CB6.2 — Plant transport (Edexcel 1SC0)
Xylem and phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Substance transported | Water and mineral ions | Sugars (sucrose) and other solutes |
| Direction | Upwards only (roots → leaves) | Both directions |
| Cells | Dead, hollow, lignified | Living, with companion cells |
| Process | Transpiration pull | Translocation |
Transpiration
Transpiration is the loss of water vapour from leaves through stomata by evaporation and diffusion.
Transpiration stream: water evaporates from leaves → tension pulls water up xylem → water absorbed from soil by root hair cells via osmosis.
Factors affecting transpiration rate:
- Temperature (higher → faster evaporation → faster rate)
- Humidity (higher → smaller gradient → slower rate)
- Wind speed (higher → faster removal of water vapour from surface → faster rate)
- Light intensity (higher → stomata open wider → faster rate)
Guard cells and stomata
Stomata open when guard cells take up water by osmosis → become turgid → bend open. Stomata close in dry/dark conditions → guard cells lose water → flaccid → close.
Importance: stomata open during the day for CO₂ entry (photosynthesis) but allow water loss; balance between gas exchange and water conservation.
Required practical — transpiration rate with a potometer
A potometer measures water uptake (an estimate of transpiration rate) by tracking movement of an air bubble in a capillary tube. Different conditions are tested systematically.
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