2.7.6Inquiry Process
Adaptations of Xylem Vessels — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 78 of 160
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.
Section 1 of 4Structural Adaptations
Xylem has two jobsmoving water upwards and supporting the plant.
Hollow lumen
- Hollow lumen Mature xylem vessel cells are dead and contain no cytoplasm or organelles, so water can flow through without obstruction.
No end walls
- No end walls The cross walls between vessel elements are broken down, creating a continuous tube from root to leaf.
Lignified walls
- Lignified walls Lignin is a strong, waterproof polymer that makes the walls rigid and stops them collapsing inwards under tension.
Pits
- Pits Unthickened gaps in the wall allow water to move sideways between vessels and into living cells, and to bypass blocked vessels.
Section 2 of 4Wall Patterns and Support
Wall Patterns and Support
Rings and spirals
- Rings and spirals Lignin deposited in rings or spirals leaves flexible gaps, allowing the stem to grow and bend while keeping the tube open.
Reinforcement
- Reinforcement Thickened walls give high tensile strength to resist the pulling force that develops during transpiration.
Support
- Support In woody plants, large amounts of lignified xylem form the bulk of wood and hold the plant upright.
Narrow vessels
- Narrow vessels Narrow diameter increases capillary effects and makes columns less likely to break, though it increases resistance to flow.
| Feature | Benefit |
|---|---|
| Dead hollow cells | Low resistance to flow |
| Lignin | Strength, waterproofing |
| Pits | Lateral movement of water |
Section 3 of 4Why These Features Matter Under Tension
1
Evaporation from leaves lowers pressure in the xylem to values below that of the atmosphere.
2
Without strong walls, this negative pressure would cause the vessels to collapse inwards.
3
Cohesion between water molecules allows the column to be pulled without breaking, provided the tube is smooth and continuous.
4
If air enters and breaks the column, an air bubble can block a vessel, and water has to move sideways via pits instead.
5
Adhesion between water and the walls helps keep water in place when transpiration is low.
Section 4 of 4Exam Checklist & Tips
2.7.6 – Key Points for the Exam
- Xylem vessel A tube made of dead, lignified cells that carries water and minerals up the plant.
- Lignin A strong, waterproof polymer deposited in cell walls of xylem.
- Pit A thin area in a cell wall where water can pass between cells.
- Tension
- A pulling force;
- in xylem it is a negative pressure.
- A pulling force;
Common Mistakes
- Describing xylem as living tissue; the vessel cells are dead at maturity.
- Confusing xylem (water, upwards) with phloem (sugars, both directions).
- Saying lignin makes the wall permeable; it makes the wall waterproof.
- Listing a feature without explaining how it helps function.
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