How Water Passes Up The Stem

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How Water Passes Up The Stem

  1. 1. How water passes up the stem Movement of water and mineral ions through the xylem
  2. 2. How water passes up the stem <ul><li>The tallest trees are over 60m high and may need as much as 250 dm 3 of water an hour to be transported upwards. </li></ul>
  3. 3. How water passes up the stem <ul><li>In leaves </li></ul><ul><li>Water evaporates from spongy mesophyll cells so the air spaces become saturated with water. </li></ul>
  4. 4. How water passes up the stem <ul><li>The air outside the stoma is not saturated so water diffuses out from an area of high water potential to region of lower water potential. </li></ul>
  5. 5. How water passes up the stem <ul><li>As water evaporates from mesophyll walls more is drawn into cells to replace it. </li></ul><ul><li>Water can pass from the xylem to the leaf cells by both the apoplast and symplast pathway. </li></ul>
  6. 6. Water
  7. 7. Water Water is drawn up the xylem
  8. 8. Water Water is drawn up the xylem cytoplasm
  9. 9. Water Water is drawn up the xylem cytoplasm cell wall
  10. 10. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata
  11. 11. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma
  12. 12. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole
  13. 13. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell
  14. 14. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel
  15. 15. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space
  16. 16. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space
  17. 17. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space
  18. 18. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space Apoplast pathway (cell wall)
  19. 19. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space Apoplast pathway (cell wall)
  20. 20. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space Apoplast pathway (cell wall)
  21. 21. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space Apoplast pathway (cell wall)
  22. 22. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space Apoplast pathway (cell wall) Symplast pathway (cytoplasm)
  23. 23. Water Water is drawn up the xylem cytoplasm cell wall plasmodesmata stoma vacuole epidermal cell xylem vessel air space Apoplast pathway (cell wall) Symplast pathway (cytoplasm) Evaporation of water vapour
  24. 24. How water passes up the stem <ul><li>Cohesion-tension theory </li></ul><ul><li>As water is removed from xylem more water molecules are ‘pulled up’ to replace them – this pulling force is known as transpiration pull (a bit like sucking on a straw). </li></ul>
  25. 25. How water passes up the stem <ul><li>This mass flow of water relies upon two properties of water. </li></ul><ul><li>cohesion – water molecules tend to stick together </li></ul><ul><li>adhesion – the molecules also stick to the inside of the xylem vessels. </li></ul>
  26. 26. How water passes up the stem <ul><li>Drawing up a column of water through the xylem in this way is called the cohesion-tension theory . </li></ul>
  27. 27. How water passes up the stem <ul><li>It requires a large force to break such a water column – the xylem vessels would collapse under such tension if they were not thickened </li></ul><ul><li>The cohesion-tension theory is considered the main way water reaches the leaves from the roots. </li></ul>
  28. 28. How water passes up the stem <ul><li>Root pressure also contributes to flow of water up the plant – the roots push water into the xylem and help to push up the water column. </li></ul><ul><li>(Evidence: if you cut a plants stem at soil level water will continue to come out of the cut stem for some time) </li></ul>
  29. 29. How water passes up the stem <ul><li>Capillarity is a third force contributing to the movement of water. Water will move up narrow tubes by capillary action as water molecules will stick to the walls of the xylem vessels. </li></ul><ul><li>This force is important in small plants but of little significance in large trees. </li></ul>

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