Exam questions · Biology
Plant Defences and Disease
- 18 exam questions
- 61 marks
- 27 quick checks
Physical Defences of Plants
Just this lesson-
1 Name [2 marks]
Name two physical barriers that protect a leaf against pathogens.
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Model answer
The waxy cuticle and the cell wall.
Mark scheme
- Waxy cuticle — 1 mark
- Cell wall — 1 mark
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2 State [2 marks]
State what the cell walls of plants are made of, and state the function of the cuticle.
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Model answer
Cell walls are made of cellulose. The cuticle is a waxy layer that is waterproof, and reduces water loss and stops pathogens entering.
Mark scheme
- Cellulose — 1 mark
- Waxy waterproof barrier — 1 mark
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3 Explain [3 marks]
Explain how the waxy cuticle helps to protect a leaf from fungal infection.
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Model answer
The cuticle is waterproof, so water runs off the leaf and the surface stays dry. Fungal spores need water to germinate, so they are less likely to grow. The cuticle is also a tough, smooth layer, so it is difficult for fungi to get through.
Mark scheme
- Waterproof, surface dry — 1 mark
- Spores need water to germinate — 1 mark
- Tough layer difficult to get through — 1 mark
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4 Describe [3 marks]
Describe two physical defences that protect plants against animals that eat them.
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Model answer
Thorns or spines are sharp, so they hurt animals that try to eat the plant. Hairs on the leaves, like those of nettles, can irritate the skin of an animal or stop insects getting to the leaf.
Mark scheme
- Thorns / spines — 1 mark
- Hairs — 1 mark
- A function for each — 1 mark
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5 Calculate [3 marks]
In a sample of 60 plants, 45 have waxy leaves. Of the waxy plants, 3 are infected with a fungus. Of the other plants, 6 are infected. Calculate the percentage of each group that is infected.
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Model answer
Waxy plants: 3 ÷ 45 × 100 = 6.7%. Other plants: 6 ÷ 15 × 100 = 40%.
Mark scheme
- 15 plants without wax — 1 mark
- 6.7% — 1 mark
- 40% — 1 mark
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6 Evaluate [6 marks]
Explain why stomata are a weak point in the physical defences of a leaf, and describe how a plant can protect itself.
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Model answer
Stomata are pores in the leaf that must be open to let carbon dioxide in for photosynthesis, and oxygen and water vapour out. This means that bacteria and fungi on the surface can get into the leaf through them, so the waxy cuticle and the cell walls are bypassed. The plant can protect itself by closing the stomata, with the guard cells, when it detects bacteria on the surface. This keeps the bacteria out, but it also stops the leaf from taking in carbon dioxide, so photosynthesis slows down. The plant can also strengthen its cell walls and produce chemicals to fight any pathogens that do get in.
Mark scheme
- Stomata let gases in — 1 mark
- Pathogens can enter — 1 mark
- Bypass cuticle and cell wall — 1 mark
- Close stomata using guard cells — 1 mark
- Cost: less photosynthesis — 1 mark
- Other responses (cell wall, chemicals) — 1 mark
Quick check
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1
What is the cuticle of a leaf made of?
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C: Wax
The cuticle is a layer of wax.
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2
What is the cell wall of a plant made of?
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A: Cellulose
Cell walls are made of cellulose.
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3
Which structure forms the outer barrier of a tree trunk?
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B: Bark
Bark is a layer of dead cells that forms a tough barrier around the trunk.
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4
How does a thorn protect a plant?
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D: It stops animals eating the plant
Thorns are sharp, so animals are hurt when they try to eat the plant.
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5
Why are stomata a weak point in a leaf?
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C: Pathogens can get in through them
Pathogens can enter the leaf through the open stomata.
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6
How can a plant defend itself at a stoma?
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A: By closing it with the guard cells
The guard cells can close the stoma when bacteria are detected.
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7
Why does the waxy surface protect against fungal spores?
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B: The surface stays dry, and spores need water
The surface stays dry, and spores need water to germinate.
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8
In a sample of 50 plants, 10 are infected. What is the percentage infected?
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D: 20%
\(10 \div 50 \times 100 = 20\%\).
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9
Which statement best describes physical defences of plants?
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A: They are always present, but pathogens can enter by weak points
Physical defences are structures that are always in place to stop pests and pathogens, but they can be bypassed.
Chemical Defences of Plants
Just this lesson-
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.
Detecting Plant Diseases
Just this lesson-
1 State [2 marks]
State two symptoms that could show that a plant has a disease.
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Model answer
Any two from: spots on the leaves, yellowing, wilting, mould or fungal growth, lesions on bark or stems, stunted growth, galls.
Mark scheme
- First symptom — 1 mark
- Second symptom — 1 mark
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2 State [2 marks]
State two environmental causes that could make the leaves of a plant turn yellow.
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Model answer
Any two from: lack of magnesium, lack of nitrate, too little light, too little or too much water, frost, chemical damage.
Mark scheme
- First — 1 mark
- Second — 1 mark
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3 Explain [3 marks]
Explain how the pattern of affected plants in a field can help to show whether a disease is caused by a pathogen.
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Model answer
If a pathogen is the cause, the affected plants will often be clustered around a point and spread outwards, as the pathogen is passed from plant to plant, and may follow the direction of the wind. If the whole field is affected evenly, the cause is more likely to be an environmental factor such as the soil or the weather.
Mark scheme
- Pathogen: clustered and spreading — 1 mark
- May follow wind or water — 1 mark
- Even pattern suggests environment — 1 mark
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4 Calculate [3 marks]
A random sample of 80 plants from a field of 12 000 plants shows that 20 have symptoms of disease. Calculate the percentage of the sample with symptoms, and estimate the number of diseased plants in the field.
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Model answer
20 ÷ 80 × 100 = 25%. 25% of 12 000 = 3000 plants.
Mark scheme
- 25% — 1 mark
- 0.25 × 12 000 — 1 mark
- 3000 plants — 1 mark
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5 Describe [4 marks]
Describe four ways of identifying the pathogen that is causing a disease in a plant.
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Model answer
A sample of infected tissue can be looked at under a microscope to find fungal threads or bacteria. The pathogen can be grown on agar and identified from how it grows. DNA tests can be used to compare the pathogen with known ones. A test kit using monoclonal antibodies can identify a specific pathogen from the colour change or line.
Mark scheme
- Microscopy — 1 mark
- Culturing — 1 mark
- DNA testing — 1 mark
- Antibody test kit — 1 mark
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6 Evaluate [6 marks]
A gardener finds that her tomato plants have yellow leaves and spots. Describe a series of tests that could be used to find out if a pathogen is the cause, and explain the possible results.
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Model answer
She should first look at the symptoms and compare them with a guide to find a likely disease. Then she should rule out the environment by checking the soil for minerals such as magnesium and nitrate, checking the watering and the temperature, and she could add fertiliser to see if the plants recover. She should look at where the affected plants are. If they are clustered and spreading, this suggests a pathogen, but if all are equally affected, then the cause is probably the environment. A sample could be taken to a laboratory, to be looked at under a microscope, grown on agar, or tested using DNA or an antibody test, which would identify any pathogen.
Mark scheme
- Observe symptoms and compare — 1 mark
- Check the soil, water and temperature — 1 mark
- Test by adding fertiliser — 1 mark
- Look at the distribution — 1 mark
- Cluster suggests a pathogen, even suggests environment — 1 mark
- Laboratory tests — 1 mark
Quick check
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1
What is the first step in detecting a plant disease in the field?
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A: Observe the symptoms
The first step is to observe the symptoms of the plant.
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2
Which of these could make leaves turn yellow without a pathogen?
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C: A shortage of minerals
A lack of magnesium or nitrate causes yellowing, which is an environmental cause.
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3
What does a cluster of diseased plants spreading outward suggest?
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B: A pathogen
A pathogen that spreads from one place to others would give this pattern.
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4
Which test uses monoclonal antibodies?
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D: A test kit that shows a line or colour change
Antibody test kits use monoclonal antibodies that bind to a specific pathogen.
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5
Why is a random sample taken from a large field?
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C: Because not every plant can be checked
It is not possible to check all of the plants, so a sample is used to estimate the amount of disease.
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6
In a sample of 40 plants, 8 have symptoms. What is the percentage?
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A: 20%
\(8 \div 40 \times 100 = 20\%\).
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7
Why is DNA testing a useful way to identify a plant pathogen?
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B: It is accurate, and can identify viruses
It is accurate, and can identify viruses that cannot be cultured or seen easily.
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8
What would evenly distributed damage across a whole field suggest?
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D: An environmental factor
An even pattern suggests that the soil, weather or a chemical is the cause.
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9
Adding fertiliser cures yellowing in the plants. What does this show?
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A: The cause was a mineral deficiency
It shows that a mineral deficiency was the cause, so a pathogen is not needed to explain the symptoms.