# Plant Organs and Transport
Plants have specialised transport systems to move water, minerals, and sugars around the organism. Understanding xylem, phloem, transpiration, and translocation is essential for GCSE Biology.
1. Plant Organ System
| Organ | Function |
|---|---|
| Root | Absorbs water and mineral ions |
| Stem | Supports plant; contains transport vessels |
| Leaf | Photosynthesis and gas exchange |
2. Transport Tissues
Xylem
- Transports water and mineral ions from roots to leaves
- Dead cells; hollow tubes; thickened with lignin
- Movement by transpiration pull (passive)
- One direction: roots → leaves
Phloem
- Transports dissolved sugars (sucrose and amino acids)
- Living cells; sieve tubes with companion cells
- Movement by translocation (requires energy — active)
- Both directions: from sources (leaves) to sinks (roots, fruits, growing tips)
3. Transpiration
The loss of water vapour from the leaf surface through stomata.
Transpiration stream: water enters root → moves up xylem → evaporates from leaves
Factors Affecting Transpiration Rate
| Factor | Effect on Rate |
|---|---|
| Temperature ↑ | Rate ↑ (more evaporation) |
| Wind speed ↑ | Rate ↑ (removes humid air) |
| Light intensity ↑ | Rate ↑ (stomata open wider) |
| Humidity ↑ | Rate ↓ (lower concentration gradient) |
Stomata and Guard Cells
- Stomata: pores in leaves for gas exchange
- Guard cells: open stomata in light (for photosynthesis), close in dark (to conserve water)
- Guard cells turgid → stomata open
- Guard cells flaccid → stomata closed
4. Required Practical: Transpiration
Use a potometer to measure rate of water uptake:
- Cut shoot under water
- Set up potometer (sealed, no air bubbles)
- Measure how far air bubble moves in set time
- Change conditions (fan, light, humidity) and compare
5. Root Hair Cells
Adapted for absorption:
- Long extension increases surface area
- Thin cell wall for faster diffusion
- Many mitochondria for active transport of minerals
6. Practice Questions
- Compare xylem and phloem (structure and function).
- Explain how transpiration helps transport water in plants.
- How do guard cells control the opening of stomata?
- Name three factors that increase the rate of transpiration.
- Why do root hair cells have many mitochondria?
Want to check your answers and get step-by-step solutions?
Summary
- Xylem: dead, hollow, carries water UP (transpiration pull)
- Phloem: living, carries sugars both ways (translocation, active)
- Transpiration: evaporation from leaves; affected by temperature, wind, light, humidity
- Guard cells control stomata opening
- Root hair cells: large surface area for absorption