Explain how the adaptations of a mangrove tree allow it to survive in a saline, waterlogged environment.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mangrove trees grow in coastal intertidal zones where sediment is frequently anaerobic and soil water has a salt concentration that would be lethal to most terrestrial plants.
Model answer (4 marks)
Mangrove trees have specialised aerial roots such as pneumatophores that stick out of the waterlogged sediment, allowing oxygen to reach the root tissues for aerobic respiration.
At the root endodermis the Casparian strip acts as an ultrafiltration barrier, preventing most sodium and chloride ions from entering the xylem and therefore keeping the shoot free of toxic salt concentrations.
The leaves contain salt glands that actively excrete excess NaCl onto the leaf surface; the salt is then removed by rain or wind, keeping intracellular ion levels safe.
Finally, mangrove leaves are thick, waxy and often have sunken stomata, giving a low surface‑area‑to‑volume ratio and reduced stomatal aperture. This limits water loss by transpiration and osmotic water movement, counteracting the hypertonic soil environment.
At the root endodermis the Casparian strip acts as an ultrafiltration barrier, preventing most sodium and chloride ions from entering the xylem and therefore keeping the shoot free of toxic salt concentrations.
The leaves contain salt glands that actively excrete excess NaCl onto the leaf surface; the salt is then removed by rain or wind, keeping intracellular ion levels safe.
Finally, mangrove leaves are thick, waxy and often have sunken stomata, giving a low surface‑area‑to‑volume ratio and reduced stomatal aperture. This limits water loss by transpiration and osmotic water movement, counteracting the hypertonic soil environment.
Examiner tips
- Use the exact terms (pneumatophores, Casparian strip, salt glands, sunken stomata) to earn full marks.
- Show the causal link between each adaptation and the problem (anaerobic sediment, high salinity, water loss).
- Keep each point concise – one sentence per adaptation – to fit the 4‑mark structure.
Common mistakes
- Confusing pneumatophores with prop roots; only pneumatophores provide aerial oxygenation.
- Forgetting to mention the Casparian strip as the ultrafiltration mechanism.
- Over‑expanding the leaf adaptation description beyond the key features (thick, waxy, sunken stomata).
Mark scheme (4 marks)
- Mangroves possess aerial roots (pneumatophores / prop roots / knee roots) that project above the anaerobic sediment, allowing oxygen to diffuse into the root tissues for aerobic respiration.
- Ultrafiltration at the root endodermis / Casparian strip excludes most salt ions from entering the xylem, preventing toxic salt concentrations from reaching the shoot.
- Salt glands on leaves actively secrete excess sodium chloride / salt ions to the leaf surface, which are then washed or blown away, preventing toxic ion accumulation in cells.
- Thick, succulent / waxy leaves with low surface-area-to-volume ratio and reduced stomatal aperture / sunken stomata limit water loss by osmosis / transpiration, counteracting the tendency to lose water to the hypertonic soil solution.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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