The structure and role of the endodermis

• why must water leave the apoplast pathway?

  as it can only pass into the xylem from the symplast or vacuolar pathways
• what is the vascular tissue, in the centre of the root, surrounded by?

  its surrounded by a region called the pericycle
• what is the pericycle surrounded by?

  its surrounded by a single layer of cells, the endodermis
• what is the endodermis?

  a single layer of cells around the pericycle and vascular tissue of the root - each cell has an impermeable waterproof barrier in its cells wall
• what are the endodermis cell walls impregnated by?

  they are impregnated with a waxy material, suberin, forming a distinctive band on the radical and tangential walls, called the casparian strip
• what is the casparian strip?

  the impermeable band of suberin in the cell walls of endodermal cells, blocking the movement of water in the apoplast, so it moves into the cytoplasm
• why does the casparian strip prevent water from moving further in the apoplast?

  as the suberin is hydrophobic
• what happens to water and the dissolved minerals it contains when leaving the apoplast?

  they leave and enter the cytoplasm before they move further across the root
• whats the role of the casparian strip?

  • to block the apoplast pathway between the cortex and xylem so water and dissolved nitrate ions have to pass through cell membranes through transporter proteins
  • nitrate ions are actively transported from the cortex to the xylem, lowering the water potential of the xylem
  • so water always follows (from the cortex to the xylem)
  • once in, water can‘t pass back
• what is suberin?

  a waxy material that impregnates the endodermis cell walls
• when water reaches the endodermis, what does the casparian strip force it to do?

  it forces it to pass through the plasma membrane into the cytoplasm of the endodermal cells
• what is this an image of?

  a diagram of a root cortex

  
• what are the 2 explanations for the movement of water from the root endodermis into the xylem across the endodermal cell membranes?

  • increased hydrostatic pressure in root endodermis cells pushes water into the xylem
  • decreased water potential in the xylem, below that of the endodermal cells, draws water in by osmosis across endodermal cell membranes
• what increases the hydrostatic pressure in root endodermal cells that causes it to push water into the xylem?

  • active transport of ions, especially sodium ions, into the endodermal cells, which reduces their water potential, drawing in more water by osmosis
  • diversion of water into endodermal cells from the apoplast pathway by the casparian strip
• what decreases the water potential of the xylem, below that of endodermal cells that causes it to draw in water by osmosis across endodermal cell membranes?

  • water being diverted into the endodermal cells by the casparian strip
  • active transport of mineral salts, mainly sodium ions, from the endodermis and pericycle into the xylem