What counts as a drug, and antibiotics
Define a drug as any substance taken into the body that modifies or affects chemical reactions in the body. Antibiotics are drugs that kill bacteria (or stop them growing) and are used to treat bacterial diseases. The exam point students miss: antibiotics have no effect on viruses, because viruses reproduce inside the body's own cells and lack the bacterial structures antibiotics target. So a cold or flu (viral) cannot be treated with antibiotics. A recurring application question.
Antibiotic resistance: natural selection in a new costume
This is the topic's flagship six-marker, and it is the natural selection chain applied to bacteria:
- In a population of bacteria there is variation; by chance, a few have a mutation giving resistance to the antibiotic.
- When the antibiotic is used, it kills the non-resistant bacteria.
- The resistant bacteria survive and reproduce.
- They pass on the resistance allele to their offspring.
- Over time the proportion of resistant bacteria increases, so the antibiotic becomes less effective.
Reducing the development of resistant strains is a standard add-on: only use antibiotics when necessary (not for viral infections), and always complete the full course. Note the precise causation. The antibiotic does not create resistance; it selects for bacteria that were already resistant. Saying 'the antibiotic makes the bacteria resistant' loses the mark.
Alcohol: effects to state precisely
Alcohol is a depressant that slows down reactions. Examined effects: it slows down reaction times (dangerous when driving), affects judgement and self-control, and long-term, excessive drinking damages the liver (cirrhosis. The liver is where alcohol is broken down). Questions often give a reaction-time experiment or a road-safety scenario; link the named effect to the consequence (slower reactions → longer stopping distance → more accidents).
Tobacco smoke: name the component, name the effect
Marks come from matching each named substance to its specific effect:
| Component | Effect |
|---|---|
| Nicotine | Addictive; increases heart rate; raises blood pressure |
| Tar | Carcinogen (causes lung cancer); damages cilia in the airways, so mucus and pathogens build up (smoker's cough, bronchitis) |
| Carbon monoxide | Combines with haemoglobin, reducing oxygen the blood can carry |
The link to emphysema: damage to alveoli walls reduces surface area for gas exchange, so the smoker becomes breathless. As with alcohol, a vague 'smoking is bad for the lungs' scores nothing. Name the component and the precise effect.