Drugs
Drug definition; antibiotics — what they are, why they do not affect viruses; antibiotic resistance development via natural selection; causes of increased resistance; Extended molecular resistance mechanisms and MRSA; heroin effects including addiction and needle transmission; alcohol effects on CNS and liver; Extended liver damage pathway and reaction time mechanism.
Drugs
CORE EXTENDEDDefinition of a Drug
A drug is any substance taken into the body that modifies or affects chemical reactions in the body.
Drugs include both medicinal drugs (used to treat or prevent disease — e.g. antibiotics, paracetamol, vaccines) and recreational / misused drugs (taken for non-medical purposes — e.g. heroin, alcohol, nicotine, cannabis).
Antibiotics
Antibiotics are substances that kill or inhibit the growth of bacteria. They are a class of medicinal drug used to treat bacterial infections.
| Feature | Detail |
|---|---|
| Target | Bacteria only — antibiotics have no effect on viruses |
| How they work | Interfere with bacterial processes (e.g. cell wall synthesis, protein synthesis) that either do not exist in human cells or differ enough to be targeted selectively |
| Example | Penicillin — inhibits bacterial cell wall synthesis; bacteria lyse (burst) as they cannot maintain structural integrity |
| Selectivity | Designed to harm bacteria without harming the host’s cells (target bacterial-specific structures) |
Viruses are not cells — they have no cell walls, no ribosomes of their own, and no metabolism of their own. They replicate by hijacking host cell machinery. Since antibiotics target bacterial structures (cell walls, bacterial ribosomes), they have nothing to act on in a viral infection. Taking antibiotics for a cold or flu is ineffective and contributes to antibiotic resistance.
Antibiotic Resistance
Antibiotic resistance occurs when bacteria evolve so that antibiotics can no longer kill them. This is one of the most serious threats to global public health.
1. Within a population of bacteria, natural genetic variation exists — some bacteria may carry a mutation that gives resistance to an antibiotic.
2. When antibiotics are used, they select against non-resistant bacteria (kill them).
3. Resistant bacteria survive and reproduce, passing on the resistance gene to offspring.
4. Over many generations, the proportion of resistant bacteria in the population increases.
5. Eventually the antibiotic is no longer effective against that bacterial strain.
Causes of Increased Antibiotic Resistance
| Cause | Why it promotes resistance |
|---|---|
| Not completing a course of antibiotics | The weakest bacteria die first; stopping early leaves the most resistant bacteria alive to multiply |
| Overprescribing antibiotics | Unnecessary use exposes bacteria to antibiotics more often, increasing selection pressure for resistance |
| Using antibiotics in livestock farming | Widespread prophylactic use in animals promotes resistant strains that can transfer to humans |
| Using antibiotics for viral infections | Has no therapeutic benefit but still selects for resistant bacteria in the microbiome |
Resistance Mechanism in Detail — Extended
Resistance can arise through several molecular mechanisms:
Some bacteria produce enzymes (e.g. β-lactamase) that break down antibiotic molecules before they can act. Penicillin-resistant bacteria produce β-lactamase which hydrolyses the β-lactam ring of penicillin, rendering it inactive.
Mutations alter the bacterial structure the antibiotic targets (e.g. altered penicillin-binding proteins) so the antibiotic can no longer bind and has no effect.
Some bacteria develop membrane pumps that actively expel antibiotic molecules before they reach their target concentration inside the cell.
MRSA (methicillin-resistant Staphylococcus aureus) is a bacterium that has developed resistance to most common antibiotics including methicillin and other β-lactam antibiotics. It is a major problem in hospitals worldwide. Treatment options are very limited — only a few last-resort antibiotics remain effective. This illustrates why antibiotic stewardship is critical.
Heroin — Effects on the Body
Heroin is an illegal opioid drug derived from morphine. It is highly addictive and has severe effects on the body and behaviour.
| Effect | Mechanism / consequence |
|---|---|
| Slows reaction time | Depresses the central nervous system; slows nerve impulse transmission; impairs reflexes and coordination |
| Causes addiction / dependency | Activates opioid receptors in the brain’s reward centres; the brain adapts, requiring the drug to feel normal; withdrawal is extremely painful |
| Suppresses pain | Binds opioid receptors, blocking pain signals; initially produces intense euphoria |
| Slows breathing | Depresses the respiratory centre in the brainstem; overdose causes respiratory failure and death |
| Transmitted infection risk | Sharing needles transmits HIV, hepatitis B and C directly through contaminated blood |
Alcohol — Effects on the Body
Alcohol (ethanol) is a legal but widely misused depressant drug. Its effects range from short-term impairment to long-term organ damage.
| System affected | Short-term effects | Long-term effects (misuse) |
|---|---|---|
| Nervous system | Depresses CNS; slows reaction time; impairs coordination and balance; reduces inhibition; blurred vision; slurred speech | Brain damage; memory loss; mental health disorders; dependency (alcoholism) |
| Liver | Liver metabolises alcohol; acute poisoning at high doses | Cirrhosis (irreversible scarring); liver failure; liver cancer |
| Cardiovascular | Vasodilation (flushing, feeling warm); increased heart rate | High blood pressure; increased risk of stroke and heart disease |
Liver Damage from Alcohol — Extended
The liver is the primary site of alcohol metabolism. When alcohol is consumed:
1. The liver converts ethanol to acetaldehyde — a toxic intermediate that damages liver cells (hepatocytes).
2. Repeated damage triggers inflammation (alcoholic hepatitis).
3. Continued damage causes dead hepatocytes to be replaced with fibrous scar tissue — cirrhosis.
4. Cirrhotic liver cannot perform its normal functions (deamination, glycogen storage, bile production, detoxification) — liver failure results.
Cirrhosis is irreversible — unlike early-stage alcoholic hepatitis, which can recover if drinking stops. This is why early intervention is critical.
Alcohol and Reaction Time — Extended
Alcohol is a CNS depressant — it inhibits neurotransmitter release and slows synaptic transmission. This impairs the entire reflex arc:
- Slower detection of stimuli by sensory receptors
- Slower processing in the brain
- Slower transmission of motor impulses to effectors
- Net result: significantly increased reaction time
Method: Use a ruler-drop test to measure reaction time before and after consuming alcohol (or use a computer-based reaction time test). Record the distance the ruler falls before it is caught — shorter distance = faster reaction time.
Variables to control: Same ruler, same starting position, same time of day, same person for pre/post comparison, caffeine intake, fatigue level.
Ethical consideration: In school investigations, simulated impairment (e.g. distraction tasks or fatigue-based comparisons) would be used rather than actual alcohol administration to students.
A patient is prescribed a 10-day course of antibiotics for a bacterial throat infection. After 5 days they feel better and stop taking the medication. Why is this a problem?
- A. The remaining bacteria will immediately develop new mutations when the antibiotic is stopped
- B. The most resistant bacteria may survive and multiply, increasing the risk of antibiotic resistance
- C. The antibiotic will no longer be effective because the body has become immune to it
- D. Stopping early means the antibiotic cannot be used for viral infections in future
Explain, using the concept of natural selection, how a population of bacteria can become resistant to an antibiotic over time. [4 marks]
- Within the bacterial population, there is natural genetic variation — some individuals carry a mutation that gives resistance to the antibiotic [1 mark]
- When antibiotics are applied, non-resistant bacteria are killed (antibiotic acts as a selection pressure) [1 mark]
- Resistant bacteria survive and reproduce, passing on the resistance gene to offspring [1 mark]
- Over many generations, the proportion of resistant bacteria increases in the population until the antibiotic is no longer effective [1 mark]
Comprehensive Practice Questions
Mixed questions across Topic 15.
A doctor refuses to prescribe antibiotics to a patient with influenza (flu). Which statement correctly explains this decision?
- A. Antibiotics are too expensive to prescribe for minor illnesses
- B. Influenza bacteria are already resistant to all available antibiotics
- C. Influenza is caused by a virus, and antibiotics have no effect on viruses
- D. The patient’s immune system will be weakened by the antibiotic
(a) Define the term ‘drug’. [1 mark]
(b) State two effects of alcohol on the nervous system. [2 marks]
(c) State two ways in which the misuse of heroin can lead to the transmission of disease. [2 marks]
(d) Describe two ways in which overuse of antibiotics leads to increased antibiotic resistance. [2 marks]
- (a) A substance taken into the body that modifies or affects chemical reactions in the body [1 mark]
- (b) Any two of: slows reaction time / impairs coordination / depresses CNS / reduces inhibitions / slurred speech / blurred vision [2 marks]
- (c) Sharing contaminated needles transmits HIV / hepatitis B / hepatitis C directly through blood [1 mark]; unprotected sexual behaviour increased due to lowered inhibitions [1 mark]
- (d) Any two of: overprescribing for minor/viral infections exposes bacteria unnecessarily to antibiotics, increasing selection for resistance [1 mark]; using antibiotics in livestock creates resistant strains that can spread to humans [1 mark]; not completing courses leaves resistant bacteria to multiply [1 mark] — any two for [2 marks]
(a) Explain how long-term alcohol misuse leads to cirrhosis of the liver. [3 marks]
(b) A new antibiotic is discovered that is effective against MRSA. Suggest why doctors are advised to use this antibiotic only as a last resort. [2 marks]
- Alcohol is metabolised in the liver, producing toxic acetaldehyde which damages hepatocytes (liver cells) [1 mark]
- Repeated damage causes inflammation (alcoholic hepatitis); continued damage leads to liver cell death [1 mark]
- Dead hepatocytes are replaced by fibrous scar tissue (cirrhosis) — this is irreversible and prevents the liver from functioning normally [1 mark]
- Overuse of the new antibiotic would increase the selection pressure on MRSA bacteria [1 mark]
- Resistant mutants would survive and multiply → resistance to this antibiotic would develop and spread, leaving no effective treatment for MRSA infections [1 mark]
High-Frequency Mistakes — Topic 15 Overall
- 💊Saying antibiotics kill virusesAntibiotics are effective only against bacteria. They target bacterial-specific structures (cell walls, bacterial ribosomes) that viruses do not have. Viruses replicate using host cell machinery, so antibiotics have no target. This is one of the most frequently tested facts in this topic.
- 🦠Saying bacteria "learn" or "choose" to become resistantResistance arises from random pre-existing mutations — not in response to the antibiotic. The antibiotic does not cause mutations; it selects for bacteria that already carry a resistance mutation. This is natural selection, not directed evolution or adaptation.
- 💉Ext: Saying cirrhosis is reversibleCirrhosis (replacement of liver tissue with fibrous scar tissue) is irreversible. Early-stage alcoholic hepatitis (inflammation) can recover if drinking stops, but once fibrous scarring is established, the damage is permanent. Always distinguish: hepatitis = reversible if drinking stops; cirrhosis = irreversible.
- 🏢Saying completing the antibiotic course prevents the patient becoming resistantHumans do not develop antibiotic resistance — bacteria do. Completing the course ensures all the bacteria in that infection (including the more resistant ones that survive longer) are killed. The resistance is in the bacteria, not in the human body.
- 💉Forgetting needle sharing as a route of heroin-related disease transmissionSharing needles transmits HIV, hepatitis B, and hepatitis C directly through blood-to-blood contact. This must be stated explicitly — saying "heroin weakens the immune system" is not the mechanism of disease transmission and will not gain the mark.
- ⚕Ext: Confusing how resistance mechanisms workThere are multiple resistance mechanisms: enzyme inactivation (e.g. β-lactamase breaking down penicillin); target modification (altered binding proteins); efflux pumps (pumping antibiotic out of cell). All three involve pre-existing genetic changes, not a response to antibiotic exposure.
- 💨Saying alcohol is a stimulantAlcohol is a CNS depressant — it inhibits neurotransmitter release and slows nerve transmission. Although low doses may initially appear stimulating (by suppressing inhibitory pathways), alcohol fundamentally depresses CNS function: slows reaction time, impairs coordination, slurs speech.
Highest-yield Core items: drug definition; antibiotics kill bacteria but not viruses (+ reason); antibiotic resistance development via natural selection (four-step chain: variation → selection → survival/reproduction → increased proportion); causes of increased resistance (not completing courses, overprescribing, livestock use); effects of heroin (addiction, slowed reaction, needle transmission of HIV/hepatitis); effects of alcohol (CNS depressant, slowed reaction, liver damage). For Extended: the resistance mechanism detail (β-lactamase, target modification, efflux pumps); the liver damage chain (acetaldehyde → hepatitis → cirrhosis — irreversible); and alcohol as a CNS depressant with synaptic mechanism. The natural selection / antibiotic resistance question is one of the most reliably appearing Paper 4 topics.