AQA GCSE Biology Foundation (8461), Foundation tier · Infection and response › Plant disease
Practise Plant defence responses. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The physical, chemical and mechanical ways in which plants defend themselves against pathogens and against animals that eat them. This subtopic is GCSE Biology only (not Combined Science). Questions ask you to name defences or to explain how an unfamiliar adaptation helps a plant survive.
Key facts
Physical: cellulose cell walls, tough waxy cuticle, layers of dead cells (e.g. bark) that fall off
Chemical: antibacterial chemicals, poisons to deter herbivores
Mechanical: thorns and hairs, leaves that droop or curl when touched, mimicry
Herbivore = animal that eats plants
Mimicry = looking like something else to trick animals
Notes
Physical defences against microorganisms
diagram
Cellulose cell walls: a strong layer around each cell that pathogens find difficult to get through.
A tough waxy cuticle on the leaves: a barrier over the leaf surface.
Layers of dead cells on the surface of stems, e.g. bark on trees. They form a barrier, and they fall off, taking any pathogens on them with them.
Physical barriers keep microorganisms out; thorns and hairs deter animals.
Chemical defences
Antibacterial chemicals kill bacteria. Some plants, such as mint, produce them.
Poisons deter herbivores (animals that eat plants), e.g. foxgloves are poisonous.
Mechanical adaptations against animals
Thorns and hairs deter animals from touching or eating the plant, e.g. the thorns on a rose stem.
Leaves that droop or curl when touched, e.g. in the sensitive plant. This can knock insects off and makes the plant look less appealing to eat.
Mimicry tricks animals. Some plants have spots that look like insect eggs, so the insects do not lay their eggs there. Some plants look like a stinging plant, so animals avoid them.
Why defences matter
Fewer pathogens infect the plant and fewer animals eat it.
So it keeps more leaves for photosynthesis and is more likely to survive and reproduce.
How to answer each type of question
Name and classify defences
1 to 3 marksGrade 3
Learn which defences are physical, chemical and mechanical.
Give one clear example for each type asked for.
Example. Give one physical defence, one chemical defence and one mechanical adaptation that plants use.
Show the model answerHide the model answer
Physical: any one from cellulose cell walls, a tough waxy cuticle, layers of dead cells or bark (1). Chemical: antibacterial chemicals or poisons (1). Mechanical: any one from thorns, hairs, leaves that droop or curl when touched, mimicry (1).
Explain how a physical defence protects a plant
2 marksGrade 5
Say that it forms a barrier to pathogens.
Add any extra feature, e.g. dead cells fall off, taking pathogens with them.
Example. Explain how bark helps to protect a tree from pathogens.
Show the model answerHide the model answer
The layers of dead cells form a barrier, so pathogens cannot reach the living cells (1). The dead cells fall off, taking pathogens on their surface with them (1).
Don’t lose marks
Saying the cuticle or cell wall kills pathogens. Physical defences are barriers; antibacterial chemicals kill bacteria.
Calling thorns a defence against microorganisms. Thorns are mechanical adaptations that deter animals.
Stopping at 'animals don't eat it' in a 2-mark 'suggest' question, without linking it to the plant's survival.
Saying plants make antibodies or have white blood cells. Plants defend themselves with barriers, chemicals and mechanical adaptations.
More tips
Memory tricks
Two targets: physical barriers and antibacterial chemicals stop microorganisms; poisons and mechanical adaptations stop animals.
Physical = 'wall, wax, bark': cellulose cell walls, waxy cuticle, dead cells that fall off.
Always end with the pay-off: fewer pathogens get in or fewer animals eat it → more photosynthesis → the plant survives.
Exam technique
Decide the type of defence first (physical, chemical or mechanical), then say what it stops (pathogens or animals).
Unfamiliar examples are common: use the information given and link it to survival.
Use 'barrier' for defences against microorganisms and 'deter' for defences against animals.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name physical defences of plantsCellulose cell walls, a tough waxy cuticle on leaves, and layers of dead cells on stems (e.g. bark) that fall off.
Grade 4
Name chemical defences of plantsAntibacterial chemicals, and poisons that deter herbivores.
Grade 4
Name mechanical adaptations of plantsThorns and hairs, leaves that droop or curl when touched, and mimicry.
Grade 5
Explain how physical barriers stop pathogensThey are barriers that pathogens cannot easily get through, and dead cells take pathogens with them when they fall off.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Plants have physical, chemical and mechanical defences.
(a) Give two physical defences that help plants to resist invasion by microorganisms.[2]
(b) Some plants make poisons. Suggest how making poisons helps these plants to survive.[1]
(c) Which of these is a mechanical defence? Tick (✓) one box.[1]
Antibacterial chemicals
Bark
Thorns
Waxy cuticle
Show the answer and mark scheme
(a)
cellulose cell walls
a tough, waxy cuticle on the leaves
layers of dead cells on the surface of stems / bark, which fall off
(b)
poisons deter herbivores / animals from eating the plant
(c)Answer: Thorns
Question 2Medium8 marks
Plants defend themselves against pathogens and animals in different ways.
(a) Mint plants produce antibacterial chemicals. Explain how these chemicals help the plant.[2]
(b) The leaves of the sensitive plant droop and fold up when they are touched. Suggest two ways this could help the plant.[2]
(c) Explain how a waxy cuticle helps to protect a leaf against pathogens.[2]
(d) The bark of some trees is made of layers of dead cells that fall off. Explain how this helps to protect the tree.[2]
Show the answer and mark scheme
(a)
the chemicals kill bacteria / pathogens
(so) bacteria cannot infect the plant / cause disease
(b)
insects may be knocked off the leaves
the plant looks wilted / less appealing, so herbivores are less likely to eat it
a sudden movement may scare / deter animals
(c)
the cuticle is a barrier (covering the leaf surface)
(so) pathogens cannot enter the cells of the leaf
(d)
the dead cells form a barrier to pathogens
pathogens on the surface are lost when the dead cells fall off