Transpiration and Translocation

• Translocation


  Plants transport sugars made through photosynthesis in leaves to rest of plant
• Phloem cells


  Arranged end to end to form long columns called phloem tubes to transport cell sap (water and sugar)
• Phloem tubes


  • Have small pores/gaps between adjacent cells to enable movement of cell sap
  • Can transport substances in either direction (up or down the plant)
• Transpiration


  Movement of water from roots to leaves driven by evaporation of water from leaves
• Transpiration stream


  Chain of water molecules pulled up the plant as water evaporates from leaves
• Factors influencing rate of transpiration
• High light intensity


  More photosynthesis, more stomata open, higher rate of transpiration
• No photosynthesis at night

  Stomata closed, very little transpiration
• High temperature


  Water particles have more energy, higher rate of transpiration
• High airflow


  Water molecules blown away quickly, higher concentration gradient, higher rate of transpiration
• High humidity


  Lower concentration gradient, lower rate of transpiration
• Xylem tubes are made of dead xylem cells with no ends, strengthened with lignin, to transport water and mineral ions from roots to leaves
• Transpiration


  The process of water constantly evaporating from the surfaces of leaves
• Transpiration process


  1. Water evaporates from cells inside the leaf

  2. Water vapor diffuses through air spaces in the spongy mesophyll

  3. Water vapor exits the leaf through the stomata

  4. Water passes from the xylem into the leaf to replace the lost water

  5. Water is drawn into the root hair cells and up the xylem vessels to the leaf
• Importance of transpiration


  • Brings water to the leaf for photosynthesis
  • Transports dissolved mineral ions
  • Cools the leaf down in warm weather
• Factors affecting rate of transpiration

  • Higher temperatures
  • Dry conditions (low humidity)
  • Windy conditions
  • Higher light intensity
• High light intensity


  Stomata open to allow carbon dioxide to enter for photosynthesis, allowing water vapor to pass out
• Stomata


  Pores on the leaf surface surrounded by guard cells
• Stomata opening and closing

  1. Guard cells swell and change shape in high light, causing stomata to open

  2. Plant closes stomata under hot conditions to reduce water loss
• Stomata and guard cells play roles in gas exchange and water loss in plants