Plant Tissues, Organs and Systems (B2.3)
Plant tissues
| Tissue | Location | Function |
|---|---|---|
| Epidermal | Outer surface of leaf/root/stem | Protection; minimise water loss; gas exchange (via stomata) |
| Palisade mesophyll | Upper leaf layer | Main photosynthesis site — packed chloroplasts |
| Spongy mesophyll | Middle leaf layer | Gas exchange — air spaces allow diffusion |
| Xylem | Vascular bundles | Transport water and mineral ions upward from roots |
| Phloem | Vascular bundles | Transport dissolved sugars (sucrose) up and down |
| Meristem | Root/shoot tips | Cell division and growth |
Xylem vs phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Substance transported | Water and mineral ions | Dissolved sugars (sucrose) and amino acids |
| Direction | Roots → stem → leaves (upward) | Both directions (from leaves to roots/growing points) |
| Cells | Dead hollow tubes (lignified) | Living sieve tube cells + companion cells |
| Process | Transpiration pull | Translocation |
Transpiration
Transpiration is the loss of water vapour from leaves (mainly through stomata) as a by-product of gas exchange. Water evaporates from spongy mesophyll cells → diffuses out through stomata → creates a tension that pulls water up through xylem (transpiration stream).
Factors affecting transpiration rate:
| Factor | Effect on rate | Reason |
|---|---|---|
| Temperature ↑ | Increases | Water molecules have more kinetic energy → faster evaporation |
| Humidity ↑ | Decreases | Reduces concentration gradient for water vapour to diffuse out |
| Wind speed ↑ | Increases | Removes water vapour from leaf surface → steeper gradient |
| Light intensity ↑ | Increases | Stomata open wider to allow CO₂ in for photosynthesis |
Measuring transpiration: a potometer measures the rate at which a leafy shoot takes up water (as a proxy for transpiration). The air bubble moves along a capillary tube. Clamp the cut end underwater to avoid air entering xylem.
Translocation
Translocation is the movement of dissolved sugars (mainly sucrose) and amino acids through the phloem, in both directions, from source (leaf — where sugars are made) to sink (roots, fruits, growing tips — where sugars are used or stored).
Common exam errors
- Confusing xylem (water up) with phloem (sugars in both directions).
- Saying transpiration is "water transport" — it is specifically water vapour loss through leaves.
- Not linking stomata opening to light — stomata close at night to conserve water.
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