causes coughing

Name 4 ways the nonspecific immune system responds to infection

- phagocytosis

- inflammation

- digestive action of lysozymes

- Interferons (proteins produced by invaded cells that anct as antiviral agents).

Outline the process of inflammation

1. damaged vessels release histamines, causing vasodilation

2. Blood flow & permeability of blood vessels increase

3. white blood cells & plasma move into the infected tissue

How does blood clotting occur?

1. Blood platelets form plug & release chemicals that enhance clotting e.g. thromboplastin

2. prothrombin changes into thrombin, its active form

3. Fibrinogen changes into insoluble fibrin which covers wound.

Name 2 types of white blood cells involved in phagocytosis

- Neutrophils

- Macrophages (can become antigen-presenting cells)

How does phagocytosis destroy pathogens?

1. phagocyte moves towards pathogen which may have been marked by opsonins via chemotaxis.

2. Phagocyte engulfs pathogen via endocytosis to form a phagosome

3. Phagosome fuses with lysosome (phagolysosome)

4. lysozymes digest the pathogen

5. phagocyte absorbs the products from pathogen hydrolysis

Explain the role of antigen-presenting cells (APC's)

- macrophages displays antigen from pathogen on its surface (after hydrolysis in phagocytosis)

- Enhances recognition by Th cells, which cannot directly interface with pathogens / antigens in body fluid

- secrete cytokines that are involved in stimulating specific immune response

what are lysosomes

Digestive enzymes. Found in lysosomes as well as many secretions e.g. tears & mucus. Damage bacterial cell walls, causing osmotic lysis.

Outline how to prepare blood to be observed under a microscope

1. Smear a drop of blood onto a slide using a spreader held at 45º.

2 Add leishman stain then a buffer. Rinse

Name the 2 types of specific immune response

cell mediated and humoral

Outline the process of the cell mediated response

1. Complementary TH lymphocytes bind to foreign antigen on APC.

2. Cell signalling via secretion of interleukins stimulates:

a. clonal expansion of complementary TH cells (rapid mitosis): become memory cells or trigger humoral response

b. clonal expansion of cytotoxic T cells (Tc): secrete enzyme perforin to destroy infection cells

Outline the process of the humoral response

1. Complementary TH lymphocytes bind to foreign antigen on antigen-presenting T cells.

2. Release cytokines that stimulate clonal expansion (rapid mitosis) of complementary B lymphocytes.

3. B cells differentiate into plasma cells.

4. Plasma cells secrete antibodies with complementary variable region to antigen.

Describe the structure and function of B and T lymphocytes

- Many specific receptors & immunoglobulins on surface

- B cells differentiate into plasma cells to secrete antibodies

- 2 types of T helper (secrete cytokines), T killer (secrete perforin), T regulator (suppress other immune cells to prevent autoimmune disease).

What is an antibody

Proteins secretes by plasma cells

describe the structure of an antibody

- quaternary structure : 2 light chains held by disulfide bridges, 2 longer heavy chains

- Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen

- the rest of the molecule is known as the constant region.

How do antibodies lead to the destruction of a pathogen?

- Agglutinins form antigen-antibody complexes to enhance phagocytosis

- Activation of complement

- Opsonins mark microbes for phagocytes

- Antitoxins make toxin insoluble via precipitation / neutralisation

what are memory cells

- specialised TH / B cells produced from primary immune response

- remain in low levels in the blood

- can divide very rapidly by mitosis if organism encounters the same pathogen again

What are the risks of using antibiotics to treat bacterial infection?

Overuse of antibiotics increases selection pressure of resistant strains of bacteria. Antibiotic-resistant infections e.g. caused by MRSA & clostridium difficile are difficult to treat.

Contrast the primary and secondary immune response

Secondary response:

- Faster rate of antibody production

- Shorter time lag between exposure & antibody production

- Higher concentration of antibodies

- Antibody level remains higher after the secondary response

- Pathogen usually destroyed before any symptoms

Describe passive immunity

- Involves antibodies and can be natural or artificial

- No memory cells & antibodies not replaced when broken down = short-term

- Immediate

- Antibodies from external source

- No direct contact with antigen necessary

Describe active immunity

- Involves antibodies and can be natural or artificial

- Memory cells produced = long-term

- time lag

- lymphocytes produce antibodies

- needs direct contact with antigen

Give examples of passive and artificial immunity

PASSIVE NATURAL - antibodies in breast milk / across placenta

PASSIVE ARTIFICIAL - anti-venom, needle stick injections

ACTIVE NATURAL - humoral response to infection

ACTIVE ARTIFICIAL - vaccination

Define autoimmune disease and give examples

Immune system provides antibodies against its own tissues

RHEUMATOID ARTHRITIS - immune system targets synovial lined joints, causing inflammation.

LUPUS - results in inflammation throughout the whole body

Explain the principles of vaccination

1. Vaccine contains dead/ inactive form of a pathogen or antigen.

2. Triggers primary immune response.

3. Memory cells are produced and remain in the bloodstream, so secondary response is rapid & produces higher concentration of antibodies.

4. Pathogen is destroyed before it causes symptoms.

Define endemic

disease occurs routinely in a geographical area

Define epidemic

temporary rapid increase in incidence of disease in a geographical area

What role do vaccines play in preventing epidemics?

- Routine vaccination of 80-90% of population reduced available carriers of pathogen, resulting in herd immunity. Limited by country's resources

- Vaccinating close contacts of infected individual minutes spread of pathogen, but raises issues of distributive justice

- Programs have changed to account for informed consent and maximum beneficence even during epidemic

List possible natural sources of medicines

• microorganisms e.g. streptomycin, neomycin, chloramphenicol = various species of Streptomyces genus

• fungi e.g. penicillin

• plants e.g. Taxol for chemotherapy = yew, quinine for malaria = cinchona, digoxin for heart arrhythmia = foxglove

Maintaining biodiversity means new natural treatments can be discovered in the future.

What is personalised medicine?

genome sequencing has enabled scientists to predict an individual's response to disease/ certain medicines so prescriptions can be targeted

what is synthetic biology?

engineering that targets biochemical processes

What are the benefits of using antibiotics to treat bacterial infection?

- Effectively reduce population of bacteria colony. Used widely since discovery of penicillin in mid 20th century

- Bacteriostatic antibiotics prevent protein synthesis / inhibit formation of nucleic acids = inhibit growth

- Bactericidal antibiotics prevent formation of peptidoglycan cross-links in cell walls = osmotic lysis

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