The xylem structure
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of plant tissue organisation (from GCSE). You can test your knowledge on this below.
What is the main function of the xylem?
The xylem transports water and mineral ions from the roots to the leaves in one direction (upwards) via the transpiration stream.
What is the main function of the phloem?
The phloem transports sugars (sucrose) both up and down the plant (this is called translocation).
What is lignin and what is its role in the xylem?
Lignin is a waterproof polymer that provides strength and support to xylem walls, preventing collapse under pressure and ensuring efficient water transport.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the xylem structure or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Structure of the Xylem
The xylem is a tissue made up of hollow, dead cells that transport water and mineral ions from the roots to the leaves. It consists of:
1. Xylem Vessels
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Long, hollow tubes with no end cell walls, forming a continuous column for water transport.
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The wall is strengthened by lignin, which prevents collapse and provides structural support when under tension.
2. Pits
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Small non-lignified regions in the xylem wall.
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Allow lateral movement of water between adjacent xylem vessels.
3. Lignification Patterns
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Lignin can be arranged in spirals or rings to provide flexibility and prevent breaking under pressure.
4. No cytoplasm / no organelles (e.g. no nuclei) in xylem cells
- So as not to impede / obstruct flow / allows easier water flow.
Differences Between Xylem and Phloem
|
Feature |
Xylem |
Phloem |
|---|---|---|
|
Function |
Transports water & minerals |
Transports organic substances (such as sucrose and amino acids) |
|
Direction |
One-way (roots to leaves) |
Two-way (source to sink) |
|
Cell Type |
Dead cells |
Living cells |
|
Structure |
Hollow tubes with lignin |
Sieve tubes & companion cells |
|
Process |
Transpiration stream |
Translocation |
Key Terms
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Xylem: Plant tissue that transports water and mineral ions.
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Lignin: A waterproof polymer that strengthens xylem walls.
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Pits: Small gaps in lignin that allow sideways movement of water.
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Cohesion-Tension Theory: Model explaining how water is pulled up the xylem due to cohesion between water molecules and tension created by transpiration.
- Transpiration: The evaporation of water from plant leaves, driving water movement.
No answer provided.
Exam Tip
Remember, xylem vessels are made up of dead cells with no organelles or cytoplasm. They form hollow tubes and the cells have no end cell walls. You must link these features to the transport of a continuous column of water in the xylem vessels.
No answer provided.
Explain how the structure of xylem vessels enables them to transport water efficiently. (4 marks)
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Xylem vessels are hollow with no end walls forming a continuous column for unrestricted water flow.
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Walls are thickened with lignin which strengthens the vessel and prevents it from collapsing under tension.
-
Pits in xylem walls allow lateral movement of water between adjacent vessels.
-
no cytoplasm / no organelles / no named organelle to impede / obstruct flow / allows easier water flow
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!