Cambridge IGCSE0610

Drugs

Biology 0610 Chapter Notes

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Drugs
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In this chapter

  1. 1Defining Drugs and Their Effects
  2. 2Antibiotics: Nature's Bacterial Killers
  3. 3How Penicillin Kills Bacteria
  4. 4Making Antibiotics on a Large Scale
  5. 5The Problem of Antibiotic Resistance
  6. 6Fighting Back Against Resistance

1. Defining Drugs and Their Effects

A drug is defined as any chemical substance that is taken into the body and affects or changes the chemical reactions that happen within it. This is a very broad definition that includes everything from medical drugs like painkillers and antibiotics, to recreational drugs like alcohol and nicotine. Drugs can be beneficial, helping to fight disease or manage symptoms, but they can also be harmful, causing addiction, damage to organs, or other negative side effects. In biology, we are often concerned with medicinal drugs, which are designed to have a specific, positive effect on the body's processes.

Key term

Drug: A chemical substance taken into the body that affects chemical reactions.

Worked example 12 marks

Aspirin is a painkiller, and heroin is an illegal narcotic. Explain why both are classified as drugs. [2]

  1. 1

    Step 1: Define the term 'drug'. A drug is any substance that alters chemical reactions in the body.

  2. 2

    Step 2: Apply the definition to both substances. Both aspirin and heroin, when taken, change chemical processes in the body (e.g., by blocking pain signals or affecting neurotransmitters). Therefore, both fit the definition of a drug, regardless of their legal status or intended use.

Recap

  • A drug is any substance that changes chemical reactions in the body.
  • The term 'drug' includes medicinal substances like antibiotics and recreational substances like alcohol.
  • Drugs can have beneficial or harmful effects.
  • Medicinal drugs are used to treat or prevent disease.

Quick check

  1. State the definition of a drug. [1]1 mark

2. Antibiotics: Nature's Bacterial Killers

Antibiotics are a special class of drug used to treat bacterial infections. They are substances that either kill bacteria (bactericidal) or prevent them from multiplying (bacteriostatic). Crucially, antibiotics are ineffective against viruses, fungi, or other pathogens. Many antibiotics are naturally produced by microorganisms, particularly fungi and some soil bacteria. They produce these chemicals to compete with other microbes for resources. The first antibiotic discovered, penicillin, is produced by the fungus Penicillium.

Key term

Antibiotic: A substance that kills bacteria or inhibits their growth, and is used to treat bacterial infections.

Examiner insight

Examiners frequently ask why antibiotics don't work on viruses. A good answer will mention that viruses lack the cell structures and metabolic pathways that antibiotics target, such as cell walls.

Fun fact

The first antibiotic, penicillin, was discovered by accident in 1928 by Alexander Fleming when he noticed a mould (Penicillium) contaminating a bacterial culture plate and killing the bacteria around it.

Worked example 12 marks

A doctor diagnoses a patient with influenza (the flu), which is caused by a virus. Explain why the doctor does not prescribe antibiotics. [2]

  1. 1

    Step 1: State what antibiotics target. Antibiotics are drugs that are specifically designed to kill or inhibit the growth of bacteria.

  2. 2

    Step 2: State what antibiotics do not target. They do not work on viruses because viruses have a different structure and replicate inside host cells, lacking the metabolic processes and structures (like cell walls) that antibiotics attack.

Recap

  • Antibiotics are drugs that treat bacterial infections.
  • They work by killing bacteria or stopping their growth.
  • Antibiotics are not effective against viruses.
  • Many antibiotics, like penicillin, are naturally produced by fungi.

Quick check

  1. Name the type of microorganism that antibiotics are used to treat. [1]1 mark
  2. From which type of microorganism was penicillin originally discovered? [1]1 mark

3. How Penicillin Kills Bacteria

Penicillin has a very specific way of killing bacteria. Bacteria have a strong cell wall made of peptidoglycan, which protects them and maintains their shape. This wall is essential, especially to prevent the cell from bursting due to the high internal pressure caused by osmosis. Penicillin works by inhibiting the enzymes that bacteria use to build and repair their cell walls. When a bacterium treated with penicillin tries to divide, it cannot form a complete, strong new cell wall. As a result, the cell membrane is weak and cannot withstand the osmotic pressure. Water enters the cell by osmosis, causing it to swell and eventually burst (a process called lysis). This is why penicillin is so effective.

Key term

Lysis: The bursting of a cell, for example, a bacterial cell bursting due to a weakened cell wall and the intake of water by osmosis.

Examiner insight

High-scoring answers clearly link the inhibition of cell wall synthesis to the influx of water by osmosis and the subsequent bursting (lysis) of the bacterial cell.

Common pitfall

Stating that penicillin destroys the existing cell wall. It doesn't; it prevents the formation of new cell walls during bacterial reproduction.

Worked example 13 marks

Explain why penicillin is a safe antibiotic to use in humans, even though it is lethal to bacteria. [3]

  1. 1

    Step 1: State the target of penicillin. Penicillin works by preventing the formation of the bacterial cell wall.

  2. 2

    Step 2: Describe the relevant human cell structure. Human cells (and all animal cells) do not have a cell wall; they only have a cell membrane.

  3. 3

    Step 3: Conclude why humans are unaffected. Since human cells lack the specific structure (the cell wall) that penicillin targets, the antibiotic has no effect on them and is therefore safe for us to use.

Recap

  • Penicillin stops bacteria from building their cell walls.
  • Without a strong cell wall, bacteria absorb too much water by osmosis.
  • The influx of water causes the bacterial cell to burst and die (lysis).
  • Human cells are not harmed because they do not have cell walls.

Quick check

  1. What specific part of a bacterial cell does penicillin target? [1]1 mark

4. Making Antibiotics on a Large Scale

To meet global demand, antibiotics like penicillin must be produced in huge quantities. This is done using large containers called fermenters or bioreactors. The process involves growing the Penicillium fungus in a massive, sterile tank. The fungus is provided with all the necessary conditions for growth and antibiotic production. These conditions are carefully controlled: a nutrient-rich liquid medium (a 'food' source, often containing sugars and amino acids), a controlled temperature (around 25-27°C), a controlled pH, and a constant supply of sterile oxygen (as the fungus respires aerobically). The fermenter is stirred to keep the fungus, nutrients, and oxygen evenly mixed. After several days, the penicillin is filtered from the mixture and purified.

Key term

Fermenter: A large, sterile vessel used to grow microorganisms under controlled conditions for the industrial production of substances like antibiotics.

Examiner insight

For questions on fermenters, be specific. Instead of just saying 'nutrients', give an example like 'glucose' or 'amino acids'. Instead of 'right conditions', list the specific controlled variables: temperature, pH, oxygen, and sterility.

Worked example 14 marks

Describe the conditions inside an industrial fermenter used for producing penicillin. [4]

  1. 1

    Step 1: Nutrients. The fermenter contains a sterile nutrient broth, providing substances like glucose for respiration and amino acids for growth.

  2. 2

    Step 2: Temperature and pH. The temperature and pH are monitored and kept at the optimum level for the Penicillium fungus's enzymes to work efficiently (e.g., around 26°C).

  3. 3

    Step 3: Oxygen. A supply of sterile, filtered air is bubbled through the mixture to provide oxygen for aerobic respiration.

  4. 4

    Step 4: Aseptic Conditions. The entire system is kept sterile to prevent contamination by other microorganisms, which would compete with the Penicillium and could spoil the product.

Recap

  • Antibiotics are mass-produced in large vessels called fermenters.
  • The Penicillium fungus is grown in a sterile nutrient medium.
  • Key conditions like temperature, pH, and oxygen are carefully controlled for maximum yield.
  • Stirrers ensure even mixing of microorganisms, nutrients, and oxygen.

Quick check

  1. List two conditions that are controlled in an industrial fermenter. [2]2 marks

5. The Problem of Antibiotic Resistance

Antibiotic resistance is a major threat to global health. It occurs when bacteria evolve to the point where they are no longer affected by an antibiotic. This happens through the process of natural selection. Within any large population of bacteria, there is natural variation due to random mutations in their DNA. By chance, a mutation might make a bacterium resistant to an antibiotic. When that antibiotic is used, it kills all the non-resistant (susceptible) bacteria. However, the single resistant bacterium survives. With no competition, it multiplies rapidly, passing the resistance gene to all its offspring. Soon, the entire population of bacteria is resistant to the antibiotic, making the infection very difficult to treat.

Key term

Antibiotic Resistance: The ability of bacteria to survive exposure to an antibiotic that would normally kill them.

Common pitfall

Thinking that the antibiotic creates the resistance. The mutation for resistance happens randomly and independently of the antibiotic's presence; the antibiotic simply acts as a 'selection pressure' that allows the resistant ones to thrive.

Worked example 15 marks

Using the theory of natural selection, explain how a population of bacteria can become resistant to an antibiotic. [5]

  1. 1

    Step 1: Variation. Within the bacterial population, there is genetic variation. A few individual bacteria may have a random mutation that makes them resistant to the antibiotic.

  2. 2

    Step 2: Selection Pressure. When the antibiotic is applied, it acts as a selection pressure, killing the non-resistant bacteria.

  3. 3

    Step 3: Survival. The resistant bacteria are not killed and survive.

  4. 4

    Step 4: Reproduction. The surviving resistant bacteria reproduce, passing the resistance gene to their offspring.

  5. 5

    Step 5: Population Change. Over time, the frequency of the resistance allele increases, and the whole population of bacteria becomes resistant to the antibiotic.

Recap

  • Antibiotic resistance is a result of natural selection in bacteria.
  • Random mutations can create resistant bacteria.
  • Antibiotics kill non-resistant bacteria, allowing resistant ones to survive and multiply.
  • The resistant gene is passed on to offspring, leading to a resistant population.

Quick check

  1. What is the initial cause of variation that can lead to antibiotic resistance? [1]1 mark

6. Fighting Back Against Resistance

The spread of antibiotic resistance is accelerated by human actions. Over-prescription of antibiotics, using them for viral infections where they have no effect, and patients not completing the full course of their medication all contribute to the problem. If a course is stopped early, some bacteria may survive and develop resistance. To combat this, several strategies are essential. Doctors should only prescribe antibiotics when necessary. Patients must always complete the full prescribed course. Good hygiene practices, especially in hospitals, can prevent the spread of resistant strains like MRSA (Methicillin-resistant Staphylococcus aureus). Finally, there is an urgent need for research to develop new antibiotics to which bacteria are not yet resistant.

Key term

MRSA: Methicillin-resistant Staphylococcus aureus, a 'superbug' that is resistant to multiple antibiotics and causes difficult-to-treat infections.

Examiner insight

When asked how to reduce resistance, give specific, practical examples. 'Use antibiotics less' is too vague. 'Only prescribe antibiotics for bacterial infections' and 'Ensure patients complete the full course' are excellent, specific points.

Worked example 13 marks

A patient with a bacterial throat infection is prescribed a 7-day course of antibiotics. After 4 days, they feel much better and consider stopping the medication. Explain why it is important for them to complete the full course. [3]

  1. 1

    Step 1: Explain the initial effect. After 4 days, the antibiotic has killed most of the susceptible bacteria, which is why the patient feels better.

  2. 2

    Step 2: Explain what remains. However, some more tolerant or partially resistant bacteria may still be alive.

  3. 3

    Step 3: Explain the consequence of stopping. If the patient stops taking the antibiotic now, these remaining bacteria will survive and reproduce. This could lead to the infection returning and the new population of bacteria being more resistant to the antibiotic, making it harder to treat in the future.

Recap

  • Misuse and overuse of antibiotics accelerate the development of resistance.
  • Always complete the full course of prescribed antibiotics.
  • Antibiotics should not be used for viral infections like colds or flu.
  • Good hygiene helps prevent the spread of resistant bacteria like MRSA.
  • Developing new antibiotics is crucial for future health.

Quick check

  1. State one reason why a patient should always finish a full course of antibiotics. [1]1 mark

End-of-chapter exercise

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

  1. Define the term 'drug'. [1]1 mark
  2. Name the type of microorganism that produces penicillin. [1]1 mark
  3. Explain why antibiotics are not effective against viruses like the flu. [3]3 marks
  4. Describe the role of a fermenter in the production of antibiotics. List three conditions that must be controlled within it. [4]4 marks
  5. Explain, in terms of osmosis, how penicillin kills bacteria. [3]3 marks
  6. A patient is prescribed a 10-day course of antibiotics for a severe bacterial infection but feels better after 5 days and stops taking them. Explain why this action could be dangerous for the patient and the wider community. [5]5 marks
  7. Describe two ways to reduce the rate at which antibiotic resistance develops. [2]2 marks
  8. Explain the process of natural selection that leads to a population of bacteria becoming resistant to an antibiotic. [6]6 marks
  9. What does the abbreviation MRSA stand for? [1]1 mark
  10. Explain why penicillin does not harm human cells. [2]2 marks

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