Revision notes · Organisation
Plant tissues, organs and systems
Plant tissues4.2.3.1
Definition: A plant tissue is a group of specialised plant cells that work together to perform a shared function.
The leaf is a plant organ built from several distinct tissues, adapted for photosynthesis and gas exchange.
| Tissue | Function |
|---|---|
| Epidermis (upper & lower) | Covers and protects the leaf; the waxy cuticle on top reduces water loss |
| Palisade mesophyll | Tightly packed with chloroplasts near the top of the leaf — the main site of photosynthesis |
| Spongy mesophyll | Large air spaces let gases diffuse to and from cells easily |
| Xylem | Transports water and mineral ions, from the roots up through the plant |
| Phloem | Transports dissolved sugars around the plant (translocation) |
| Guard cells | Control the opening and closing of stomata, regulating gas exchange and water loss |
- •Meristem tissue, found at the tips of roots and shoots, contains unspecialised cells that keep dividing to produce new tissue for growth.
⚠️ Common mistake: Thinking a leaf is a single tissue — it's a plant organ, made of several different tissues working together.
🧠 Remember: Palisade = Photosynthesis (top, more light); Spongy = Spaces (for gas exchange).
Plant organ system4.2.3.2
Definition: Transpiration is the loss of water vapour from a plant, mainly through the stomata in its leaves.
Roots, stems and leaves work together as an organ system, transporting water, minerals and sugars around the plant.
- •Root hair cells have a large surface area to absorb water by osmosis and mineral ions by active transport.
- •Water and dissolved minerals move up from the roots to the leaves through xylem vessels, driven by transpiration.
- •As water evaporates from cells inside the leaf, it diffuses out through open stomata, pulling more water up through the xylem.
- •Translocation is the movement of dissolved sugars, made by photosynthesis, through phloem tissue — from the leaves to other parts of the plant, in any direction.
| Increase in... | Effect on transpiration rate |
|---|---|
| Temperature | Increases — faster evaporation and diffusion |
| Air movement (wind) | Increases — removes water vapour from the leaf surface, keeping the diffusion gradient steep |
| Humidity | Decreases — smaller diffusion gradient between the leaf and the air |
| Light intensity | Increases — more stomata open for photosynthesis, so more water vapour is lost |
⚠️ Common mistake: Confusing transpiration (water moving one-way, up through xylem) with translocation (sugars moving in phloem, in any direction).
🧠 Remember: Xylem: water, one way up. Phloem: food, any way.
Feel like you’ve got it?
Practise it now →