A level Biology B · Edexcel 9BI0 · Topic 4F
Transport in plants
Key points
- Xylem vessels are dead, hollow, lignified tubes carrying water and mineral ions up from the roots.
- Water enters root hair cells by osmosis and crosses the root by the apoplast (cell walls) or symplast (cytoplasm) pathway; the Casparian strip forces it into the symplast at the endodermis.
- Cohesion–tension: water evaporates from leaves (transpiration), pulling a continuous column of water up the xylem thanks to cohesion and adhesion.
- Transpiration rate rises with light, temperature and wind, and falls with humidity; measure water uptake with a potometer.
- Phloem sieve tube elements and companion cells carry sucrose from source to sink by mass flow (translocation).
- Sucrose is actively loaded at the source, lowering water potential so water enters and creates high pressure that pushes sap to the sink.
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Plants have two transport tissues. Xylem moves water and mineral ions up from the roots, pulled by evaporation from leaves; phloem moves sugars and other solutes in both directions, pushed by pressure from where they are made to where they…
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