Water Transport

• How does water enter the plant

  Water enters the plants from the soil , through the root hair cells , by osmosis
• Root hair , xylem and phloem cells are specialised to transport water, minerals and sugars around the plant
• How is water transported through the plant

  • Water is contains dissolved minerals 
  • The water and minerals are transported up the xylem tissue / vessels , from the roots to the stems and leaves
• What is transpiration

  • Water is constantly evaporating at the surface of the leaves and will diffuse out of the stomata 
  • The loss of water from the leaves is called transpiration - Transpiration helps o draw water up the xylem vessels from the roots
• Explain how transpiration occurs inside the cells of leaves (The transpiration stream)

  • Transpiration starts with evaporation of water from cells inside the leaf 
  • The water vapour then diffuses through the air spaces in the spongy mesophyll and out of the leaf through the stomata 
  • Now water passes from the xylem into the leaf to replace the water that has been lost 
  • Finally water is drawn into the root hair cells and up the xylem tissues / vessels to the leaf 
  • Scientists call this whole process the transpiration stream

  
• Factors that affect the rate of transpiration

  • Higher temperature 
  • Dry conditions 
  • Increased humidity / Humid conditions 
  • Faster air flow / Windy conditions 
  • Increased light intensity
• How does high temperature affect the rate of transpiration

  • An increase in temperature increases the rate of transpiration 
  • Evaporation is faster when the temperature is faster 
  • This is because more energy is transferred to the water to allow it to evaporate
• How do dry conditions affect the rate of transpiration

  • Dry conditions increase the rate of transpiration 
  • Evaporation is faster under dry conditions 
  • This is because the air contains less water vapour , so the concentration gradient for diffusion is higher
• How do dry conditions / increased humidity affect the rate of transpiration

  • Increased humidity / Humid conditions decrease the rate of transpiration 
  • Evaporation is slower under dry conditions 
  • This is because the air contains more water vapour , so the concentration gradient for diffusion is lower
• How do windy conditions / faster air flow affect the rate of transpiration

  • Faster air flow / Windy conditions increase the rate of transpiration 
  • This is because windy conditions blow away water vapour , allowing more water to evaporate
• How does increased light intensity affect the rate of transpiration

  • Increased light intensity increases the rate of transpiration 
  • This is because high light intensity increases the rate of photosynthesis: 
  • High light intensity causes the stomata to open and allow carbon dioxide to enter 
  • Once the stomata have opened , water vapour can pass out of the leaf
• How does light intensity affect the stomata

  • When the light intensity is high e.g. during the day , the guard cells swell and they change their shape 
  • This causes the stomata to open 
  • Now carbon dioxide can diffuse into the leaf and be used in photosynthesis 
  • When the light intensity is low e.g. during the night , the guard cells become limp and the stomata closes  
  • This is because the plants can no longer photosynthesise (because the sun isn’t out and it is dark), and so carbon dioxide is no longer needed , so the stomata closes to reduce water loss

  
• How does the amount of water affect the stomata

  • When water is plentiful , the guard cells take up water and bend - this causes the stomata to open 
  • So now gases for photosynthesis are now free to move in and out of the stomata along with water from transpiration 
  • When water is scarce , the guard cells lose water and change shape , becoming limp - this causes the stomata to close 
  • This stops the plant from losing water through transpiration

  
• What happens to the stomata under hot conditions

  • Under hot conditions the stomata close to reduce water loss by transpiration 
  • However this means that the plant can no longer photosynthesise