1. Define an antigen.
Answer
A molecule that stimulates an immune response.
2. What are phagocytes able to do once they have engulfed and ingested a pathogen ?
Answer
Present the antigens of the pathogen on their cell surface membrane.
3. People suffer from infections like colds and flu multiple times, but rarely suffer from infections such as chickenpox more than once in their life time. Use your understanding of antibodies and antigens to explain why.
Answer
Some pathogens show antigenic variability, meaning their antigens' tertiary structure changes shape. Antibodies produced by plasma cells have a specific tertiary structure complementary to the antigen. If the antigen is no longer complementary, a new primary immune response must be initiated. The time taken to mount this response allows symptoms of the pathogen to appear.
4. Define "herd immunity".
Answer
When the majority of a population have been vaccinated against a disease making it difficult for the disease to be transmitted.
5. Explain why herd immunity gives some protection to people who have not been vaccinated.
Answer
Majority of organisms (people) in a population have memory B cells for pathogen. Likelihood of infected person meeting a susceptible person is low.
6. The figure below shows the structure of penicillin and another substance called clavulanic acid. If clavulanic acid is given to a patient with penicillin, the penicillin is effective at killing bacteria that produce penicillinase. Explain why.
Answer
Penicillin and clavulanic acid are similar in shape. Clavulanic acid acts as a competitive inhibitor: it fits into the active site of the enzyme and prevents the enzyme from hydrolysing penicillin. As a result, penicillin is not broken down.
7. Penicillin prevents the cross-linking of small peptide chains in peptidoglycan, the polymer that is needed to make new cell walls in bacteria. It stops new cell walls forming but does not affect old cell walls. Explain why.
Answer
Bonds already formed in old cell walls.
8. Penicillin does not damage human cells. Explain why.
Answer
Human cells do not have cell walls/peptidoglycan.
9. Explain how penicillin kills bacteria.
Answer
If new cell walls do not form, daughter cells cannot separate after division. The new cells have weak cell walls and take in water by osmosis, causing them to burst (lyse).