Exam questions · Biology · Plant Defences and Disease
Chemical Defences of Plants
- 6 exam questions
- 22 marks
- 9 quick checks
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1 Name [2 marks]
Name two medicines that were developed from chemicals made by plants.
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Model answer
Any two from: aspirin, digitalis, morphine, quinine, artemisinin, taxol.
Mark scheme
- First — 1 mark
- Second — 1 mark
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2 Explain [3 marks]
Explain why many plants make poisons or bitter-tasting chemicals.
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Model answer
Plants cannot move away from animals that eat them. Poisons and bitter chemicals deter herbivores, or harm them, so the plant is less likely to be eaten. This protects the plant and gives it a better chance of survival and reproduction.
Mark scheme
- Plants cannot move away — 1 mark
- Poison or bitter taste deters / harms — 1 mark
- Plant survives / reproduces — 1 mark
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3 Describe [4 marks]
Describe how a student could investigate whether an extract of garlic has antibacterial properties.
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Model answer
Spread bacteria on an agar plate in sterile conditions. Place a paper disc soaked in garlic extract on the agar, and a control disc soaked only in the solvent. Incubate the plate at about 25 degrees Celsius for two days. Measure the clear zone around the discs. A clear area around the garlic disc and not the control shows that garlic is antibacterial.
Mark scheme
- Bacteria spread on sterile agar — 1 mark
- Disc with extract and a control disc — 1 mark
- Incubate — 1 mark
- Measure the clear zone — 1 mark
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4 Calculate [3 marks]
On an agar plate, the zone of inhibition around a disc of plant extract has a diameter of 14 mm. Calculate the area of the zone, using \(\pi r^2\). Use \(\pi = 3.14\).
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Model answer
The radius is 7 mm. Area = 3.14 × 7 × 7 = 153.86 mm², which is about 154 mm².
Mark scheme
- Radius is 7 mm — 1 mark
- 3.14 × 7² — 1 mark
- 154 mm² — 1 mark
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5 Explain [4 marks]
Explain why the chemicals made by plants are a useful source of new medicines.
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Model answer
Plants have evolved many chemicals to kill or deter pests and pathogens, and some of these may act on human cells or bacteria in a useful way. Many medicines, such as aspirin and artemisinin, have been developed from plant chemicals. Large numbers of plants have not yet been tested, so there may be new medicines to discover.
Mark scheme
- Plants make chemicals to defend themselves — 1 mark
- These may act in human cells or on pathogens — 1 mark
- Examples of existing medicines — 1 mark
- Many plants not yet tested — 1 mark
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6 Evaluate [6 marks]
Evaluate the use of plants as a source of medicines.
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Model answer
Plants have given us important medicines, such as aspirin, digitalis, morphine and artemisinin, and there are thousands of plant species that have not been tested, so there are likely to be more. Plant chemicals are often effective, because they have evolved to act on living things. However, some are poisonous, so the dose has to be controlled. It can be difficult to grow enough of the plant, and some, such as the yew, are slow growing, so the supply can be a problem. Scientists can also make the chemical in a lab, or modify it to make it safer, as with aspirin. Plants in the wild are in danger from deforestation, so conserving them is important to protect possible sources of new medicines.
Mark scheme
- Examples of medicines from plants — 1 mark
- Many plants not yet tested — 1 mark
- Some are poisonous, dose control — 1 mark
- Supply or growing is difficult — 1 mark
- Chemicals can be made or modified in the lab — 1 mark
- Conservation of plants — 1 mark
Quick check
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1
Which medicine comes from willow bark?
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B: Aspirin
Aspirin was developed from salicylic acid, which is found in willow bark.
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2
Which plant is the source of digitalis?
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A: Foxglove
Digitalis is made from the foxglove, and is used for some heart conditions.
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3
What is the main use of artemisinin?
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C: Malaria
Artemisinin, from sweet wormwood, is used to treat malaria.
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4
Why do plants make bitter-tasting chemicals?
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D: To deter herbivores
They put animals off eating the plant.
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5
What does a clear zone around a disc on an agar plate show?
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B: That the extract stopped bacteria growing
The extract has stopped bacteria growing in that area.
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6
Why is a control disc with only the solvent used?
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A: To show that the solvent alone has no effect
It shows that any clear zone is caused by the plant extract and not by the solvent.
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7
Which plant gives the painkiller morphine?
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D: Opium poppy
Morphine comes from the opium poppy.
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8
A zone of inhibition has a diameter of 10 mm. What is its radius?
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C: 5 mm
The radius is half of the diameter, so 5 mm.
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9
Why is it important to conserve plants in the wild?
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B: They may be a source of new medicines
Many plants have not been tested, so they may contain chemicals that could be used as new medicines.