movement of water through plants

• Pathways taken by water

  • cellulose cell walls are fully permeable to water
  • apoplast, symplast and vacuolar pathways
• Apoplast Pathway

  • water passes through spaces in the cell walls + between the cells; does not pass through any plasma membranes into the cells (also dissolved mineral ions and salts can be carried with water)

  → this means that the water moves by mass flow rather than by osmosis
• Symplast Pathway

  • water enters cell cytoplasm through plasma membrane

  →can be passed through the plasmodesmata from one cell to the next
• Vacuolar Pathway

  • water enters and passes through vacuole

  → similar to symplast pathway however water is not confined in cytoplasm
• Movement from cell to cell

  Water Potential

  • measure of tendency of water molecules moving from one place to another
  • water moves from a region of higher water potential to a region of lower water potential
  • water potential of pure water is 0
  • in a plant cell, cytoplasm contain mineral ions and sugars (solutes) that will reduce water potential due to fewer ‘free’ water molecules available than in pure water

  →therefore water potential in plants are always negative (lower water potential)
• Water uptake

  if you place a plant cell in pure water, it will take up water molecules by osmosis

  • this is because water potential in the cell is more negative (lower) than that of pure water

  →water molecules will move down the water-potential gradient into the cell

  • cell will not absorb water until it bursts as the cell has a strong cellulose cell wall
  • once cell is full of water it is called turgid; the water inside the cell starts to exert pressure on cell wall, called pressure potential

  → as pressure potential builds up, it reduces the influx of water
• Water loss

  if a plant cell in asalt solution with a very negative (low) water water potential, it will lose water by osmosis

  →due to water potential of cell is higher than of solution, so water moves down water potential gradient out of cell

  • cytoplasm and vacuole begin to shrink when water loss occurs

  → cytoplasm will then no longer push the cell wall and cell wall won’t be turgid

  • as water continues to lose water, the plasma membrane will lose contact with cell wall - plasmolysis

  → tissue would now be flaccid
• Movement of water between cells

  when plant cells touch each other, water molecules pass from one cell to another

  → water molecules move from a region of high water potential to a region with lower water potential (osmosis)