Plant diseases defences (Edexcel GCSE Biology Combined Science): Revision Notes

Plant diseases defences

Plants have evolved a range of physical, chemical, and mechanical defences to protect themselves from invading pathogens and predators.

Physical Defences: Preventing Microorganism Invasion

• Waxy Cuticle: The tough, waxy cuticle on the surface of leaves acts as a barrier, preventing pathogens from entering the plant.
• Cellulose Cell Walls: The cellulose in plant cell walls forms a strong physical barrier that protects the cells from pathogen invasion.
• Layers of Dead Cells: Plants develop layers of dead cells around their stems, such as bark, which act as a shield against pathogens. These dead cells eventually fall off, taking any attached pathogens with them.

Chemical Defences: Deterring Predators and Killing Bacteria

• Poisons: Certain plants produce poisons, like those found in foxgloves, tobacco plants, deadly nightshade, and yew, to deter herbivores from eating them.
• Antibacterial Compounds: Some plants, such as mint and witch hazel, produce antibacterial compounds that kill bacteria and protect the plant from infections.

Mechanical Defences: Protecting Against Predators

• Thorns and Hairs: Many plants have thorns or hairs that make it difficult and painful for animals to consume them, though these defences are less effective against insect pests.
• Drooping or Curling Leaves: Some plants have leaves that droop or curl when touched. This movement can dislodge insects or move the plant away from harm.
• Mimicry:
  • Unhealthy Appearance: Some plants droop to mimic the appearance of an unhealthy plant, discouraging animals from eating them.
  • Butterfly Egg Mimicry: Certain plants have patterns that resemble butterfly eggs, deterring butterflies from laying their eggs on the leaves to avoid competition.
  • Stone and Pebble Mimicry: Plants in the 'ice plant family' have evolved to resemble stones or pebbles, making them less likely to be eaten by herbivores.

Medicines from plants

Quinine	Comes from bark of cinchona tree Was the main treatment of malaria
Aspirin	Relieve pain & fever Developed from chemical found in bark & leaves of willow trees