AHL Content Statements

• D2.3.8

  Water potential as the potential energy of water per unit volume
  

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  Students should understand that it is impossible to measure the absolute quantity of the potential energy of water, so values relative to pure water at atmospheric pressure and 20°C are used. The units are usually kilopascals (kPa).
  

• D2.3.9

  Movement of water from higher to lower water potential
  

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  Students should appreciate the reasons for this movement in terms of potential energy.
  

• D2.3.10

  Contributions of solute potential and pressure potential to the water potential of cells with walls

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  Use the equation ψw = ψs + ψp. Students should appreciate that solute potentials can range from zero downwards and that pressure potentials are generally positive inside cells, although negative pressure potentials occur in xylem vessels where sap is being transported under tension.
  

• D2.3.11

  Water potential and water movements in plant tissue

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  Students should be able to explain in terms of solute and pressure potentials the changes that occur when plant tissue is bathed in either a hypotonic or hypertonic solution.
  

• B3.2.17

  Generation of root pressure in xylem vessels by active transport of mineral ions
  

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  Root pressure is positive pressure potential, generated to cause water movement in roots and stems when transport in xylem due to transpiration is insufficient, for example when high humidity prevents transpiration or in spring, before leaves on deciduous plants have opened.
  

• B3.2.18

  Adaptations of phloem sieve tubes and companion cells for translocation of sap

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  Include sieve plates, reduced cytoplasm and organelles, no nucleus for sieve tube elements and presence of many mitochondria for companion cells and plasmodesmata between them. Students should appreciate how these adaptations ease the flow of sap and enhance loading of carbon compounds into phloem sieve tubes at sources and unloading of them at sinks.