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Exam questions · Biology · Plant Defences and Disease

Physical Defences of Plants

  • 6 exam questions
  • 19 marks
  • 9 quick checks
  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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

  1. 1

    What is the cuticle of a leaf made of?

    1. ACellulose
    2. BProtein
    3. CWax
    4. DStarch
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    C: Wax

    The cuticle is a layer of wax.

  2. 2

    What is the cell wall of a plant made of?

    1. ACellulose
    2. BWax
    3. CCork
    4. DLipid
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    A: Cellulose

    Cell walls are made of cellulose.

  3. 3

    Which structure forms the outer barrier of a tree trunk?

    1. AThe cuticle
    2. BBark
    3. CA stoma
    4. DA root hair
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    B: Bark

    Bark is a layer of dead cells that forms a tough barrier around the trunk.

  4. 4

    How does a thorn protect a plant?

    1. AIt makes poison
    2. BIt makes food
    3. CIt attracts insects
    4. DIt stops animals eating the 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.

  5. 5

    Why are stomata a weak point in a leaf?

    1. AThey make wax
    2. BThey lose water
    3. CPathogens can get in through them
    4. DThey are made of cellulose
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    C: Pathogens can get in through them

    Pathogens can enter the leaf through the open stomata.

  6. 6

    How can a plant defend itself at a stoma?

    1. ABy closing it with the guard cells
    2. BBy making it larger
    3. CBy making more wax
    4. DBy removing it
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    A: By closing it with the guard cells

    The guard cells can close the stoma when bacteria are detected.

  7. 7

    Why does the waxy surface protect against fungal spores?

    1. AThe wax kills the spores
    2. BThe surface stays dry, and spores need water
    3. CThe wax is made of cellulose
    4. DThe spores cannot be seen
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    B: The surface stays dry, and spores need water

    The surface stays dry, and spores need water to germinate.

  8. 8

    In a sample of 50 plants, 10 are infected. What is the percentage infected?

    1. A5%
    2. B10%
    3. C40%
    4. D20%
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    D: 20%

    \(10 \div 50 \times 100 = 20\%\).

  9. 9

    Which statement best describes physical defences of plants?

    1. AThey are always present, but pathogens can enter by weak points
    2. BThey only work when the plant is attacked
    3. CThey kill all pathogens
    4. DThey are made of chemicals
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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.