water transport

• water movement

  -turgor pressure= as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves.

  -turgor also drives cell expansion= its the force that enables plant roots to force their way through tarmac and concrete.

  -loss of water by evaporation helps keep plant cool.

  -mineral ions and products of photosynthesis are transported in aqueous solutions.

  -water is a raw material for photosynthesis.
• movement of water into roots

  root hair cells= the exchange surface in plants where water is taken into the plant from the soil.

  root hair= long thin extension from a root hair cell, specialised epidermal cell.

  • adaptations:

  -microscopic size= can penetrate easily between soil particles

  -large SA:V

  -thin surface layer= fast diffusion and osmosis

  -concentration of solutes in cytoplasm of root hair cell maintains a water potential gradient between soil water and cell.
• symplast pathway

  = continous cytoplasm of cells is connected through the plasmodesmata by osmosis.

  • root hair cell has a higher water potential than next cell along because of water diffusing in from soil- makes cytoplasm dilute.
  • so water moves from the root hair cell into the next cell- process continues until it reaches the xylem.
  • as water leaves root hair cell- water potential of cytoplasm falls again- maintains a steep water potential gradient so as much water as possible continues to move into the cell from the soil.
• apoplast pathway

  = movement of water through the cell wall and the intracellular spaces.

  • water fills the spaces between the loose, open network of fibres in the cellulose cell wall. As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between water molecules.
• apoplast continued

  • the pull from water moving into the xylem and up the plant along with the cohesive forces between the water molecules creates a tension that means there's a continuous flow of water through the open structure of the cellulose wall, this offers little/no resistance.
• movement of water into xylem

  -water moves across root in apoplast and symplast pathways until it reaches the epidermis= layer of cells surrounding the vascular tissue.

  • casparian strip= a band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer.
  • water in apoplast pathway can't go any further and it's forced into the cytoplasm of the cell, joining the symplast pathway.
  • water must pass through selectively permeable cell surface membranes, excluding any potentially-toxic solutes in the soil water from reaching living tissues in the cytoplasm.
• continued

  • solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to cells in the xylem.
  • endodermal cells move mineral ions into the xylem by active transport~ the water potential of the xylem cells is lower than water potential of endodermal cells ~
  • this increases the rate of water moving into the xylem by osmosis
  • once inside vascular bundle, water returns to apoplast pathway to enter xylem and move up the plant.
  • the active pumping of minerals into the xylem to produce movement of water by osmosis results in root pressure= gives water a push up the xylem.
• evidence for role inactive transport in root pressure

  -some poisons= cyanide, affect mitochondria and prevent production of ATP. If applied to root cells there is no energy supply so root disappears

  -root pressure increases with a rise in temp and falls with a decrease in temp= suggests chemical reactions are involved

  -if levels of oxygen or respiratory substrates fall, root pressure falls.

  -xylem sap may exclude from the cut end of stems at certain times.

  -xylem sap is forced out of special pores at end of leaves in some conditions.