Ch 1 · Drugs · IGCSE Biology 0610
Topic 1 of 1 in Drugs — IGCSE Biology 0610 — 58 lessons.
Introduction
The word "drug" covers far more than illegal substances: caffeine, paracetamol and prescribed antibiotics are all drugs in the Cambridge sense. This lesson sets out the definition, explains why antibiotics work on bacteria but not viruses, and describes the damage done by excessive alcohol, tobacco smoke and heroin. Examiners reward precise mechanisms: which chemical in smoke does what, and why a virus is untouched by penicillin.
Key ideas — What a drug is
A drug is any substance taken into the body that modifies or affects chemical reactions in the body. This includes medicinal drugs (antibiotics, painkillers), legal recreational drugs (alcohol, nicotine, caffeine) and illegal drugs (heroin). What matters is the effect on the body's chemistry, not the law.
🧑🏫 Teacher's explanation
Think of your body as a busy city where thousands of chemical messengers (enzymes, hormones, neurotransmitters) are constantly running errands. A drug is like a new, uninvited worker who walks into the city and changes how those errands are done. It might speed up a reaction, block a pathway, or shut down a factory. The key point is that the definition is biological, not legal: whether it is prescribed by a doctor, bought at a shop, or smuggled, if it alters the body's chemistry, it is a drug.
📖 Definitions
🔍 Worked example — Easy
A student takes a paracetamol tablet for a headache. Explain why paracetamol is classified as a drug according to the Cambridge definition.
🔍 Worked example — Standard
A patient with a viral flu is prescribed an antibiotic. Explain why this treatment is ineffective and potentially harmful.
🔍 Worked example — Exam-style
Compare the effects of nicotine and heroin on the body, referencing their classification as drugs.
⚠️ Common errors
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🎯 Exam question
Key ideas — Antibiotics and antibiotic resistance
Antibiotics are drugs used to treat bacterial infections. Penicillin, for example, stops bacteria building cell walls, so growing bacteria burst; others block bacterial ribosomes or enzymes. Human cells have no cell wall and different ribosomes, so the patient is unharmed.
🧑🏫 Teacher's explanation
Think of an antibiotic as a very specific key that only fits a bacterial lock. Bacteria are single-celled organisms with a rigid cell wall and their own ribosomes (protein factories). Antibiotics like penicillin work by jamming the key into the cell wall-building machinery, causing the bacterium to burst. Because human cells do not have cell walls and use different ribosomes, the key does not fit us, so we are safe while the bacteria die.
📖 Definitions
🔍 Worked example — Easy
A patient has a sore throat caused by a virus. A doctor prescribes antibiotics. Explain why this treatment is ineffective and potentially harmful.
🔍 Worked example — Standard
Explain how the development of antibiotic-resistant bacteria, such as MRSA, can be minimised by patients and healthcare providers.
🔍 Worked example — Exam-style
A hospital outbreak of MRSA is occurring. A bacteriologist tests the bacteria and finds they are resistant to methicillin but sensitive to vancomycin. Explain why MRSA is resistant to methicillin and why vancomycin is effective.
⚠️ Common errors
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🎯 Exam question
Key ideas — Excessive alcohol consumption
Alcohol is a depressant: it slows the nervous system. Its effects include:
🧑🏫 Teacher's explanation
Think of your nervous system like a car's engine management computer. Alcohol is like pouring sand into the gears; it doesn't break the engine, but it makes it run sluggishly and unpredictably. This is why it is called a depressant: it slows down the transmission of nerve impulses, making you feel relaxed but also clumsy and slow to react.
📖 Definitions
🔍 Worked example — Easy
Explain why alcohol is classified as a depressant rather than a stimulant, and state one immediate physical effect on a driver.
🔍 Worked example — Standard
Describe how long-term excessive alcohol consumption leads to cirrhosis of the liver.
🔍 Worked example — Exam-style
A patient with alcohol addiction stops drinking abruptly. Explain why they may experience withdrawal symptoms such as shaking and anxiety, referencing the concept of dependence.
⚠️ Common errors
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🎯 Exam question
Key ideas — Smoking: nicotine, tar and carbon monoxide
Tobacco smoke contains three named components.
🧑🏫 Teacher's explanation
Think of your lungs and heart as a delivery service. Nicotine is like a boss screaming at the drivers to drive faster and harder, causing stress and wear on the engine (heart) and roads (arteries). Tar is like a thick, sticky glue that clogs the delivery routes (airways) and damages the boxes (cells), eventually causing a fire (cancer) or blocking the path completely (COPD). Carbon monoxide is like a thief who steals the seats in the delivery trucks (haemoglobin), so fewer packages (oxygen) can be carried, forcing the drivers to work twice as hard to deliver the same amount.
📖 Definitions
🔍 Worked example — Easy
Identify which component of tobacco smoke is primarily responsible for addiction and explain its immediate physiological effect on the cardiovascular system.
🔍 Worked example — Standard
Explain how tar in tobacco smoke contributes to the development of Chronic Obstructive Pulmonary Disease (COPD).
🔍 Worked example — Exam-style
A smoker has a high level of carboxyhaemoglobin. Explain how this leads to an increased risk of coronary heart disease (CHD).
⚠️ Common errors
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🎯 Exam question
Key ideas — Heroin
🧑🏫 Teacher's explanation
Imagine your nervous system is like the central heating in a house. Normally, it runs at a comfortable, steady temperature. Heroin acts like a powerful valve that jams the heating off. It forces the system to shut down, making you feel warm and relaxed (euphoria) but also very sleepy and slow to react. This is why it is classified as a depressant; it slows down the messages traveling through your nerves.
📖 Definitions
🔍 Worked example — Easy
Explain why heroin is classified as a depressant rather than a stimulant.
🔍 Worked example — Standard
Describe the process of addiction to heroin, including the concepts of tolerance and withdrawal.
🔍 Worked example — Exam-style
Explain two health risks associated with the use of injected heroin, one related to the drug itself and one related to the method of use.
⚠️ Common errors
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🎯 Exam question
Worked example
(a) Antibiotics act on structures or reactions found in bacterial cells, such as the cell wall [B1]; viruses have no cell wall or metabolism of their own and reproduce inside host cells, so there is nothing for the antibiotic to act on [B1].
🔍 Worked example — Easy
A student asks: 'Why can't I take antibiotics for a common cold?' Explain the biological reason in one sentence.
🔍 Worked example — Standard
A hospital notices that a new strain of *Staphylococcus aureus* is no longer killed by methicillin. Explain the process by which this resistant strain became dominant in the patient's body.
🔍 Worked example — Exam-style
Explain why the overuse of antibiotics in agriculture and medicine is a major public health concern, specifically referencing the concept of selection pressure.
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