AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Physical defence responses to resist invasion of microorganisms.
Full specification wording
Plant defence responses: Physical defence responses to resist invasion of microorganisms.
Physical defences are structures that block microorganisms from entering plant tissues or limit their spread once inside.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Physical defences are structures that block microorganisms from entering plant tissues or limit their spread once inside. 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.
What earns the marks
- 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.
Common mistakes and how to put them right
Treating the waxy cuticle as waterproofing only.
Put it right: Also stating that it is a physical barrier to microorganism entry.
Saying cellulose cell walls kill microorganisms.
Put it right by stating that they are strong barriers that resist penetration.
Forgetting that bark falls off.
Put it right by explaining that shedding dead cell layers removes pathogens from the stem.
Exam 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.
Try the lesson
The start of the Plant defence responses: Physical defence responses to resist invasion of microorganisms. lesson, made from checked teaching notes for this specification point.
Have a go first
Which physical barrier on the surface of a leaf helps to stop microorganisms entering the plant?
Listen
Physical defences are structures that block microorganisms from entering plant tissues or limit their spread once inside. 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. A strong answer earns marks for these 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 common slip: Treating the waxy cuticle as waterproofing only. Instead, also stating that it is a physical barrier to microorganism entry. Exam tip: Use the phrase physical defence only for structures, and keep chemical examples such as poisons separate.
Key words
Say what each word means before you turn the card.
Structural barriers to pathogens
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Cellulose cell walls
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Waxy cuticle on leaves
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Dead cell layers and shedding bark
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See every definition
- Structural barriers to pathogens
- Physical defences that block microorganisms from entering plant tissues or limit their spread once inside.
- Cellulose cell walls
- Strong and rigid structures surrounding each cell that resist penetration by fungal hyphae and bacteria.
- Waxy cuticle on leaves
- A tough covering on the leaf surface that repels water, so pathogens cannot easily settle and enter.
- Dead cell layers and shedding bark
- Layers of dead cells around stems, forming bark on trees, that create an outer barrier and eventually fall off, carrying pathogens away.
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