top of page

6.3 Discovering Drugs

GCSE Biology · AQA · Chapter 6: Preventing and Treating Disease

A free GCSE Biology lesson for AQA (8461) from OBA School: drugs from plants and microorganisms, the discovery of penicillin, and how new drugs are found today. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Give examples of drugs that were first extracted from plants
  • Describe how penicillin was discovered from a mould
  • Explain how most new drugs are found today
  • Explain why so few compounds become approved medicines
  • Calculate percentages and success rates from drug discovery data

Key words

  • Drug: A chemical that changes the way the body works; medicines are drugs used to treat or prevent disease.
  • Extract: To separate a useful chemical from a plant or microorganism.
  • Digitalis: A heart drug that came from foxglove plants.
  • Aspirin: A painkiller developed from a chemical found in willow bark.
  • Penicillin: The first widely used antibiotic, discovered from the mould Penicillium.
  • Drug company: A pharmaceutical company: it researches, develops and makes medicines.

Drugs from plants

For thousands of years people have used plants to treat illness. Many plants make chemicals to defend themselves against pests and pathogens, and some of these chemicals affect the human body. Scientists have extracted the active chemical and turned it into a medicine.

  • Digitalis; Original source Foxglove plants; Use Treats heart conditions (strengthens the heartbeat)
  • Aspirin; Original source Willow bark (salicylic acid); Use Painkiller; reduces fever; thins the blood
  • Penicillin; Original source Penicillium mould (a microorganism); Use Antibiotic: kills bacteria

Penicillin: a chance discovery

In 1928 Alexander Fleming was growing bacteria on agar plates. He noticed that one plate had been contaminated by a mould, Penicillium. Around the mould there was a clear zone where no bacteria were growing. The mould was making a substance that killed the bacteria. This was penicillin.

It took more than ten years before other scientists found a way to extract and purify penicillin in large amounts. It was then used to treat infected soldiers in the Second World War and has saved millions of lives since.

Key fact: Fleming noticed the clear zone and suggested the mould was killing the bacteria. Good science often starts with a careful observation.

Finding new drugs today

Most new drugs are now synthesised (made) by chemists in the pharmaceutical industry. The starting point may still be a chemical extracted from a plant or microorganism, which chemists then change to make it work better or have fewer side effects. Scientists also design molecules to fit a particular target, such as an enzyme in a pathogen, and use robots to screen thousands of compounds quickly.

Worked example: success rate

Using the numbers above: percentage of screened compounds that reach clinical trials = 5 ÷ 5000 × 100 = 0.1%.
Percentage of compounds in clinical trials that are approved = 1 ÷ 5 × 100 = 20%.

Answer: 0.1% and 20%

Why so few compounds succeed

  • Many compounds do not work well enough on the target (low efficacy).
  • Some are toxic – they harm cells or organs.
  • Some are broken down too quickly in the body, or cannot reach the place where they are needed.
  • Some cause too many side effects, or cannot be made cheaply.

Exam tip: Developing one new medicine can take over 10 years and cost hundreds of millions of pounds. This is why drug testing (6.4) matters so much.

Common mistakes

  • Saying Fleming “invented” penicillin. He discovered it; the mould already made it.
  • Mixing up the sources: digitalis is from foxgloves, aspirin from willow, penicillin from mould.
  • Thinking all modern drugs come straight from plants. Most are now synthesised by chemists, though a plant chemical may be the starting point.
  • Calling penicillin a painkiller. It is an antibiotic.

Revision questions

Try these, then check your answers below each one.

Question 1. Who discovered penicillin?

Answer: Alexander Fleming

Working: Alexander Fleming discovered penicillin in 1928.

Question 2. What did Fleming observe on his contaminated plate? (Choose from: A clear zone with no bacteria around the mould, Bacteria growing faster near the mould, The mould dying near the bacteria, A blue colour around the mould.)

Answer: A clear zone with no bacteria around the mould

Working: No bacteria grew around the mould, suggesting the mould made a substance that killed them.

Question 3. True or false? Most new drugs today are synthesised by chemists in the pharmaceutical industry.

Answer: True

Working: True. Most new drugs are made by chemists, though the starting point may be a plant chemical.

Related lessons

Continue in the full course

This lesson is section 6.3 (AGCBH0603) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.

 

 

 

 

 

 

 

Previous
Next
bottom of page