2.5Unit 2 — Multicellular Organisms
Transport Systems in Plants — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 2 parts Lesson 5 of 7
Section 1 of 2Xylem and Phloem transport tissues
| Feature | Xylem Vessels | Phloem Tissue |
|---|---|---|
| Substance transported | Water and dissolved mineral ions | Dissolved sugars (sucrose) and amino acids |
| Direction of transport | One-way upwards only (from roots to leaves) | Two-way transport (up and down to storage organs and growing tips) |
| Cellular nature | Dead, hollow tubes with no cytoplasm or end walls | Living cells (sieve tube elements and companion cells) |
| Structural reinforcement | Lignin spiral rings or mesh (withstands water tension) | Sieve plates with perforations connecting sieve tube cells |
| Energy requirement | Passive process (driven by transpiration evaporation) | Active process requiring energy (translocation) |
Section 2 of 2Transpiration stream and stomata
1Transpiration2 points
:
- The loss of water vapour by evaporation from plant leaves through open stomata.
- Creates a suction pull (tension) that draws water upwards continuously from root hairs through xylem vessels in the stem to mesophyll cells in leaves (the transpiration stream).
2Environmental factors affecting transpiration rate4 points
:
- Wind speedIncreases rate (blows away humid air layer, maintaining steep water vapour concentration gradient).
- TemperatureIncreases rate (provides thermal energy for faster evaporation of water molecules).
- Light intensityIncreases rate (stimulates stomata to open for photosynthesis).
- HumidityDecreases rate (high moisture in air reduces the concentration gradient between leaf interior and surroundings).
3Stomata and guard cells2 points
:
- Stomata are microscopic pores located predominantly on the underside of leaves to reduce excessive water loss.
- Flanked by two guard cells: when turgid (water enters), guard cells curve open; when flaccid (water lost), they close the stoma to prevent wilting.
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