Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

14.1.1Drugs

Drugs for Medicine — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 2

📚 Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 8Key ideas

Antibiotics are used to treat bacterial infections only.
Antibiotics do not affect viruses, so they cannot treat colds or flu.
Some bacteria are resistant to antibiotics, which reduces their effectiveness.
Development of resistant bacteria such as MRSA is minimised by using antibiotics only when essential and by completing the course.
Bacteria reproduce very quickly, so resistance can spread through a population rapidly.
Supplement
Supplement antibiotics work by interfering with structures or processes found only in bacterial cells, not in human or viral structures.
Percentage of bacteria killed = (initial number − final number) ÷ initial number × 100.

Part 2 of 8Learning objectives

1
Define the term drug.
2
Describe the use of antibiotics in treating bacterial infections.
3
Explain why antibiotics do not affect viruses (Supplement).
4
Explain how antibiotic-resistant bacteria such as MRSA develop and how this can be minimised.

Part 3 of 8Essential knowledge

1Core
Core a drug is any substance taken into the body that modifies or affects chemical reactions in the body.
2Core
Core antibiotics are used to treat bacterial infections.
3Core
Core some bacteria are resistant to antibiotics, which reduces the effectiveness of antibiotics.
4Core
Core antibiotics kill bacteria but do not affect viruses.
5Supplement
Supplement viruses have no cell wall or metabolism of their own and reproduce inside host cells, so antibiotics, which target bacterial structures, cannot affect them.
6Supplement
Supplement the development of resistant bacteria such as MRSA can be minimised by using antibiotics only when essential and ensuring treatment is completed.
7
Resistance is caused by a random mutation; the antibiotic acts as a selection pressure.
8
Overuse (e.g. for viral infections or in livestock) increases the chances of resistant strains being selected.
9
Stopping a course early may leave the more resistant bacteria alive to multiply.
10
Resistant infections are harder to treat and may need different or stronger antibiotics.

Part 4 of 8Process & sequence

    • A population of bacteria varies; a random mutation makes one bacterium resistant to an antibiotic.
    • The antibiotic is used; susceptible bacteria are killed.
    • The resistant bacterium survives because it is better adapted.
    • It reproduces rapidly by binary fission, passing on the resistance allele.
    • The resistant strain becomes more common in the population.
    • The antibiotic no longer works against this strain (e.g. MRSA).

    Development of antibiotic resistancewrite these steps in this order; each step is a marking point.

    1. Random mutation produces a resistant bacterium.
    2. Antibiotic kills non-resistant bacteria (selection pressure).
    3. Resistant bacterium survives and reproduces.
    4. Resistance allele is passed on to offspring.
    5. Resistant population grows; antibiotic becomes ineffective.

Part 5 of 8Worked examples

97.5%
An antibiotic reduces a culture from 2.0 × 106 to 5.0 × 104 bacteria. Calculate the percentage killed.
  • Killed = 2.0 × 106 − 5.0 × 104 = 1.95 × 106. % = 1.95 × 106 ÷ 2.0 × 106 × 100 = 97.5%.
64 bacteria
A bacterium divides every 20 minutes. Starting from one resistant cell, how many are there after 2 hours?
  • 2 hours = 120 min = 6 divisions. Number = 26 = 64 bacteria.

Part 6 of 8Summary

    • A drug is any substance that modifies or affects chemical reactions in the body.
    • Antibiotics treat bacterial infections only.
    • Antibiotics do not affect viruses.
    • Resistance arises by mutation and spreads by natural selection.
    • Limit resistance: use only when essential and complete the course.

Part 7 of 8Keep going

  1. Drugs in Medicine IGCSE Biology topic: what to do next

    These Drugs in Medicine IGCSE Biology topic notes covered the key ideas, worked examples and exam technique for this topic. Check your Drugs in Medicine IGCSE Biology topic understanding with the practice questions below, then reread any Drugs in Medicine IGCSE Biology topic point you were unsure about before moving on.

    Drugs in Medicine IGCSE Biology topic: revision checklist

    • Revise the main Drugs in Medicine IGCSE Biology topic definitions until you can write them from memory.

Part 8 of 8Compare: Compare

Why antibiotics kill bacteria but not viruses
FeatureBacteriaViruses
Cellular?Yes (cell wall, ribosomes, own metabolism)No — use host cell machinery
Antibiotic targets present?Yes (e.g. cell wall, bacterial ribosomes)No
Treated with antibiotics?YesNo

Supplementviruses reproduce inside host cells using the host's enzymes and ribosomes, so antibiotics that target bacterial structures have no effect.

Key terms

Command terms

Command terms
State / Name
Give a short answer only; no explanation is needed (1 mark = 1 fact).
Define
Give the precise meaning. Learn the syllabus definitions word-for-word: every keyword in the definition is a marking point.
Describe
Say what happens / what the data show, with key features and figures. Do not give reasons.
Explain
Give reasonswhat happens AND why/how, linked with “because”, “so”, “therefore”.
Suggest
Apply knowledge to an unfamiliar situation; any sensible, scientifically-based answer can score.
Compare
Give similarities AND differences, each as a direct comparison (“X is … whereas Y is …”).
Outline
Set out the main points briefly.
Calculate
Work out from given facts; show working, give the unit and the requested significant figures.
Identify
Name/select from given information.
Predict
Give an expected result without a full justification.
Evaluate
Judge the importance or quality of the evidence/method, with reasons.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Drug · Medicinal drug · Antibiotic · Bacterium · Virus · Antibiotic resistance · MRSA · Mutation · Natural selection · Pathogen · Course of antibiotics
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Use the exact scientific term and learn syllabus definitions word-for-word.
2
Write “rate of … increases”, not “… increases”.
3
Say “concentration gradient”, never just “gradient”; name particles precisely (atoms, molecules, ions) — never “it”.
4
Give units with every numerical answer and use the significant figures asked for; show all working.
5
Compare = a similarity AND a difference; describe = what happens; explain = why it happens.
6
Repeats let you identify anomalies and calculate a mean — they do not make an experiment “valid”.

Words to avoid

1
antibiotics kill virusesantibiotics do not affect viruses.
2
the bacteria become immune to the antibioticthe bacteria are resistant.
3
the antibiotic causes the mutationthe mutation occurs randomly; the antibiotic selects resistant bacteria.
4
people become resistant to antibioticsbacteria become resistant.
5
finish the course so you get better fastercomplete the course so all bacteria are killed and resistant ones are not left to multiply.

Last updated: