4.1 - Communicable Diseases

Question 1

What is a pathogen?

Answer 1

An organism that causes disease.

Question 2

What are the four types of pathogens?

Answer 2

Bacteria, viruses, fungi and protoctista.

Question 3

What is a communicable disease?

Answer 3

A disease that can spread between organisms.

Question 4

Name the 2 animal and 1 plant diseases caused by bacteria.

Answer 4

Tuberculosis, bacterial meningitis and ring rot.

Question 5

Name the 2 animal and 1 plant diseases caused by a virus.

Answer 5

HIV, influenza and Tobacco mosaic virus.

Question 6

Name the 2 animal and 1 plant diseases caused by fungi.

Answer 6

Athletes foot, ringworm and Black sigatoka.

Question 7

Name an animal and a plant disease caused by protoctista.

Answer 7

Malaria and late blight.

Question 8

What is the difference between direct and indirect transmission?

Answer 8

Indirect transmission requires an intermediate to transmit one disease from one organism to another. Direct transmission does not.

Question 9

Give two examples of direct transmission and indirect transmission.

Answer 9

Direct – droplet infection (coughing or sneezing tiny droplets of mucus or saliva directly onto someone), sexual intercourse.
Indirect - intermediates include air, water, food or another organism (known as a vector).

Question 10

Suggest how living conditions, climate and social factors affect disease transmission.

Answer 10

Living conditions – overcrowding and poor ventilation will increase transmission rates.
Climate – wet summers increase transmission of diseases that need water to spread.
Social factors - poverty will reduce access to drugs for treatment. Poverty will also reduce access to healthcare means you will be less likely to be diagnosed. Both are factors that will increase transmission.

Question 11

Describe the non-specific defence mechanism.

Answer 11

Does not distinguish between one type of pathogen and another.
Responds to all in the same way.
Acts immediately.
Two forms: a barrier to the entry of pathogens or phagocytosis.

Question 12

Describe and explain each of the following non-specific defence mechanisms: skin, mucus membranes, blood clotting, inflammation, wound repair and expulsive reflexes.

Answer 12

Skin – physical barrier, blocking pathogens from entering; chemical barrier, producing antimicrobial chemicals and lowering pH to inhibit the growth of pathogens.
Mucus membranes – protect body openings exposed to the environment. Cells produce mucus, mucus traps pathogens, ciliated cells move the mucus.
Blood clotting – a blood clot is a mesh of (insoluble) fibrin fibres. Plug wounds to prevent entry and blood loss; formed by a series of chemical reactions that take place when platelets are exposed to damaged vessels.
Inflammation – swelling, pain, heat and redness. Triggered by tissue damage. Chemical signals (histamine) released which increase the permability of blood vessels, making them leak fluid to the surrounding area. The resulting swelling isolates any pathogens that may have entered damaged tissue. Signals also cause vasodilation, increasing the blood flow to the damaged area – making area hot and brings WBC to the area to fight off any pathogens.
Wound repair – the outer layer of skin cells divide and migrate to the edges of the wound. The tissue below the wound then contracts to bring the edges closer together. Collagen fibres aid the repair process – too many result in a scar.
Expulsive reflexes – expel foreign objects (including pathogens) from the body. Sneezing and coughing – irritation in the nasal passage or the respiratory tract.

Question 13

Describe the process by which a pathogen is destroyed after it has become attached to the surface of a phagocyte.

Answer 13

Pathogen is recognized as foreign.
Pathogen is engulfed by the phagocyte.
Forms a phagosome.
Lysosome fuses with phagocyte and releases enzymes and lysins.
These enzymes break down the pathogen by hydrolysis.
Macrophages present antigen on surface and present to specific immune system – called antigen presenting cells (APC).

Question 14

Why is the response involving phagocytes regarded as non-specific?

Answer 14

Phagocytes are able to break down many different pathogens.

Question 15

Name 3 white blood cells (WBC) that are phagocytic.

Answer 15

Neutrophils, Monocytes and Macrophages.

Question 16

Explain how neutrophils are able to pass from the blood into the tissue fluid.

Answer 16

Neutrophils have a lobed nucleus so can squeeze through the walls of capillaries into the tissue fluid.

Question 17

What is the shape of a monocyte nucleus?

Answer 17

Kidney-shaped.

Question 18

What is the role of T-helper cells in the specific immune response?

Answer 18

Pathogen engulfed by phagocytes, antigen presented on its cell surface by antigen presenting cells (APC).
T-helper cells with complementary receptor bind to antigen on surface of APC.
This activates the T cell - this is called clonal selection.
Activated T-helper cell divides by mitosis (clonal expansion) into clones.
The T cell clones differentiate into: T-helper cells, T-killer cells, T-regulatory cells and T-memory cells.
T helper cells release interleukins (a type of cytokine) that bind to receptors on B cells and activates B cells.

Question 19

What is the function of each T cell?

Answer 19

T helper cells – release interleukins to activate other cells.
T killer cells – attach and kill cells infected with the specific pathogen.
T regulatory cells – suppress the immune response from other WBC – preventing the immune system from attacking itself.

Question 20

What is the role of B cells in the specific immune response?

Answer 20

When a B-cell meets a pathogen with a specific antigen that is complementary to its specific cell surface receptor – it binds to it.
This, together with the interleukin signal (from activated T cell) activates the B cell – B cell clonal selection.
The activated B cell divides by mitosis (clonal expansion) into B cell clones.
Some clones differentiate into plasma cells and others into B memory cells.
Plasma cells secrete many antibodies into the blood. All are specific to the antigen.

Question 21

Describe 3 ways how antibodies help to clear an infection.

Answer 21

Opsonins bind to the antigen on a pathogen and aid phagocytosis.
Agglutinins – each antibody has two binding sites and so can bind to two pathogens at the same time. Agglutination clump pathogens together and immobilises them – aiding phagocytosis.
Anti-toxin antibodies bind to toxins making them harmless.

Question 22

How is the structure of an antibody related to its function?

Answer 22

Constant region is the part that attaches to the cell-surface membrane and is the same in all antibodies.
Variable region is unique in each antibody – this region has is specific to each antigen – it is complementary shape to the antigen.
Hinge region is flexible and allows the antibody and antigen to fit together.
Disulphide bridges – hold polypeptide chains together to maintain the quaternary structure.

Question 23

Explain the importance of memory cells in immunity.

Answer 23

Memory cells are made during the primary immune response. They remain in the body for a long time.
Memory T cells remember the specific antigen and will recognise it a second time round.
Memory B cells remember the specific antibodies needed to bind to the antigen.
Memory B cells quickly divide into plasma cells and quickly produce many antibodies that are specific to the antigen.
Both memory cells quick divide and go into clonal expansion.
The secondary response is much quicker and stronger

Question 24

What is an autoimmune disease (and give two examples)?

Answer 24

The body does not recognise self-antigens and launches an immune response against its own tissues. Causes pain and inflammation.
Lupus – is when the immune system attacks connect tissues.
Arthritis – is when the immune system attacks cells in the joints.

Question 25

What is meant by the word artificial in the term artificial active immunity?

Answer 25

Antigens are injected.

Question 26

Describe how a vaccination can produce active immunity to a disease.

Answer 26

Injection of antigen or attenuated / weakened / dead form of the pathogen.
Triggers an immune response.
Injected antigens are engulfed by phagocytes and the antigens are presented to the immune system by APC.
T cell selection – activation – T cell expansion (cloning) by mitosis.
Secretion of cytokines.
Activation of B cells – B cell clone by mitosis – production of plasma cells which produce antibodies and B memory cells.
Memory cells remain in the body – upon secondary response these cells remember the specific antigen.
This results in the secondary response being quicker and greater to infection resulting in no symptoms when infected.

Question 27

What is herd vaccination?

Answer 27

Vaccinate most people to stop the infection spreading within a population.

Question 28

What is ring vaccination?

Answer 28

Vaccinate all people around a victim to contain the spread of infection.

Question 29

Most children have antibodies to measles in their bloodstream at birth giving them natural immunity. What is the term given to this type of natural immunity? How do children acquire these antibodies before and after birth?

Answer 29

Passive.
Before - Antibodies cross the placenta.
After – Antibodies present in breast milk.

Question 30

What are the physical barrier in plants that decrease the chance of a pathogen entering a cell?

Answer 30

Bark - physical barrier and contains tannins (chemical barrier) which inhibit digestion in insects.
Waxy cuticle – stops water collecting on the leaf, reducing indirect transmission.
Cellulose cell wall – makes it harder for pathogens to get inside the cell.
Lignin thickening of cell walls - waterproof and almost completely indigestible.
Stomatal closure - guard cells close when pathogens are detected in that part of the plant.

Question 31

Why is callose deposited in the sieve tubes at the end of the growing season?

Answer 31

To block the flow of phloem to prevent a pathogen spreading around a plant.

Question 32

When is callose deposited between plant cell walls and plasma membranes?

Answer 32

When a plant is stressed (e.g. pathogen invasion) – callose strengthens the cell walls and blocks the plasmodesmata.

Question 33

What is necrosis and why do plant cells do it?

Answer 33

Healthy cells around an infection commit suicide to prevent the infection spreading throughout the plant.

Question 34

Give an example of a chemical defence in plants.

Answer 34

Antimicrobial chemicals inhibit pathogen growth.

Tannins are toxic to insects.

Question 35

Why may it become increasingly more difficult to develop new drugs in the future?

Answer 35

New drugs often originate from plants. Biodiversity is reducing - due to deforestation of the rainforest for crop growth.
Note – always name the habitat lost and state the reason for habitat destruction.

Question 36

Why do researchers trying to develop new drugs often concentrate their efforts on traditional medicines?

Answer 36

The plants used have already been identified at having potential medicinal benefits.
Reduces cost - saves time and effort and often has few side effects.

Question 37

What are personalised medicines?

Answer 37

They are tailored to an individual’s DNA.

Genetic information is used to predict individual responses to different drugs.

Question 38

What is synthetic biology?

Answer 38

Uses technology for design and synthesis of new medicines.

Question 39

What is the genetic basis to explain bacterial resistance to antibiotics?

Answer 39

Genetic mutations make some bacteria naturally resistant to an antibiotic.
Extensive use of antibiotics in hospitals provide a selective pressure, to which resistant bacteria have a selective advantage as they are better able to survive in a host even if treated with antibiotics.
Resistant bacteria survive and pass the resistant allele to many offspring.
Resistant allele becomes more common in a population of bacteria over time - the allele frequency increases.

Question 40

What is MRSA?

Answer 40

Meticillin-resistant Staphylococcus aureus.

Question 41

Where does Clostridium difficile infect and when?

Answer 41

The digestive system – usually causes problems for people after antibiotic treatments. C. difficile is resistant and so flourishes in the gut at this time.