2.5.1Unit 2 — Multicellular Organisms
Transport System Structure in Plants — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 4 parts Lesson 20 of 39
Part 1 of 4Why Plants Need Transport Systems
Why Plants Need Transport Systems
Size problem
- Diffusion, osmosis and active transport alone are too slow to supply every cell of a large multicellular organism.
Organs linked
- Transport systems link the roots, stems and leaves so that materials can be moved between the organs of the plant.
Water and ions
- Water and mineral ions are taken in at the roots and moved up through the stem to the leaves.
Sugars and amino acids
- These are moved from where they are made or stored to the places in the plant where they are needed.
Part 2 of 4Leaf Structure
Cuticle and epidermis
- The waxy cuticle reduces water loss, and the thin transparent epidermis lets light reach the cells below.
Palisade mesophyll
- Tightly packed cells near the top surface contain many chloroplasts, so they absorb a large amount of light.
Spongy mesophyll
- Loosely packed cells with large air spaces allow carbon dioxide and oxygen to diffuse quickly through the leaf.
Veins
- Each vein contains xylem carrying water into the leaf and phloem carrying the products of photosynthesis away.
Stomata
- Pores in the leaf surface let gases move in and out, and each is opened or closed by two guard cells.
Part 3 of 4Guard Cells and Stomata
1
Carbon dioxide needed for photosynthesis diffuses into the leaf through open stomata and oxygen produced in photosynthesis diffuses out.
2
Water vapour also escapes through open stomata, so a plant loses water whenever its stomata are open.
3
Guard cells change shape to open the stomata when conditions are suitable for photosynthesis.
4
In dry conditions the guard cells change shape to close the stomata, which reduces the amount of water lost.
5
The plant therefore balances the need to take in carbon dioxide against the risk of losing too much water.
| Leaf structure | Function |
|---|---|
| Cuticle | Waxy layer that reduces evaporation from the leaf surface |
| Palisade mesophyll | Absorbs light for photosynthesis |
| Spongy mesophyll | Air spaces for gas diffusion |
| Guard cells | Open and close the stomata |
Part 4 of 4Exam Checklist & Tips
2.5.1 – Key Points for the Exam
- Stoma A pore in the leaf surface that allows gases to move in and out.
- Guard cell A cell that changes shape to open or close a stoma.
- Palisade mesophyll A layer of chloroplast-rich cells where most photosynthesis occurs.
- Xylem Tissue that transports water and mineral ions upwards from the roots.
- Phloem Tissue that transports sugars and amino acids around the plant.
Common Mistakes
- Saying that the leaf takes in oxygen for photosynthesis; carbon dioxide is the gas needed.
- Describing guard cells as the pores; the pore is the stoma and guard cells control it.
- Forgetting that the waxy cuticle reduces water loss, and instead calling it a layer for absorbing light.
- Stating that the spongy mesophyll has few air spaces; its spaces are what make gas diffusion fast.
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