Transport In Flowering Plants: Transpiration And Translocation
7.2 Transpiration And Translocation
Transpiration
- Definition: Loss of water vapour from leaves.
- Process:
- Water evaporates from mesophyll cell surfaces into air spaces.
- Diffuses out through stomata.
- Factors Affecting Transpiration Rate:
- Wind speed ↑ → transpiration ↑.
- Temperature ↑ → evaporation ↑.
- Light intensity ↑ → stomata open → transpiration ↑.
- Humidity ↑ → transpiration ↓.
Consequences Of Transpiration
- Water transport up plant (transpiration pull).
- Provides minerals to leaves.
- Cools plant.
- Supports leaves by turgor.
Written And Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions And 11 World Records For Educate A Change O Level And IGCSE Biology Full Scale Course
Wilting
- Occurs when water loss > uptake.
- Guard cells become flaccid → stomata close.
- Plant droops to reduce water loss.
Transpiration Stream Mechanism
- Water molecules stick together (cohesion).
- Pulled as a continuous column in xylem.
- Maintained by transpiration pull.
Translocation
- Definition: Movement of sucrose and amino acids in phloem.
- Direction: Sources (leaves) → Sinks (roots, fruits, storage organs).
- Phloem transport is bidirectional (both up and down).
Structure Of Xylem And Phloem
| Tissue | Structure | Function |
|---|---|---|
| Xylem | Thick lignin walls, no end walls, no contents | Water + mineral transport, support |
| Phloem | Living cells, sieve tubes + companion cells | Transport sucrose + amino acids |
Written And Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions And 11 World Records For Educate A Change O Level And IGCSE Biology Full Scale Course