Home · A-Level Biology 9700 · Infectious diseases · Antibiotics: penicillin and resistance · Practice
AS · Practice questions · Antibiotics: penicillin and resistance

Block the cross-links, then slow the selection.

Six original Cambridge-style questions on antibiotics, from explaining osmotic lysis to interpreting the rise of resistant infections in a hospital.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep the chain of reasoning

Enzyme, wall, water, burst.

01
Explain
[4 marks]

Outline how penicillin acts on bacteria.

  • Penicillin acts on bacteria that are growing (making new cell wall). ✓
  • It inhibits transpeptidase enzymes that form cross-links between peptidoglycan chains. ✓
  • The new cell wall is weak. ✓
  • Water enters the cell by osmosis; the wall cannot resist the pressure, so the cell bursts (osmotic lysis). ✓
02
Explain
[3 marks]

Explain why antibiotics are not effective in treating diseases caused by viruses, such as HIV/AIDS.

  • Viruses have no cell wall or peptidoglycan for penicillin to act on. ✓
  • Viruses have no ribosomes or metabolic enzymes of their own, which other antibiotics target. ✓
  • Viruses replicate inside host cells using the host's machinery, so there is nothing for the antibiotic to inhibit. ✓
03
Recall
[2 marks]

Suggest why penicillin does not harm the cells of the person taking it.

  • Human (animal) cells do not have a cell wall. ✓
  • They contain no peptidoglycan and no transpeptidase that makes it, so there is no target for penicillin. ✓
04
Explain
[4 marks]

Explain how a population of bacteria that is resistant to an antibiotic can develop in a patient who is being treated with that antibiotic.

  • A random mutation produces a gene giving resistance (for example coding for an enzyme that breaks down the antibiotic). ✓
  • The antibiotic kills the susceptible bacteria but resistant bacteria survive. ✓
  • The resistant bacteria reproduce by binary fission with less competition, passing the resistance gene to their offspring. ✓
  • The gene may also be passed to other bacteria on plasmids by conjugation, so the proportion of resistant bacteria increases. ✓
05
Analysis
[4 marks]

Discuss the consequences of antibiotic resistance for health care.

  • Infections such as MRSA or MDR-TB become hard or impossible to treat, so more patients die. ✓
  • Patients stay in hospital longer, and second-line drugs are more expensive and may have more side effects. ✓
  • Operations, cancer chemotherapy and transplants become riskier because infections cannot be controlled. ✓
  • New antibiotics are needed, but developing them is slow and costly. ✓
06
Data
[4 marks]

In the lab's resistance model, 1% of bacteria are resistant at the start. After 20 years of antibiotic use with no steps taken, almost 100% are resistant. With all five steps taken, only about 4% are resistant. Describe and explain how two of the steps reduce the rise in resistance.

  • Prescribing antibiotics only when needed (not for viral infections): the antibiotic is used less often, so there is less selection pressure favouring resistant bacteria. ✓
  • Finishing the course: all the bacteria, including less susceptible ones, are killed, so fewer survive to reproduce. ✓
  • Hospital hygiene: resistant bacteria are not passed from patient to patient, so they spread less. ✓
  • (Also: less use in farming reduces selection in animals; rotating antibiotics means no single antibiotic selects continuously.) Any two, each with an explanation. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.