Plant defence responsesSpec 4.3.3.2
In short
Plants defend themselves with physical, chemical and mechanical adaptations. Physical defences include cellulose cell walls, a tough waxy cuticle on leaves and layers of dead cells such as bark, which fall off. Chemical defences include antibacterial chemicals and poisons to deter herbivores. Mechanical adaptations include thorns and hairs, leaves that droop or curl when touched, and mimicry.
Plants have defences that resist the invasion of microorganisms and deter animals that might eat them. You need to be able to describe physical and chemical responses, and mechanical adaptations.
| Type | Examples |
|---|---|
| Physical defences against microorganisms | Cellulose cell walls; a tough waxy cuticle on leaves; layers of dead cells around stems (bark on trees) which fall off |
| Chemical defences | Antibacterial chemicals; poisons to deter herbivores |
| Mechanical adaptations | Thorns and hairs deter animals; leaves which droop or curl when touched; mimicry to trick animals |
Layers of dead cells around the stem, such as bark, act as a barrier. They fall off, which helps to remove any pathogens that have settled on them.
Mimicry means looking like something else so that animals are tricked, for example into avoiding the plant.
Group your answer under physical, chemical and mechanical, and give a specific example for each. A plain 'plants have thorns' needs the idea 'to deter animals'.
Quick check
Which ion deficiency causes stunted growth, and why?
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Nitrate. Nitrate ions are needed for protein synthesis and therefore growth.
Which ion deficiency causes chlorosis?
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Magnesium, because magnesium ions are needed to make chlorophyll.
Name three physical plant defences.
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Cellulose cell walls, a tough waxy cuticle on leaves, and layers of dead cells around stems.
Give two mechanical adaptations of plants.
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Any two of: thorns and hairs, leaves that droop or curl when touched, mimicry.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026