Plant defence responses — AQA GCSE Biology
Test yourself on Plant defence responses with AQA GCSE practice questions.
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Plant defence responses explained
Physical defences are structures that block microorganisms from entering plant tissues or limit their spread once inside.
Read the full explanation
Cellulose cell walls surround each cell and are strong and rigid, so they resist penetration by fungal hyphae and bacteria. A tough waxy cuticle covers the leaf surface and repels water, so pathogens cannot easily settle and enter. Layers of dead cells around stems, forming bark on trees, create an outer barrier that eventually falls off, carrying pathogens away. These defences work together: a thick wall slows invasion, the cuticle reduces surface entry, and shedding bark removes established pathogens. A useful method is to trace a pathogen from the leaf surface inward and identify which physical barrier it meets at each stage.
Your focus
- Identify physical plant defences that resist invasion by microorganisms.
- Explain how cellulose cell walls, a waxy cuticle and dead bark layers reduce entry or spread of pathogens.
- Distinguish physical defences from chemical defences when describing plant responses.
Plant defence responses exam tips
Marking Points
- Physical defences are structural barriers that prevent microorganisms entering plant tissues or spreading within them.
- Cellulose cell walls are strong and rigid and resist penetration by microorganisms such as fungi and bacteria.
- A tough waxy cuticle on leaves repels water and forms a surface barrier against pathogen entry.
- Layers of dead cells around stems, seen as bark on trees, form an outer barrier and fall off, removing pathogens.
- A full description links each structure to reduced invasion or spread of microorganisms rather than naming it alone.
Examiner Tips
- 💡Use the phrase physical defence only for structures, and keep chemical examples such as poisons separate.
- 💡Explain the mechanism, for example the cuticle repels water so pathogens cannot settle and penetrate.
- 💡Link each barrier to the invasion process: entry, spread or removal of the pathogen.
Common Mistakes
- Treating the waxy cuticle as waterproofing only: correct this by also stating that it is a physical barrier to microorganism entry.
- Saying cellulose cell walls kill microorganisms: correct this by stating that they are strong barriers that resist penetration.
- Forgetting that bark falls off: correct this by explaining that shedding dead cell layers removes pathogens from the stem.