Transpiration

• Uses of water

  • photosynthesis
  • solution for transport of ions and assimilates
  • cooling
  • turgor pressure - hydrostatic skeleton to keep stems and leaves up, drives cell expansion so roots can force through things
• Water entry at roots

  • soil -> root hair cells -> root cortex -> endodermis -> xylem
  • moves by osmosis as soil has a higher water potential compared to the plant
  • ATP is required if soil has lower water potential
• Apoplast pathway


  • through the absorbent cellulose cell walls
  • very fast - no membranes are crossed
• Symplast pathway


  • through cytoplasm and plasmodesmata
  • slower than apoplast, membranes have to be crossed
• Vacuolar pathway


  • through the cytoplasm and vacuole
  • slowest, crosses both plasma membrane and tonoplast
• Casparian strip


  • blocks the apoplast pathway in the endodermis cell layer, before vascular tissue
  • SUBERIN - impermeable, waxy substance
  • forces water to use the symplast pathway over the apoplast pathway
  • controls rate of water and solutes entering the xylem
• How water moves UP plants:

  1. root pressure
  2. transpiration pull
  3. capillary action
• Root pressure


  • low rate of transpiration at night
  • root cells still take up water via osmosis
  • volume of water in xylem increases, increasing pressure so water is forced up the xylem
• Transpiration pull


  • water evaporates out of stomata in leaves
  • creates a water potential gradient
  • water is pulled upwards
• Capillary action and tension

  • water evaporates - tension pulls water up
  • cohesion causes all water molecules to move upwards
  • adhesion allows water to pull up using xylem walls
• Stomata


  • open to allow carbon dioxide in for photosynthesis
  • water exits due to water potential gradient
  • a side effect of gas exchange lol
• Factors effecting rate of respiration

  • light
  • temperature
  • humidity
  • wind
• Light and respiration

  • stomata closes in the dark as there is no light available for photosynthesis as no carbon dioxide has to be taken in
  • less transpiration pull
• Temperature and respiration


  • higher temperatures increases kinetic energy of molecules so they evaporate faster
  • this increases water potential gradient so rate of transpiration increases
• Humidity and respiration


  • increased humidity increases the water potential of air
  • this reduces the water potential gradient, so rate of transpiration decreases
• Wind


  • molecules are taken away from the stomatal opening
  • water potential of air decreases
  • water potential gradient increases, so rate of transpiration increases
• Potometer - determining rate of transpiration

  • record distance travelled by bubble per unit time
  • control factors that effect rate of respiration