Movement of water through xylem

• How is water pulled up the water column in the xylem

  Evaporation of water through stomata causes a negative pressure / tension

  So water moves from high to low pressure

  Hydrogen bonds btwn water molecules cause them to stick together by cohesion
• What is the transpiration stream

  Continuous unbroken column of water
• What is cohesion tension theory

  Theory of how water moves up the xylem in the root and stem

  • water molecules stick together and tension by the negative pressure created by evaporation of water out of leaf
• Evidence of cohesion tension theory

  If water column is broken and an air gap appears, water below the gap cannot be pulled up

  But it can move laterally into adjacent xylem vessels and continue up

  In the day when transpiration is highest, there’s more negative pressure or tension in the xylem, causing xylem walls to move in and decreasing the diameter of tree trunk
• Relationship btwn rate of transpiration from leaves and diameter if tree trunk over 24hrs 


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• What’s the third mechanism that allows water to move up the xylem

  capillarity
• How does capillarity allow water to move up the xylem

  Adhesive forces form btwn water molecules and the walls of the xylem vessels

  Adhesive forces are stronger than cohesive attraction

  This movement is known as capillarity and is important in reducing the forces necessary for transpiration pull
• The 3 mechanisms in allowing water to move up xylem

  1. Root pressure = push
  2. Cohesion tension theory = pull
  3. capillarity = pull
• describe cohesion tension theory by using its name

  cohesion- hydrogen bonds btwn water molecules

  tension- evaporation of water through stomata = negative pressure
• how does water move up the xylem in terms of pressure

  from high pressure at the bottom, to low pressure at the top
• describe relationship of the rate of transpiration and diameter of tree

  as rate of transpiration increases, the diameter of the tree decreases
• why does the rate of transpiration not start at 0 in the graph

  some water evaporates through the cuticle
• what happens at the peak of the transpiration graph

  most stomata are open for CO2 for PS

  warmest part of the day so transpiration is at its highest
• describe root pressure theory

  PUSHes water up xylem

  • mineral ions dissolve the H2O travelling along the apoplast pathway cannot diffuse across cell membrane + r actively secreted into cytoplasm and xylem
  • this lowers water potential in the xylem = water drawn through endodermis
  • pulling water from surrounding cells = positive hydrostatic pressure inside the xylem which forces water up