Explain how the adaptations of mangrove trees allow them to survive in waterlogged, saline soils.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mangrove trees grow in coastal intertidal zones where soils are waterlogged, low in oxygen, and have high salt concentrations. Despite these extreme conditions, mangroves are highly productive and form dense forests along tropical and subtropical coastlines.
Model answer (4 marks)
Mangrove trees have specialised root systems – prop roots and pneumatophores – that extend above the water‑logged sediment, allowing oxygen to reach the root tips and supporting aerobic respiration.
Their leaves contain salt glands that actively excrete excess salt, and some species prevent salt entry at the root by ultrafiltration, so toxic ions do not accumulate in cells.
Inside the root cells the osmotic potential is lowered by the accumulation of compatible solutes or by salt exclusion, which creates a water potential gradient that draws water from the saline soil.
Finally, the leaves are thick, waxy or succulent, reducing transpiration and conserving water when osmotic stress makes water uptake difficult.
Their leaves contain salt glands that actively excrete excess salt, and some species prevent salt entry at the root by ultrafiltration, so toxic ions do not accumulate in cells.
Inside the root cells the osmotic potential is lowered by the accumulation of compatible solutes or by salt exclusion, which creates a water potential gradient that draws water from the saline soil.
Finally, the leaves are thick, waxy or succulent, reducing transpiration and conserving water when osmotic stress makes water uptake difficult.
Examiner tips
- Use the exact terms – prop roots, pneumatophores, salt glands, ultrafiltration, compatible solutes, thick/waxy leaves
- Show the causal link: adaptation → function → survival
- Keep each point concise to fit 4 marks
Common mistakes
- Confusing salt glands with salt bladders; not mentioning root ultrafiltration
- Failing to explain the osmotic gradient or water potential mechanism
- Using vague terms like ‘specialised roots’ without naming prop roots or pneumatophores
Mark scheme (4 marks)
- Prop roots / pneumatophores (aerial roots) project above the waterlogged sediment to allow gas exchange / uptake of oxygen for aerobic respiration
- Salt glands in leaves actively excrete excess salt / some species exclude salt at the root by ultrafiltration, preventing toxic accumulation in cells
- Low water potential inside root cells (due to accumulation of compatible solutes / salt exclusion mechanisms) allows osmotic uptake of water from saline soil
- Thick, waxy / succulent leaves reduce water loss by transpiration, conserving water in an environment where osmotic stress makes water uptake difficult
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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