The Transpiration Stream

• How water moves into the root from the soil: Exodermis to the Cortex

  1. Water is taken up via the root, from the surrounding soil by osmosis.
  2. Minerals are taken up by the root by active transport. The minerals lower the water potential inside the cells of the root and therefore help to steepen the water potential gradient.
  3. This water then moves across the root cortex via osmosis and the apoplast pathway.

  
• The structure of the root:

  • Root hair cells are a long thin extension from a root, a specialised epidermal cell found near the growing root tip.
  • Their microscopic size means they can penetrate easily between soil particles.
  • Have a large SA: V ratio and there are thousands on each growing root tip.
  • Each hair has a thin surface layer through which diffusion and osmosis can take place quickly.
  • The concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water.
• • Adhesion: the attraction between water molecules and the walls of the xylem vessel.
  • Cohesion: the attraction between water molecules caused by hydrogen bonds.
• Apoplast pathway


  • The movement of water through the cell walls and the intercellular spaces.
  • As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules. This creates a tension and means there is a continuous flow of water through the open structure of the cellulose wall, which offers little or no resistance.
  • Water doesn’t pass through the membrane.
• Symplast pathway


  • Water enters the cell cytoplasm through the cell membrane.
  • The continuous cytoplasm is connected through the plasmodesmata (gaps in the cell wall containing cytoplasm that connects two cells).
  • The root hair cell has a higher water potential than the next cell along. This is the result of water moving by osmosis in from the soil, which has made the cytoplasm more dilute. So water moves from the root hair cell into the next door cell by osmosis. This process continues from cell to cell across the root until the xylem is reached.
• Symplast pathway (2)

  A steep water potential gradient is maintained to ensure that as much water as possible continues to move into the cell from the soil.

  
• How water moves across the endodermis:

  • The cells of the endodermis have a waterproof strip made of a substance called suberin.
  • The strip is called the casparian strip. The casparian strip blocks the apoplast pathway, forcing water through the symplast and vacuolar pathways.
  • This ensures that water and dissolved mineral ions have to pass into the xyloplasm through the partially permeable membrane. This allows for the filtering of unwanted substances too.
• How water moves across the endodermis: (2)

  • mineral ions can then be actively transported through carrier/transporter proteins from the cytoplasm of the cortex cells into the medulla and into the xylem, decreasing the water potential in the xylem.
  • This then steepens the water potential gradient between the cortex and the xylem, allowing water to move by osmosis.
  • Once the water has entered the medulla, it cannot pass back into the cortex, as the apoplast pathway of the endodermal cells is blocked by the Casparian strip.

  
• How water moves into the xylem:

  1. Mineral ions are actively transported into the base of the xylem and decrease the water potential.
  2. Water will then move into the base of the xylem by osmosis down the water potential gradient.
  3. This results in root pressure/ high hydrostatic pressure and gives water a push up the xylem. Transpiration at the leaves creates a low hydrostatic pressure at the top of the xylem.
• How water moves up the stem:

  Transpirational pull- evaporation of water from the leaf surface must be replaced by water from the xylem, moving water up the stem.

  • Water molecules are attracted to each other by forces of cohesion which is strong enough to hold the molecules together in a long chain or column. As molecules are lost at the top of the column, the whole column is pulled up as one chain.
  • The pull from above creates tension in the column of water. This is why the xylem vessels must be strengthened by lignin. The lignin prevents the vessel from collapsing under tension.
• How water moves up the stem: (2)

  • The same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel.
  • This is called adhesion (capillary action) because the xylem vessels are very narrow, these forces of attraction can pull the water up the sides of the vessel. The water moved from a high hydrostatic pressure to a low hydrostatic pressure down a pressure gradient.
• How water leaves the leaf:

  • Most of the water that leaves the leaf exits as vapour through the stomata. Only a tiny amount leaves through the waxy cuticle.
  • Water evaporates from the cells lining the cavity immediately above the guard cells (the sub-stomatal air space). This lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells.
  • In turn, water is drawn from the xylem in the leaf by osmosis.
  • Water may also reach these cells by the apoplast pathway from the xylem.