Cambridge IGCSE Biology
Cambridge IGCSE Biology 15 Drugs Exam Questions
Free Cambridge IGCSE Biology revision for 15 Drugs: what you need to know, common mistakes and exam-style questions with answers. Covers the definition of a drug, antibiotics for bacterial infections, why antibiotics do not affect viruses, antibiotic resistance and MRSA.
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About Cambridge IGCSE Biology 15 Drugs
Drugs is a short topic in Cambridge IGCSE Biology that focuses on medicinal drugs, particularly antibiotics. You need to define a drug as any substance taken into the body that modifies or affects chemical reactions in the body, describe the use of antibiotics to treat bacterial infections, and explain why some bacteria are resistant. Extended candidates also explain why antibiotics kill bacteria but not viruses, how resistance develops by natural selection, and how the development of resistant strains such as MRSA can be reduced. This topic links closely to diseases and immunity and to variation and selection, and questions often combine the two.
What you need to know
- Definition of a drug A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
- Antibiotics Antibiotics are used to treat bacterial infections. They work by affecting structures or processes in bacteria that are not present in human cells.
- Resistance Some bacteria are resistant to antibiotics, which reduces how effective the antibiotics are. Limiting the use of antibiotics to when they are essential reduces the chance of resistant strains developing.
- Viruses (Extended) Antibiotics kill bacteria but do not affect viruses, because viruses do not have the cell structures or metabolic processes that antibiotics target, and they reproduce inside host cells.
- Development of resistance (Extended) Random mutation may give a bacterium resistance. When antibiotics are used, non-resistant bacteria are killed, but resistant bacteria survive and reproduce, passing on the resistance allele. The population becomes mostly resistant; MRSA is a strain of Staphylococcus aureus resistant to several antibiotics.
- Reducing resistance Use antibiotics only when essential, complete the full course, avoid using antibiotics routinely in farming and use good hygiene in hospitals to limit the spread of resistant strains such as MRSA.
Common mistakes in 15 Drugs
- Saying antibiotics treat colds and flu. These are caused by viruses, which antibiotics do not affect.
- Saying the antibiotic causes the mutation. Mutations happen randomly; the antibiotic only selects the resistant bacteria.
- Saying people become resistant to antibiotics. It is the bacteria that become resistant.
- Saying resistant bacteria 'learn' or 'adapt'. Use natural selection: resistant bacteria survive, reproduce and pass on the allele.
- Defining a drug as an illegal substance. A drug is any substance that modifies or affects chemical reactions in the body.
Try three 15 Drugs exam-style questions
1. Define the term drug. (1 mark)
Show the mark scheme answer
Any substance taken into the body that modifies or affects chemical reactions in the body.
2. Explain why antibiotics cannot be used to treat viral infections. (2 marks)
Show the mark scheme answer
Antibiotics target structures or processes found in bacteria (such as the cell wall); viruses do not have these structures and reproduce inside host cells, so are not affected.
3. Explain how a population of bacteria can become resistant to an antibiotic. (4 marks)
Show the mark scheme answer
A random mutation gives some bacteria resistance; the antibiotic kills the non-resistant bacteria; resistant bacteria survive and reproduce; the allele for resistance is passed on so the proportion of resistant bacteria increases (natural selection).
15 Drugs FAQs
What does Cambridge IGCSE Biology 15 Drugs cover?
It covers the definition of a drug, antibiotics and their use against bacteria, antibiotic resistance and, for Extended candidates, why antibiotics do not affect viruses and how resistant strains such as MRSA develop.
Which paper is 15 Drugs examined in?
It is examined in the theory and multiple choice papers: Papers 1 and 3 for Core candidates, or Papers 2 and 4 for Extended candidates. Practical skills linked to it can also appear in Paper 5 (Practical Test) or Paper 6 (Alternative to Practical).
How is the Drugs topic linked to natural selection?
Antibiotic resistance is a clear example of natural selection. Use the same steps as in topic 18: variation from mutation, selection pressure from the antibiotic, survival and reproduction of resistant bacteria, and passing on of the allele.
