Practise 8 exam-style questions on Plant defence responses (biology only) for AQA GCSE Biology. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
8 questions
2–3 marks each
Instant marking
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8 questions, marked as you go
Sample questions with model answers
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Question 1
2 marks
Describe how systemic acquired resistance (SAR) provides long-term protection after a local infection.
Model answer
local infection triggers signalling molecules (salicylic acid) that travel to other parts and induce broad-spectrum defence genes, giving lasting resistance
Explanation
SAR involves a mobile signal produced at the infection site that activates defensive gene expression in distant tissues, leading to sustained heightened resistance against a broad range of pathogens.
Mark scheme
1 mark
local infection produces signals (eg salicylic acid) that move to other tissues
1 mark
these signals induce expression of defence genes providing long-term systemic resistance
Question 2
2 marks
Explain why mechanical adaptations (thorns, hairs) are less effective against microbial pathogens than against large herbivores.
Model answer
microbes are microscopic and can bypass physical structures; microbes use chemical/enzymatic mechanisms to penetrate tissues, and are often spread via wounds or vectors
Explanation
Thorns and hairs deter large feeders but cannot stop microscopic microbes, which can bypass surface structures by entering small openings (stomata, wounds) or through enzymes and vector transmission, so chemical and cellular defences are needed.
Mark scheme
1 mark
microorganisms are too small to be blocked by thorns / hairs
1 mark
microbes can enter via stomata/wounds or via vectors and can use enzymes to degrade barriers
Question 3
3 marks
Explain why breeding plants for stronger physical defences (thicker cuticle, tougher cell walls) might have trade-offs for crop productivity. Give two trade-offs.
Model answer
energy/resources allocated to defences reduce growth/yield; thicker walls may reduce gas exchange and photosynthesis; less palatable crops may affect consumer preference
Explanation
Investing in stronger physical barriers can reduce energy available for yield and may impair physiological processes (like CO2 uptake), or change crop qualities, all of which can lower productivity or market value.
Mark scheme
1 mark
resources diverted to defence reduce resources for growth or seed/fruit production
1 mark
thicker cuticle/walls may reduce gas exchange/photosynthesis (lower productivity)
1 mark
reduced palatability or altered quality may affect marketability
Accept other reasonable trade-offs.
Question 4
3 marks
A pathogen secretes an enzyme that degrades plant cell walls. Suggest two plant counter-defences and explain how each reduces the enzyme's effect.
Model answer
produce enzyme inhibitors to block pathogen enzyme; reinforce cell wall with callose/lignin to prevent access
Explanation
Plants can make inhibitors that block pathogen enzymes, preventing degradation, and can rapidly thicken or modify cell walls with callose or lignin to provide a resistant barrier. Altering cell wall chemistry also reduces enzyme effectiveness.
Mark scheme
1 mark
produce inhibitors that inhibit pathogen enzymes
Accept named inhibitor or protease/ cell wall enzyme inhibitor.
1 mark
deposition of callose/lignin/suberin to strengthen / thicken walls
1 mark
sequester / modify cell wall components so enzyme cannot bind or act
Accept altering pH or producing decoy substrates.
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