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Movement of water vapour out of the leaves of a plant

Transpiration stream - from roots leaves:

  • Active transport of minerals from soil to leaves decreases water potential in the root hair cells so water travels in by osmosis
  • Water can travel by apoplast pathway (through cellulose cell walls) or the symplast pathway (through cytoplasm) until the endodermis where the Casparian ***** stops the apoplast pathway by blocking it
  • Water travels into the xylem by osmosis
  • Cohesion-tension theory - water travels up the xylem because water molecules are polar and held together by hydrogen bonds which mean when one molecule moves out of the xylem vessel by osmosis then all the water moves upwards in a continuous stream - cohesion is the forces between the molecules and adhesion is the forces between the molecules and the wall of the xylem
  • Capillary action - the xylem vessels are quite narrow which pushes the water up further than if…


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