Answers · Antibiotics

Antibiotics against bacteria — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. AQ1 · Practice

Model answer. A drug is any substance taken into the body that modifies or affects chemical reactions in the body. A substance that kills bacteria on a worktop is not taken into the body, many drugs are not prescribed, and most drugs are legal.

A full answer includes

I chose: any substance taken into the body that modifies or affects chemical reactions in the body.

Also accept

any substance taken into the body that modifies or affects chemical reactions in the body

Watch for

“Any substance that kills microorganisms, inside or outside the body.” — A disinfectant kills microorganisms on a surface but is never taken into the body, so it is not a drug. A drug is defined by what it does to the reactions in the body.
“Any medicine that a doctor prescribes to treat an illness.” — Prescribed medicines are drugs, but so are substances nobody prescribes, such as the alcohol in a drink. The definition covers every substance taken in that changes the body's reactions.
“Any illegal substance that people can become addicted to.” — Being illegal has nothing to do with it. Many drugs are legal, from cough medicines to the alcohol in a drink.

2. AQ2 · Practice

Model answer. The alcohol in wine and the antihistamine tablet are both taken into the body and change chemical reactions in it, so both are drugs. Bleach poured into a sink is never taken into the body. A plaster covers a cut but is not a substance taken in that changes the body's reactions.

A full answer includes

I chose the alcohol in a glass of wine and the antihistamine tablet.
I did not choose the bleach or the plaster.

Also accept

the alcohol in a glass of wine; an antihistamine tablet for hay fever

Watch for

“Bleach poured into a sink to kill bacteria.” — Bleach kills bacteria, but in the sink, not in the body. A drug must be taken into the body.
“A plaster stuck over a cut.” — A plaster protects the cut from outside. It is not a substance taken into the body that changes its chemical reactions.
“Not the alcohol, because it is not a medicine.” — A drug does not have to be a medicine. Alcohol is taken into the body and changes the reactions in nerve cells, so it is a drug.

3. AQ3 · Practice

Model answer. An antibiotic kills the bacteria or stops them growing. With far fewer bacteria, the body's defences can clear the rest and the patient gets better. It does not act on the patient's own cells.

A full answer includes

I chose: it kills the bacteria or stops them growing, so the body's defences can clear the rest.

Also accept

it kills the bacteria or stops them growing, so the body's defences can clear the rest

Watch for

“It makes the patient's white blood cells stronger, so they can fight any germ.” — An antibiotic acts on the bacteria themselves, not on the patient's cells.
“It kills the throat cells that the bacteria have infected.” — An antibiotic acts on something bacteria have and human cells do not, such as the cell wall, so it leaves the patient's cells alone.
“It numbs the throat so that the pain stops while the infection clears.” — That is what some throat sprays do. An antibiotic acts on the cause, the bacteria.

4. AQ4 · Practice

Model answer. The left edge of the zone meets the ruler at 17 mm and the right edge at 41 mm. The diameter is 41 − 17 = 24 mm. It includes the disc.

Checked automatically

Diameter of the clear zone: 24 mm (within ±1 mm)
- Common wrong answer 18 : That leaves out the disc. The diameter is measured from edge to edge and includes the disc.
- Common wrong answer 12 : That is only half of the zone. Measure from one edge of the zone to the other.

A full answer includes

I read both edges of the zone on the ruler: 17 mm and 41 mm.
I subtracted: 41 − 17.
I wrote 24 mm.

Also accept

24 mm

Watch for

“Subtract the disc's 6 mm.” — The zone's diameter includes the disc. Every zone is measured the same way, so nothing is subtracted.
“Measure from the centre of the disc to one edge.” — That is only half of the zone, its radius. Measure the whole width, from one edge to the other.

5. AQ5 · Practice

Model answer. A disc soaked in ethanol only. Every extract disc carries ethanol as well as the plant substances, so the ethanol might stop the bacteria growing. If the bacteria grow right up to the ethanol disc, any clear zones were caused by the plant extracts.

A full answer includes

I chose a disc soaked in ethanol only.
It shows whether the ethanol, rather than the plants, stops the bacteria growing.

Also accept

a disc soaked in ethanol only

Watch for

“A disc soaked in sterile water.” — Water shows that the paper does not stop growth, but the extract discs also carry ethanol. Only an ethanol disc tests that.
“No disc at all in the fourth place.” — With no disc, the paper and the ethanol are both left untested.
“A disc soaked in an antibiotic.” — An antibiotic disc shows what a clear zone looks like, but it cannot show whether the ethanol caused the zones.

6. AQ6 · Practice

Model answer. B. The water disc shows 6 mm, the disc alone, so the paper and soaking do not stop growth. X and Y both have zones wider than 6 mm, so both antibiotics stopped the bacteria growing. In A the water disc also has a zone, so something other than the antibiotics may have caused the zones; in C and D one antibiotic shows only the disc.

A full answer includes

I chose row B.
The water disc is 6 mm, the disc alone, and X and Y both have zones wider than the disc.

Also accept

Watch for

“Row A: every disc has a zone, so everything worked.” — The water disc has no antibiotic. A zone round it means something else on the plate stopped growth, so the zones round X and Y might not be caused by the antibiotics.
“Row C: X has the largest zone.” — In C, the zone round Y is only the disc, 6 mm, so Y did not stop the bacteria growing.
“Row D: the water disc is 6 mm, as it should be.” — The water disc is right, but X shows only the disc, 6 mm, so X did not stop the bacteria growing.

7. AI1 · Practice

Model answer. (a) Four results lie between 24 mm and 27 mm, but 11 mm is far below them, so 11 mm is anomalous. (b) (26 + 25 + 27 + 24) ÷ 4 = 102 ÷ 4 = 25.5 mm.

Checked automatically

(a) Anomalous result: 11 mm (exact value)
(b) Mean without the anomaly: 25.5 mm (3 significant figures)
- Common wrong answer 22.6 : That mean includes the anomaly: 113 ÷ 5. Leave 11 mm out.
- Common wrong answer 20.4 : That is 102 ÷ 5. You added four results, so divide by 4.

A full answer includes

(a) I chose 11 mm as the anomalous result.
(b) I added the other four: 26 + 25 + 27 + 24 = 102.
(b) I divided by 4, not 5.
(b) I wrote 25.5 mm.

Also accept

(a) 11 mm; (b) 25.5 mm

Watch for

“(b) Divide by 5, because there are five plates.” — You left one result out, so you added only four. Divide by the number of results you added.
“(b) Include every result, because it is all real data.” — An anomalous result does not fit the pattern and would pull the mean down, so it is left out.

8. AI2 · Practice

Model answer. The zone shows only the area of no visible growth on this plate. Its size also depends on how quickly each antibiotic spreads through the agar and how much was on the disc, so Q may spread slowly. The test used one kind of bacterium under one set of conditions, and treating an infection also depends on how the drug works in the body and which bacteria cause the infection.

A full answer includes

I wrote that a zone's size also depends on how quickly the antibiotic spreads through the agar or how much was on the disc.
I wrote that the test used one kind of bacterium under one set of conditions.
I wrote that how well an antibiotic treats an infection also depends on how it works in the body.

Also accept

zone size also depends on how fast the antibiotic spreads and how much was on the disc; one bacterium, one set of conditions; the body is not a plate

Watch for

“The conclusion is fine, because the zone for P is more than twice as wide.” — A wider zone means more agar with no visible growth in this test. It does not show how well P works inside the body.
“It is not justified because the student did not repeat the plate.” — Repeats would make the zone sizes more reliable, but even a reliable 30 mm zone would not show how P works in the body.

9. AI3 · Practice

Model answer. Yes, the injected insulin is a drug. It is taken into the body, by injection, and it modifies chemical reactions in the body: liver and muscle cells take up glucose and turn it into glycogen. A drug does not have to be swallowed, and it can be a substance the body also makes.

A full answer includes

I wrote that the insulin is a drug.
I wrote that it is taken into the body, by injection.
I wrote that it changes chemical reactions in the body: glucose is turned into glycogen.

Also accept

yes: it is taken into the body by injection and changes chemical reactions, glucose to glycogen

Watch for

“It is not a drug, because the body makes insulin itself.” — The definition does not ask where the substance comes from. Injected insulin is taken into the body and changes its chemical reactions, so it is acting as a drug.
“It is not a drug, because it is injected rather than swallowed.” — Injected substances are taken into the body too. A drug can be swallowed, breathed in, injected or absorbed through the skin.

10. AI4 · Practice

Options as numbered on the pupil sheet: 1 — Tape the lid on with two short strips.; 2 — Spread a safe bacterium evenly over the agar.; 3 — Incubate the plate upside down at 25 °C for two days.; 4 — Measure each zone through the centre of its disc.; 5 — Press the discs onto the agar with forceps..

Model answer. Spread a safe bacterium evenly over the agar; press the discs onto the agar with forceps straight away; tape the lid with two short strips; incubate the plate upside down at 25 °C for two days; measure each zone's diameter through the centre of its disc, without opening the plate.

A full answer includes

I put spreading the bacteria first and measuring the zones last.
I put the discs on before the lid was taped and the plate incubated.

Also accept

spread bacteria, press discs on, tape the lid, incubate upside down, measure

Watch for

“Incubate the plate first, then press the discs on.” — The discs go on before incubation, so that the antibiotic spreads into the agar while the bacteria grow, in the same way for every disc. If the lawn grew first, the test would no longer show which antibiotic stops the bacteria growing.
“Measure the zones before incubating.” — The zones only appear as the lawn grows over two days, so there is nothing to measure before incubating.

11. AT1 · Apply

Model answer. Disc S will have the wider zone. More antibiotic spreads out of S into the agar, so it is still strong enough to stop the bacteria growing further from the disc before it becomes too dilute. That is one reason zone sizes can only be compared fairly when the amount on each disc is controlled.

A full answer includes

I predicted that disc S has the wider zone.
I wrote that more antibiotic spreads out of disc S.
I wrote that it stays strong enough to stop the bacteria growing further from the disc.

Also accept

disc S; more antibiotic spreads out, so it stops growth further from the disc

Watch for

“The zones will be the same, because it is the same antibiotic.” — The kind of antibiotic is the same, but disc S carries more of it, and more antibiotic spreads further before it is too dilute to stop growth.
“Disc T, because a weaker solution spreads more easily.” — Both solutions spread through the agar in the same way. What differs is how much antibiotic there is to spread.

12. AT2 · Apply

Model answer. 9 mm is exactly half of 18 mm, so the first student probably measured from the centre of the disc to one edge of the zone, its radius. The diameter is measured across the whole zone, through the centre of the disc, from edge to edge.

A full answer includes

I wrote that 9 mm is half of 18 mm.
I suggested that the first student measured from the centre of the disc to one edge, the radius, instead of across the whole zone.

Also accept

measured from the centre to one edge, the radius, instead of the whole width

Watch for

“The first student subtracted the disc.” — Subtracting a 6 mm disc from 18 mm gives 12 mm, not 9 mm. Half of 18 mm is 9 mm.
“The first student read the ruler in centimetres.” — 18 mm is 1.8 cm, not 9. Half of the zone fits the 9 mm.

13. AT3 · Apply

Model answer. The nicotine is a drug. It is taken into the body, absorbed through the skin into the blood, and it modifies chemical reactions in nerve cells in the brain. The sunscreen is not a drug: it stays on the surface of the skin and blocks light, so it is never taken into the body.

A full answer includes

I wrote that the nicotine is a drug.
I wrote that it is taken into the body, through the skin, and changes chemical reactions in the body, in nerve cells.
I wrote that the sunscreen is not a drug, because it stays on the surface and is not taken into the body.

Also accept

the nicotine: taken in through the skin, changes reactions in nerve cells; the sunscreen stays on the surface

Watch for

“Both are drugs, because both are put on the body.” — Being put on the skin is not enough. A drug must be taken into the body, and the sunscreen stays on the surface.
“Neither is a drug, because neither is swallowed.” — A drug can be absorbed through the skin. The nicotine passes through the skin into the blood, so it is taken into the body.