Triple Science Only - Plant Defence Responses
Joe Wolfensohn
Teacher
Joe Wolfensohn
Assistant Head of Biology
Recall Questions
This topic requires prior knowledge of how plants can be affected by pathogens and the general concept of immune responses in organisms.
Name a fungal pathogen that infects plants?
Rose black spot.
State a symptom of infection with a plant pathogen?
Stunted growth, spots on leaves, areas of decay (rot), abnormal growths, malformed stems or leaves, discolouration.
What are the specific markers on the surface of cells and pathogens called?
Antigens.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains plant defence responses, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Physical, Chemical & Mechanical Plant Defences
Plants are constantly exposed to microorganisms and herbivores in their environment. Because they can’t move to escape these threats, they have developed physical, chemical, and mechanical defence responses.
Physical Defences (to prevent entry of pathogens)
- Cellulose cell walls: Provide a strong structural barrier that many microorganisms cannot penetrate.
- Waxy cuticle on leaves: A waterproof layer that stops water and pathogens from entering the leaf surface.
- Layers of dead cells: For example, bark on trees protects living tissue underneath. It falls off and removes pathogens with it.
Chemical Defences (to kill or deter pathogens and herbivores)
- Antibacterial chemicals produced by plants: These kill bacteria and help protect the plant. E.g. mint and witch hazel produce natural antiseptics.
- Poisons: Some plants produce poisons / toxins to make themselves unpalatable or harmful to animals (e.g. foxglove, deadly nightshade).
Mechanical Adaptations (to physically deter herbivores)
- Thorns and hairs: Sharp structures on stems or leaves make plants difficult or painful to eat.
- Leaves that droop or curl when touched: Mimosa pudica is a plant whose leaves quickly fold when touched, surprising animals and deterring grazing.
- Mimicry: Some plants mimic other harmful or inedible organisms. For example, some plants look like they are already infested by insect eggs, discouraging real insects from laying more.
Key Terms and Definitions
Term |
Definition |
|
Physical defence |
Structural barriers that prevent microorganisms from entering the plant. |
|
Chemical defence |
Substances produced by plants to kill or deter pathogens or herbivores. |
|
Mechanical adaptation |
Physical features or behaviours that help deter animals from eating the plant. |
|
Mimicry |
When a plant resembles something else to avoid being eaten. |
No answer provided.
Exam Tip
In longer questions, group defences clearly under physical, chemical, and mechanical headings and always use examples.
No answer provided.
Practice Question
Describe how plants defend themselves against the threat of microorganisms and herbivores. (3marks)
Model Answer:
- Plants have physical defences like cellulose cell walls, tough waxy cuticles on leaves, and layers of dead cells such as bark that prevent microorganisms from entering.
- They also use chemical defences such as antibacterial substances that kill bacteria and poisons that deter herbivores.
- Mechanical adaptations include thorns and hairs that make them difficult to eat, leaves that curl when touched to scare herbivores, and mimicry to avoid attack.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!