transpiration

• why is water needed in plants?

  • maintains turgor pressure- hydrostatic skeleton to support stems and leaves
  • drives cell expansion- so roots can force their way underground
  • lose water by evaporation to keep plants cool
  • mineral ions/photosynthetic products transported in aqueous solutions
  • water is needed for photosynthesis
• how are root hair cells adapted?

  • root hairs- maximise surface area to volume ratio
  • microscopic size
  • thin surface layer
  • high solute concentration within the cytoplasm of a root hair cell so water potential gradient is maintained, water moves into it by osmosis
• what two pathways can water continue to move through after entering the roots?

  SYMPLAST-

  • continuous cytoplasm linked by plasmodesmata by osmosis
  • the cells ahead have a higher solute concentration than the cell water is moving through, so water moves continuously through cytoplasms
  • continues ACROSS THE ROOT until the xylem is reached
  • as water leaves root hair cell, this causes fall in WP so that more water is drawn up though the root

  APOPLAST-

  • cell walls and intercellular spaces
  • as water moves into the root, more water is pulled into the apoplast by cohesive forces between water molecules, little resistance, and moves along the pathway
  • water continues moving in apoplast until it reaches root endodermis, Casparian strip (waterproof) forces water out the apoplast into the symplast pathway
  • this is because water is then forced to pass through the cell membranes, which prevent any toxins from the soil entering the cell
• how does water move from the root to the xylem?

  • moves across through the root through symplast/apoplast until it reaches endodermis of xylem
  • casparian strip forces water into endodermal cells
  • solute concentration of endodermal cells is lower than of xylem cells.
  • endodermal cells can also move minerals into the xylem by active transport
  • therefore water moves into the xylem by osmosis/AT
  • once in the xylem it returns to the apoplast pathway
  • active pumping of minerals into the xylem produces root pressure, which gives water a push up the xylem

  OVERALL- water is pulled through the apoplast pathway by cohesive forces, and through the symplast pathway through osmosis