Transport in Plants: Xylem and Transpiration

This lesson covers xylem transport and transpiration as required by the Edexcel A-Level Biology specification (9BI0). You need to understand the structure of xylem vessels, the cohesion-tension theory, factors affecting the rate of transpiration, and the use of potometers.

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Why Plants Need Transport Systems

Plants, like large animals, have a small surface area to volume ratio relative to their metabolic needs. They require transport systems to move:

• Water from the roots (where it is absorbed from the soil) to the leaves (where it is needed for photosynthesis and cell turgor)
• Mineral ions (e.g. nitrate, magnesium, potassium) from the roots to where they are needed
• Sugars (mainly sucrose) from the leaves (where they are made by photosynthesis) to all other parts of the plant (e.g. roots, growing tips, storage organs)

Plants have two separate transport tissues:

Tissue	Direction of transport	Substance transported
Xylem	Roots → leaves (upward)	Water and dissolved mineral ions
Phloem	Source → sink (any direction)	Sucrose and amino acids

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Structure of Xylem