Xylem vessels at Higher Level: The same statement as SL. HL papers also ask how xylem supports the plant, and link the polar walls to adhesion and capillary action.
Practise this as you read
- Outline two adaptations of xylem vessels for water transport.
- Explain the adaptations of xylem for support and transport.
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The big idea: A xylem vessel is a long, hollow, lignified tube of dead cells: water flows through it unimpeded, and the walls do not collapse under tension.
This only works because the xylem is built for the job. The pulling force (tension) is strong enough to collapse an ordinary tube, so xylem vessels are specially adapted.
How xylem vessels are adapted
- Dead, hollow cells stacked end to end
- The end walls are removed, so they form one long continuous pipe with nothing blocking the flow
- Walls thickened with lignin (they are lignified) keep the vessel strong, so it does not collapse under the tension of the water column
- Their narrow diameter helps keep the water column continuous
| Xylem adaptation | What it is | Why it helps water transport |
|---|---|---|
| Hollow, dead cells | The cells lose their contents and die, leaving an empty tube | Water flows through with nothing blocking the way |
| No end walls | The end walls between stacked cells break down | The cells join into one long, continuous pipe from root to leaf |
| Lignified walls | The walls are thickened and strengthened with lignin | The vessel does not collapse under the tension (pull) of the water column |
| Narrow diameter | Each vessel is a thin tube | Helps the water column stay continuous and supports cohesion |
The points to remember
- No cell contents: nothing gets in the way of the flow.
- No end walls (or incomplete ones): one continuous tube from root to leaf.
- Lignified walls: strong enough to withstand the tension without collapsing.
- Pits: thin, unlignified patches that let water enter and leave sideways.
- Walls are polar, so water adheres to them; a narrow tube helps capillary action.
Remember it as: Empty, endless, strong, with doors: no contents, no end walls, lignin, pits.
Real example: growth rings. Each ring in a cut tree trunk is a year's new xylem: wide vessels grown in spring, narrow ones in summer. Counting the rings gives the age of the tree.
Two traps: Each adaptation needs its reason: 'lignified walls' alone does not score; 'lignified walls withstand tension' does.
It is lignin, not cellulose, that stops the vessel collapsing.
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How it is asked: Expect 'outline two adaptations of xylem vessels', often beside a micrograph of xylem, or 'explain the adaptations of xylem for its functions'.
Xylem supports a plant and transports water and minerals. Explain the adaptations of xylem for its functions.
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