Answers · Antibiotics
Antibiotics — final check key
Check meaning, not exact wording. These are teaching points, not official Cambridge marks.
1. AK1 · Final check
Model answer. (a) It is a substance taken into the body that modifies chemical reactions, those in the bacterial cells, so it is a drug. (b) It kills the bacteria or stops them growing, so their number falls and the body's defences can clear the rest of the infection.
A full answer includes
- (a) I wrote that it is taken into the body.
- (a) I wrote that it modifies chemical reactions, those in the bacteria.
- (b) I wrote that it kills the bacteria or stops them growing.
- (b) I wrote that the body's defences can then clear the infection.
- (b) I did not write that it makes the patient stronger.
Also accept
- (a) taken into the body, changes chemical reactions; (b) kills the bacteria or stops them growing, then the body clears the rest
Watch for
- “(b) It boosts the patient's immune system.” — An antibiotic acts on the bacteria themselves. The body's defences then have fewer bacteria to deal with.
- “(a) It is a drug because a doctor prescribed it.” — Being prescribed does not make something a drug. It is a drug because it is taken in and changes chemical reactions.
2. AK2 · Final check
Model answer. R gave the widest zone of no visible growth under these conditions. The results do not show that R is best for every infection: zone size also depends on how the antibiotic spreads through agar, and one bacterium was tested. S did stop some growth, because 9 mm is wider than the 6 mm disc.
A full answer includes
- I chose: R gave the widest zone of no visible growth under these conditions.
Also accept
- R gave the widest zone of no visible growth under these conditions
Watch for
- “R is the best antibiotic to treat any infection.” — One kind of bacterium was tested on agar. A zone shows no visible growth on this plate, not how well R works in the body.
- “S had no effect on the bacteria.” — With no effect, the zone would be the disc alone, 6 mm. S gave 9 mm, so it stopped some growth.
- “The bacteria are resistant to the water.” — Water carries no antibiotic. The water disc is the control: it shows that the paper and soaking do not stop growth.
3. BK1 · Final check
Model answer. Antibiotics kill bacteria but do not affect viruses, so they do not shorten flu. Some bacteria are resistant to antibiotics, which reduces how effective those antibiotics are. Resistance belongs to the bacteria, not to the patient.
A full answer includes
- I chose: antibiotics kill bacteria but do not affect viruses.
- I chose: bacteria that are resistant to an antibiotic make it less effective against the infections they cause.
- I did not choose the statements about flu or about the patient becoming resistant.
Also accept
- antibiotics kill bacteria but do not affect viruses; resistant bacteria make an antibiotic less effective
Watch for
- “An antibiotic shortens flu if it is taken early enough.” — Flu is caused by a virus, and antibiotics do not affect viruses at any time.
- “Resistance means the patient's body no longer responds to the antibiotic.” — Resistance belongs to the bacteria. It is the bacteria that the antibiotic no longer kills or stops.
4. BK2 · Final check
Model answer. (a) A random mutation makes a few bacteria resistant. Each time an antibiotic is used, it kills the bacteria that are not resistant; the resistant ones survive and multiply, until most of the population is resistant. (b) Using antibiotics only when essential means fewer uses, so resistant bacteria are given the advantage less often and become common more slowly.
A full answer includes
- (a) I wrote that a few bacteria are resistant because of a random mutation.
- (a) I wrote that the antibiotic kills the bacteria that are not resistant.
- (a) I wrote that the resistant ones survive and multiply.
- (b) I wrote that fewer uses mean fewer chances for resistant bacteria to survive while the others are killed.
- (b) I did not write that the bacteria get used to the antibiotic.
Also accept
- (a) mutation; non-resistant killed, resistant survive and multiply; (b) fewer uses, fewer chances, slower development
Watch for
- “(a) The bacteria get used to the antibiotics over time.” — Bacteria do not adapt to an antibiotic they meet. The resistant ones were already there and are left when the others are killed.
- “(b) It stops the bacteria mutating.” — Mutations happen at random anyway. Fewer uses mean the resistant bacteria get the advantage less often.