SL Content Statements

• D2.3.1

  Solvation with water as the solvent

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  Include hydrogen bond formation between solute and water molecules, and attractions between both positively and negatively charged ions and polar water molecules.
  

• D2.3.2

  Water movement from less concentrated to more concentrated solutions

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  Students should express the direction of movement in terms of solute concentration, not water concentration. Students should use the terms “hypertonic”, “hypotonic” and “isotonic” to compare concentration of solutions.
  

• D2.3.3

  Water movement by osmosis into or out of cells

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  Students should be able to predict the direction of net movement of water if the environment of a cell is hypotonic or hypertonic. They should understand that in an isotonic environment there is dynamic equilibrium rather than no movement of water.
  

• D2.3.4

  Changes due to water movement in plant tissue bathed in hypotonic and those bathed in hypertonic solutions

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  AOS: Students should be able to measure changes in tissue length and mass, and analyse data to deduce isotonic solute concentration. Students should also be able to use standard deviation and standard error to help in the analysis of data. Students are not required to memorize formulae for calculating these statistics. Standard deviation and standard error could be determined for the results of this experiment if there are repeats for each concentration. This would allow the reliability of length and mass measurements to be compared. Standard error could be shown graphically as error bars.
  

• D2.3.5

  Effects of water movement on cells that lack a cell wall
  

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  Include swelling and bursting in a hypotonic medium, and shrinkage and crenation in a hypertonic medium. Also include the need for removal of water by contractile vacuoles in freshwater unicellular organisms and the need to maintain isotonic tissue fluid in multicellular organisms to prevent harmful changes.
  

• D2.3.6

  Effects of water movement on cells with a cell wall
  

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  Include the development of turgor pressure in a hypotonic medium and plasmolysis in a hypertonic medium.
  

• D2.3.7

  Medical applications of isotonic solutions
  

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  Include intravenous fluids given as part of medical treatment and bathing of organs ready for transplantation as examples.
  

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.