Cohesion tension 

root pressure 

symplastic & apoplastic pathways 


Water Potential 

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  • Created by: Daisy
  • Created on: 15-02-13 12:32

Root Pressure

  • The uptake of water by the roots creates a force. 
  • This force pushes water up the xylem
  • It can be measured by manometer and is usually 100kPa. 
  • The force is only big enough to push water up a small stem or branch but not a tree or long branch. 
  • Root pressure is the cause of guttation - droplets of water forming at the end of leaves. 
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Cohesion Tension

  • Xylem vessels contain pipes which move water up to the leaves.
  • The pipes are dead, empty spaces which means no osmosis can take place so water moves by mass flow. 
  • Transpiration in the leaves causes a drop in pressure. 
  • This causes water to be taken up through the xylem to the leaf to replace lost water. 
  • Xylem is under pressure during this
  • Water is under tension too, but water molecules bond by hydrogen bonds in cohesion which gives it tensile strength. 
  • This is the cohesion-tension mechanism. 
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Reducing transpiration in xerophytes

Xerophytes are plants which are adapted to dry habitats. 

Thick cuticle - Stops uncontrolled evaporation 

Small SA:Vol - Less area for water loss 

Low Stomata Density - Fewer gaps in leaves for transpiration. 

Stomata in lower surface only - more humid air prevents transpiration because of low water potential. 

Shedding leaves in the cold - Reduce water loss at certain times of the year. 

Sunken Stomata - Maintains humid air around stomata 

Stomata hairs -     "         "          "         "           " 

Folded leaves - Maintains humid air and limits water potential gradient. 

Succulent leaves and stem - Stores water 

Extensive roots - Maximise water uptake. 

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How water enters the root by water potential

  • Water will diffuse into the root by osmosis if the water potential of the soil is GREATER THAN that of the root. 
  • The water will then move along sucessivley more negative water potential gradients until it reaches the xylem. 
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Counter Current flow


Counter current flow is how fish get oxygen from the water. 

The blood flows in opposite directions to the water which means a constant diffusion gradient is maintained.

Allowing about 80% of water into the fish's blood, instead of 50% in parallel flow. 

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Factors Affecting Transpiration

Light            Stomata open in the light and close in the dark.

Temperature  Alters the kinetic energy of the water molecules

                     and the humidity in the air.  

Humidity   Affects water potential gradient between the air

                         spaces in the leaf and the atmosphere.

Wind   Changes the water potential gradient by altering 

                         the rate at which moist air is removed from around the 


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