Skip to content
Cambridge IGCSE0610

Drugs

Biology 0610 Chapter Notes

What this chapter covers

Drugs
ShareWhatsAppPost
Drugs notes

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Drugs notes as text: skim, search, and jump between subtopics.

~6 min read

In this chapter

  1. 1What a drug is
  2. 2Antibiotics treat bacterial infections
  3. 3Resistant bacteria
  4. 4How resistant bacteria arise
  5. 5Limiting resistance and MRSA

1. What a drug is

A drug is any substance taken into the body that modifies or affects chemical reactions in the body.

This covers medicines and non-medicinal drugs alike. Define a drug by what it does to chemical reactions, not by how harmful or legal it is.

Examiner insight

“The idea that the drug must be taken into the body was not really appreciated, although the effect on a chemical reaction of some kind was relatively well known.” “Many candidates wrote only about illegal or harmful drugs forgetting that many drugs are medicinal.”

Worked example 11 mark

What is a definition that applies to all types of drug?

A a substance taken into the body that acts as a depressant · B a substance taken into the body that affects chemical reactions in the body · C a substance taken into the body to cure illness · D a substance taken into the body to kill bacteria

  1. 1

    B: a substance taken into the body that affects chemical reactions in the body. [1]

2. Antibiotics treat bacterial infections

Antibiotics are used to treat bacterial infections. They kill bacteria.

Antibiotics do not affect viruses. Penicillin kills bacteria through its effect on the cell wall. Viruses do not have a cell wall, so they are unharmed.

Examiner insight

“Some candidates named a disease, often viral in origin, rather than state bacterial infections.” “Some candidates incorrectly believed that antibiotics can be used to treat viral infections.”

Worked example 12 marks

A patient has an infection caused by a virus. The patient asks the doctor for an antibiotic. Explain why the antibiotic will not help.

  1. 1

    antibiotics kill bacteria [1]

  2. 2

    antibiotics do not affect viruses [1]

3. Resistant bacteria

Some bacteria are resistant to antibiotics. This reduces the effectiveness of antibiotics.

An antibiotic does not kill bacteria that are resistant to it. Say the bacteria are resistant, never that they are immune.

Examiner insight

“A significant proportion of candidates incorrectly believed that antibiotics kill resistant bacteria.” “A common error was to use the term immunity instead of resistance.”

Worked example 12 marks

A bacterium is grown on agar with discs of four antibiotics. Bacteria cannot grow in the clear zone around a disc. The table gives the width of each clear zone. Name the antibiotic to which the bacterium is most resistant and give a reason.

AntibioticWidth of clear zone / mm
P14
Q0
R8
S20
  1. 1

    Q [1]

  2. 2

    there is no clear zone, so the antibiotic did not kill the bacteria [1]

4. How resistant bacteria arise

Resistance arises through variation and natural selection. A random change in the base sequence of a gene gives some bacteria resistance.

When the bacteria are exposed to the antibiotic, the resistant ones survive and reproduce. The antibiotic does not cause the change.

Examiner insight

“Another frequent error was the idea that bacteria mutated for a purpose, in this case to acquire antibiotic resistance. Another error was that the antibiotic triggered the mutation.” “The most common mark points awarded were for the differential survival of bacteria when exposed to antibiotics and the subsequent reproduction of resistant bacteria.”

Worked example 13 marks

Explain how a population of bacteria can become resistant to an antibiotic.

  1. 1

    a random change in the base sequence of a gene gives some bacteria resistance [1]

  2. 2

    the resistant bacteria survive when exposed to the antibiotic [1]

  3. 3

    the resistant bacteria reproduce [1]

5. Limiting resistance and MRSA

Resistant bacteria survive and reproduce when bacteria are exposed to an antibiotic. Using antibiotics only when essential exposes bacteria to them less often, which limits the development of resistant bacteria. Patients should also follow the instructions for the course of treatment.

MRSA is a bacterium that is resistant to antibiotics. It is not a virus.

Examiner insight

“The most commonly awarded marks were for using antibiotics only when essential or when prescribed by a medical professional and for following instructions about the course of treatment.” “Most candidates described how antibiotics should only be used when necessary and that the full course should be completed to reduce the likelihood of resistant bacteria developing.”

Worked example 12 marks

State two ways in which patients can help to reduce the development of antibiotic-resistant bacteria.

  1. 1

    use antibiotics only when essential or when prescribed by a medical professional [1]

  2. 2

    follow the instructions about the course of treatment [1]

Recap

  • A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
  • Antibiotics kill bacteria and are used to treat bacterial infections.
  • Antibiotics do not affect viruses.
  • Some bacteria are resistant to antibiotics, which reduces the effectiveness of antibiotics.
  • Extended: resistance arises by variation and natural selection. Resistant bacteria survive the antibiotic and reproduce.
  • Extended: using antibiotics only when essential, and completing the course, limits resistant bacteria such as MRSA.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term drug.2 marks
  2. A student says that a substance is only a drug if it is illegal or harmful. Use the definition of a drug to explain why the student is wrong.2 marks
  3. State the type of infection that antibiotics are used to treat.1 mark
  4. A patient has an infection caused by a virus. A friend suggests taking the antibiotic penicillin. State whether penicillin will help and give a reason.2 marks
  5. Penicillin is an antibiotic. Explain why penicillin kills bacteria but does not harm viruses.2 marks
  6. A patient is infected with bacteria that are resistant to an antibiotic. The patient takes that antibiotic. State what happens to the bacteria and the effect on the antibiotic.2 marks
  7. Equal numbers of bacteria were spread on four agar plates and left for 24 hours. The numbers of bacterial colonies that grew were: no antibiotic 85, antibiotic A 3, antibiotic B 84, antibiotic C 40. Name the antibiotic to which the bacteria are most resistant and give a reason.2 marks
  8. A patient feels better after two days of a five-day course of an antibiotic and stops taking it. Explain why this goes against advice for limiting antibiotic resistance.2 marks
  9. A scientist spread ten million bacteria on agar containing antibiotic Z. Each colony grows from one bacterium. Only 6 colonies grew. Bacteria from one of these colonies were spread on fresh agar containing Z. Thousands of colonies grew. A student concludes: "Z made the bacteria mutate so that they became immune." (a) Explain why only 6 colonies grew on the first agar. (b) Explain why thousands of colonies grew on the second agar. (c) State two errors in the student's conclusion.6 marks
  10. The table gives, for one hospital, the doses of antibiotic X given each year and the cases of infection by bacteria resistant to X. Until 2019, X was given for mild illnesses, including viral infections. After 2019, X was given only when essential. (a) Calculate the percentage increase in cases from 2015 to 2019. (b) Explain why giving X for viral infections did not help the patients but led to more bacteria resistant to X. (c) A manager says that the change after 2019 reduced the number of cases. Use the data to explain why this is wrong, and state what the change did. | Year | Doses of X | Cases | | --- | --- | --- | | 2015 | 400 | 25 | | 2017 | 650 | 40 | | 2019 | 900 | 70 | | 2021 | 500 | 76 | | 2023 | 300 | 77 |7 marks

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Characteristics of living organismsCell structureDiffusionBiological moleculesEnzymesPhotosynthesis