B6.3 Discovering Drugs

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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 and microorganisms
• 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 ratios from drug discovery data

Key words

• 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 (salicin) found in willow bark.
• Penicillin: The first antibiotic, discovered from the mould Penicillium.
• Drug company: A company in the pharmaceutical industry that researches, develops and makes medicines.

Drugs from plants and microorganisms

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 also affect the human body. Scientists extract the active chemical and turn it into a medicine.

• Digitalis; Original source Foxglove plants; Use Treats heart conditions, e.g. an irregular heartbeat
• Aspirin; Original source Willow bark (salicin); Use Painkiller; reduces fever and swelling
• 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 grew. Instead of throwing the plate away, he suggested that the mould was making a substance that killed bacteria. This was penicillin.

More than ten years later, other scientists found a way to extract and purify penicillin in large amounts. It was used to treat wounded soldiers in the Second World War and has saved millions of lives since.

Key fact: Fleming’s discovery shows the value of a careful observation and of asking why something unexpected happened.

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 from a plant or microorganism, which chemists then change so it works better or has fewer side effects. The first stage often uses computer modelling to design molecules that fit a target, such as an enzyme in a pathogen; then robots screen thousands of compounds quickly.

Worked example: success rate

Using the numbers above: percentage of screened compounds that reach clinical trials = 8 ÷ 8000 × 100 = 0.1%.
Percentage of compounds in clinical trials that are approved = 1 ÷ 8 × 100 = 12.5%.

Answer: 0.1% and 12.5%

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 medicines are often expensive and why drug testing (B6.4) 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. The diagram shows an agar plate like the one Fleming saw. What does region Y show? (Choose from: An area where a substance from the mould has killed the bacteria, A colony of the mould, An area where the bacteria are growing fastest, An area with no agar.)

Answer: An area where a substance from the mould has killed the bacteria

Working: Y is the clear zone around the mould (X). The mould made a substance, penicillin, that killed the bacteria (Z) nearby.

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

Answer: True

Working: True. A plant or microorganism chemical may be the starting point, but most drugs are now synthesised.

Related lessons

• Previous lesson: B6.2 Antibiotics and Painkillers
• Next lesson: B6.4 Developing Drugs
• In this chapter: B6.1 Vaccination

Continue in the full course

This lesson is section B6.3 (AGCSC0603) of our free GCSE Combined Science 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 Combined Science course.