GCSE · Biology · AQA · Spec 8461

Plant defence responses (biology only)

A plant can't run, hide or fight back. So why isn't every plant a free meal for microorganisms and hungry animals?

Sort the defences

What kind of defence — and who is it for?

Pick a defence, then choose the column you think it belongs in. Commit first — the reason appears once you've placed it.

Physical barrier, chemical, or mechanical adaptation?

Still to sort

Physical defence (0)

A barrier that invading microorganisms have to get past.

Where the line is: Thorns and hairs are structures too, but they are listed under their own heading, mechanical adaptations, which act against animals.

Chemical defence (0)

A substance the plant makes that acts on its attacker.

Where the line is: Being made of a substance isn't enough: the waxy cuticle contains wax but protects by being a layer in the way, so it's physical.

Mechanical adaptation (0)

A structure, a movement or a disguise that works on animals.

Where the line is: Poisons also deter animals, but they are a substance, not a structure or movement, so they sit under chemical.

8 of 8 still to sort.

Sort all eight by type first. Then switch to 'Who is it aimed at?' and sort the same eight again. Watch which columns line up.

Watch out: Poisons and antibacterial chemicals sit in the same 'chemical' column but have different targets: poisons deter herbivores, antibacterial chemicals act against bacteria.

Barriers against microorganisms

Reason it through

How do a plant's physical barriers keep microorganisms out, and why does bark falling off help?

Link 1 of 4

First link · your turn

A microorganism lands on a leaf. What is the first thing in its way?

Locked — reveal the link above first

Predict, then check

Some plants have leaves that droop or curl up the moment they are touched.

A caterpillar crawls onto one of these leaves and the leaf suddenly droops and curls. What is most likely to happen?

What do you really think?

Can a plant really defend itself?

A plant is rooted to one spot. It has no legs, no teeth and no claws.

Which of these is closest to what you think right now?

Plants can't really defend themselves. They can't move away from danger.Plant defences are really about stopping animals eating them.Mimicry means the plant pretends to be an animal.Plants defend against two kinds of enemy: microorganisms and animals.

How sure are you?

I'm sureI think soI'm guessing

Plants can't really defend themselves. They can't move away from danger.

This is a common misconception

Moving away isn't the only way to be safe. A plant stands behind defences that are there all the time: a cellulose wall around every cell, a waxy cuticle on its leaves, bark around its stems, and chemicals it makes itself. None of those needs legs.

Plant defences are really about stopping animals eating them.

This is a common misconception

That's only half the story. The cell walls, the waxy cuticle, the bark and the antibacterial chemicals are all aimed at microorganisms trying to invade — an enemy far too small to see. Only the poisons and the mechanical adaptations are aimed at animals.

Mimicry means the plant pretends to be an animal.

This is a common misconception

Mimicry means the plant looks like something that tricks the animal into leaving it alone. For example, some leaves have markings that look like insect eggs already laid, so an insect looking for somewhere to lay its own eggs moves on. The plant isn't pretending to be the animal; it's fooling it.

Plants defend against two kinds of enemy: microorganisms and animals.

This matches the evidence

Exactly. Microorganisms meet physical barriers and antibacterial chemicals; animals meet poisons and mechanical adaptations such as thorns, hairs, moving leaves and mimicry. Always ask who a defence is aimed at.

A plant has no legs, but it isn't defenceless: it has barriers and chemicals for microorganisms, and poisons and mechanical adaptations for animals.

Your turn to write

Describe it in your own words

Describe physical and chemical defence responses in plants. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Cellulose cell walls act as a barrier that microorganisms have to get through. (1 mark)
A tough waxy cuticle on leaves acts as a barrier to microorganisms. (1 mark)
Layers of dead cells around stems (bark) fall off, taking microorganisms on them with them. (1 mark)
Antibacterial chemicals act against bacteria. (1 mark)
Poisons deter herbivores from eating the plant. (1 mark)

Model answer

Physical defences are barriers that resist invasion by microorganisms. Every plant cell has a cellulose cell wall that microorganisms have to get through, and leaves have a tough waxy cuticle over their surface. Stems are surrounded by layers of dead cells, such as bark on trees, which fall off and take any microorganisms on them with them. Chemical defences are substances the plant makes: antibacterial chemicals act against bacteria, and poisons deter herbivores from eating the plant.

Name the defence, then say what it does. 'Waxy cuticle' alone is a name; 'a tough waxy cuticle on leaves acts as a barrier to microorganisms' is a description.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a plant that can't move keeps out microorganisms and puts off animals

What you need to know

Plants have defences against two kinds of enemy: invading microorganisms and animals.
Physical defences resist invasion by microorganisms: cellulose cell walls, a tough waxy cuticle on leaves, and layers of dead cells around stems (bark on trees) which fall off.
Chemical defences are substances the plant makes: antibacterial chemicals, and poisons to deter herbivores.
Mechanical adaptations deter or trick animals: thorns and hairs, leaves that droop or curl when touched, and mimicry.

The big picture

Plants defend themselves against two kinds of enemy. Invading microorganisms meet physical barriers (cellulose cell walls, a tough waxy cuticle on leaves, and layers of dead cells around stems that fall off) and antibacterial chemicals. Animals meet poisons that deter herbivores and mechanical adaptations: thorns and hairs, leaves that droop or curl when touched, and mimicry that tricks them.

Key points

1Cellulose cell walls and the waxy cuticle are barriers a microorganism must get past.

2Bark is layers of dead cells; when the outer layers fall off, microorganisms on them go too.

3Antibacterial chemicals act against bacteria; poisons deter herbivores — both are chemical defences.

4Thorns and hairs deter animals.

5Leaves that droop or curl when touched can dislodge insects and startle feeding animals.

6Mimicry tricks animals, for example leaf markings that look like insect eggs already laid.

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Worked example

Problem

A gardener scrubs some leaves on a plant and scratches through their waxy surface. A week later, only the scratched leaves have become infected. Explain why.

⚠ Watch out

Saying bark protects a tree just by being thick. The detail that earns credit is that the dead outer layers fall off, and any microorganisms on them fall away too.

🧠

Memory hook

Microbes meet walls, wax, falling bark and antibacterial chemicals. Animals meet poison, prickles, a flinch and a fake.

✓

Check yourself

Cover the page. Can you name the three physical defences, the two chemical defences and the three mechanical adaptations — and say which ones are aimed at animals rather than microorganisms?

Flashcards

(11)

What do a plant's physical defences resist?

Invasion by microorganisms.

How do cellulose cell walls protect a plant?

They are a barrier around every cell that a microorganism has to get through before it can invade the cell.

Which physical defence coats the surface of leaves?

A tough waxy cuticle.

Bark is made of layers of dead cells. Why does it matter that they fall off?

Microorganisms settle on the dead outer layers; when those layers fall off, the microorganisms fall away with them.

Antibacterial chemicals: which heading, and what target?

Chemical defence; they act against bacteria.

Poisons: which heading, and what target?

Chemical defence; they deter herbivores from eating the plant.

Thorns and hairs: which heading, and what do they do?

Mechanical adaptations; they deter animals.

How can leaves that droop or curl when touched protect a plant?

The movement can dislodge insects from the leaf and startle animals that are feeding. It's a mechanical adaptation.

What is mimicry as a plant defence?

A mechanical adaptation that tricks animals, e.g. leaf markings that look like insect eggs already laid, so an insect lays its eggs elsewhere.

Which plant defences are aimed at animals rather than microorganisms?

Poisons, thorns and hairs, leaves that droop or curl when touched, and mimicry.

Name the three headings plant defences are listed under.

Physical defences, chemical defences and mechanical adaptations.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What kind of defence — and who is it for?

Can a plant really defend itself?

Describe it in your own words

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