Roots

• There are three different pathways for water through roots:

  • apoplast pathway
  • symplast pathway
  • vacuolar pathway
• Apoplast pathway - water and inorganic ions move along the cellulose cell wall
• Symplast pathway - water and inorganic ions move through cell membrane, cytoplasm and plasmodesmata
• Vacuolar pathway - water and inorganic ions move through cell membrane, cytoplasm and vacuole tonoplast
• Endodermis:

  • Endodermis cell walls are impregnated with suberin ( a waxy material), forming a distinctive band on the radial and tangential walls, called the Casparian strip.
  • suberin is hydrophobic so the Casparian strip prevents water moving further in the apoplast.
  • Water and inorganic ions leave the apoplast and enter the cytoplasm and join the symplast.
• Root hair cells:

  • root hairs on the root increase the surface area of the root.
  • there is a high water content in the soil and a solute concentration in the vacuole of the cells. Therefore water enters the root hair cells by osmosis from a high water potential to a low water potential.
  • Ions can be actively transported into the root hair cell to lower the water potential and increase the water potential gradient.
• Root pressure:

  • Solutes diffuse with the concentration gradient through the apoplast pathway.
  • They are the actively transported into the endodermis to set up a steeper water potential gradient, resulting in a greater force of water flow into and up the xylem (positive hydrostatic pressure) called root pressure
• casparian strip:

  • Casparian strip/suberin blocks the apoplast pathway and forces water into the symplast pathway.
  • active transport of ions into the endodermis/xylem
  • this lowers the water potential of the endodermis/xylem
  • causing water to move into the endodermis/xylem by osmosis
  • producing hydrostatic pressure - root pressure