2.7.6Additional lessons
Adaptations of Xylem Vessels — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 39 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Structure of a Vessel
Dead cells
- Dead cells Mature vessels have lost their cytoplasm and nucleus, leaving a hollow tube that water can move through freely.
No end walls
- No end walls The end walls between cells have broken down, so vessel elements form one continuous pipe.
Lignin
- Lignin Walls are thickened with lignin, a strong waterproof material that resists the inward pull of tension.
Patterns
- Patterns Lignin forms rings, spirals or networks, which keep the tube open yet allow it to bend and grow.
Pits
- Pits Unthickened gaps called pits let water pass sideways between vessels and into neighbouring cells.
Part 2 of 6Why Each Feature Matters
Every adaptation relates to carrying water upwards under negative pressure.
Why Each Feature Matters
No contents
- No contents Without cytoplasm or organelles nothing obstructs the flow, so resistance to water movement is low.
Strong walls
- Strong walls Lignified walls resist collapse when transpiration pull makes the pressure inside the vessel negative.
Waterproof
- Waterproof Lignin makes walls waterproof, so water stays inside the vessel and does not leak out.
Narrow lumen
- Narrow lumen A narrow tube helps cohesion and adhesion to hold the water column together without breaking.
| Feature | Adaptation for |
|---|---|
| Hollow, dead cells | Unobstructed flow |
| Lignified walls | Withstanding tension |
| Pits | Sideways movement of water |
| Continuous tube | Unbroken water column |
Part 3 of 6Moving Water Upwards
1
Evaporation from leaf cell walls creates a pull on the water in the xylem.
2
Hydrogen bonds make water molecules cohesive, so the whole column moves upwards together.
3
Adhesion to the wall helps support the column and keeps it from falling back.
4
Strong lignified walls prevent the vessel from being squashed by the negative pressure.
Part 4 of 6Comparison with Phloem
Comparison with Phloem
Direction
- Direction Xylem carries water upwards from the roots, whereas phloem transports sugars in both directions.
Cell state
- Cell state Xylem vessels are dead and hollow, but phloem sieve tubes are living cells with companion cells.
Driving force
- Driving force Xylem relies on transpiration pull, while phloem transport needs energy from companion cells.
Part 5 of 6Evidence from Sections
Evidence from Sections
Appearance
- Appearance In a stained section, xylem vessels show thick walls of lignin that often stain red or pink.
Size
- Size The lumen of a vessel is wide compared with surrounding cells and contains no cytoplasm.
Position
- Position In a stem the xylem lies on the inside of each vascular bundle, nearer the centre than the phloem.
Part 6 of 6Exam Checklist & Tips
2.7.6 – Key Points for the Exam
- Xylem The plant tissue that carries water and mineral ions upwards from roots to leaves.
- Lignin A strong, waterproof polymer that thickens xylem walls.
- Pit A thin area in a xylem wall that allows lateral movement of water.
- Lumen The hollow central space of a xylem vessel.
Common Mistakes
- Describing xylem vessels as living cells with a nucleus; mature vessels are dead.
- Stating that lignin lets water enter through the wall, when it is waterproof.
- Saying that xylem carries sugars; its main role is water and mineral ions.
- Suggesting that xylem uses active transport; it moves water passively using transpiration pull.
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