Transport in plants

• Transpiration


  The loss of water vapour from leaves
• Pathway of water through a plant


  Can be traced using a suitable stain
• Leaf


  • Large surface area provided by cell surfaces, interconnecting air spaces and stomata encourages the evaporation of water
  • Water vapour diffuses out of the leaf through the stomata
• Rate of transpiration


  • Affected by wind speed and temperature
  • Affected by humidity
• Wilting


  Occurs when the volume of water vapour lost by leaves is greater than that absorbed by roots
• Translocation


  The movement of sucrose and amino acids in phloem from sources to sinks
• Sources


  Parts of plants that release sucrose or amino acids
• Sinks


  Parts of plants that use or store sucrose or amino acids
• Xylem


  • Carries water and mineral ions up the stem to the leaves
  • Has a structure that adapts it to carry water
• Phloem


  • Carries food (sucrose and amino acids) up or down the stem to wherever it is needed
• Root hairs


  • Make very close contact with soil particles and are the main route by which water and mineral ions enter the plant
  • Provide a large surface area that increases the rate of absorption of water and mineral ions
• Water movement through the plant

  1. Root hair cells

  2. Root cortex cells

  3. Xylem

  4. Mesophyll cells
• The movement of water and mineral ions in the xylem is always upwards, from soil to leaf. However, the solutes in the phloem may be travelling up or down the stem
• Water loss from the cell vacuoles results in the cells losing their turgour and becoming flaccid. A leaf with flaccid cells will be limp and the stem will droop. A plant that loses water to this extent is said to be wilting
• Warm air can hold more water vapour than cold air. So, evaporation or transpiration will take place more rapidly into warm air
• When the Sun shines on the leaves they will absorb heat as well as light. This warms them up and increases the rate of evaporation of water