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Biology · Plant Defences and Disease

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Biology · Edexcel Paper 1

Plant Defences and Disease

Physical Defences of Plants (Separate only)

Plants cannot run away from the animals that eat them, or from the microorganisms that infect them.

  • Cuticle The waxy layer on the surface of a leaf that reduces water loss and is a barrier to pathogens.
  • Cell wall The rigid outer layer of a plant cell, made of cellulose.
  • Physical barrier A structure that stops a pathogen or a pest from entering a plant.
A cross-section of a leaf with the waxy cuticle and the cell walls of the epidermis blocking pathogens, and a stoma as a possible way in.
The cuticle and the cell walls block most pathogens. Stomata and wounds are weak points.

Physical and chemical defences

  • Physical defences Structures that stop an attack from getting in; Always present, as part of the plant; Examples: cuticle, cell wall, bark, thorns
  • Chemical defences Substances that kill or deter attackers; Many are made when the plant is under attack; Examples: poisons, antibacterial chemicals, bitter compounds

Chemical Defences of Plants (Separate only)

Many plants make poisons, bitter chemicals and antibiotics.

  • Chemical defence A chemical made by a plant that protects it from pests and pathogens.
  • Antibacterial Able to kill bacteria or stop them growing.
  • Toxin A poison.
Four plants with the chemicals they make and the medicines they led to: willow and aspirin, foxglove and digitalis, poppy and morphine, and sweet wormwood and artemisinin.
Many important medicines began as plant defence chemicals.

Plants and their chemicals

  • How the plant uses the chemical Poison to stop herbivores eating the plant; Antibacterial or antifungal chemicals to kill pathogens; Bitter taste or irritant to deter pests
  • How people use it Medicines for pain, such as aspirin and morphine; Drugs for the heart, such as digitalis; Antimalarial drugs, such as quinine and artemisinin

Detecting Plant Diseases (Separate only)

How do you tell what is wrong with a sick plant? This lesson is for separate Biology only: Combined Science students do not need it.

  • Symptom A visible sign of disease in a plant, such as spots or wilting.
  • Chlorosis Yellowing of leaves because of a lack of chlorophyll.
  • Mineral deficiency A lack of a mineral that a plant needs, such as nitrate or magnesium.
Four steps to find the cause of a plant disease: observe symptoms, rule out environmental causes, map the distribution and run laboratory tests.
A pattern that spreads from a point suggests a pathogen, and an even pattern suggests the environment.

Pathogen or environment?

  • Signs of a pathogen Affected plants clustered, and spreading; Symptoms start in one place and move outwards; A pathogen is found in a test
  • Signs of an environmental cause Plants affected evenly across the area; A deficiency or damage is found in the soil or weather; No pathogen is found