The xylem and the transpiration stream
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of cell specialisation and transport mechanisms such as osmosis and active transport.
What is osmosis?
The movement of water molecules from a dilute solution to a concentrated solution across a partially permeable membrane.
How do mineral ions enter a root hair cell?
By active transport, using energy from respiration to move ions against their concentration gradient.
What is a specialised cell?
A cell that has adapted features to help it carry out a particular function effectively.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the xylem and the transpiration stream, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Transport in Plants
Plants have a transport system made up of tissues in the roots, stem and leaves. This system moves water, mineral ions and sugars.
Xylem Tissue – Structure and Function
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Transports water and mineral ions from the roots up the stem and to the leaves (in one direction only).
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Made of dead, hollow cells joined end-to-end to form long tubes.
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Walls are strengthened with lignin, making them rigid and waterproof.
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There are no end walls between cells, allowing for continuous water flow.
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The xylem cells have no nuclei, so the flow of water is not restricted.
The Transpiration Stream
The movement of water (and dissolved mineral ions) up a plant from the roots to the leaves via the xylem. The movement of water is a passive process as it does not require energy from respiration.
Steps in the Transpiration Stream:
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Water evaporates out through the stomata in the leaf - this is transpiration.
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Water is pulled up the xylem to the leaves.
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Water moves from the root cells into the xylem.
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Water enters root hair cells by osmosis.
This entire movement of water through the plant is known as the transpiration stream.
Diagram of the transpiration stream through a plant.
Role of Stomata and Guard Cells
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Stomata are small pores on the underside of leaves.
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They allow gas exchange (CO₂ in, O₂ and water vapour out).
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Guard cells open and close the stomata:
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In bright light and high water availability → stomata open to allow CO₂ in for photosynthesis.
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In darkness or drought → stomata close to prevent water loss via transpiration.
Key Terms
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Transpiration - loss of water vapour from leaves through the stomata.
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Transpiration stream - the entire movement of water through the plant from roots to leaves and out via the stomata.
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Exam Tip
- Remember the xylem carries water and mineral ions in one direction only, up the plant.
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Practice Question
Describe the structure and function of xylem tissue. (4 marks)
Structure
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The xylem is made of dead cells that form a tube.
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Xylem walls are strengthened with lignin.
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Xylem cells have no end cell walls.
Function
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Xylem vessels transport water and mineral ions.
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The xylem is involved in transpiration.
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The xylem transports unidirectionally / from roots to leaves.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!