D2.3  Water Potential

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• What factors affect the movement of water into or out of cells?
• How do plant and animal cells differ in their regulation of water movement?

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​Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• What are the implications of solubility differences between chemical substances for living organisms?
• What variables influence the direction of movement of materials in tissues?

D2.3.1— Solvation with water as the solvent.

• Identify solvent and solutes of a solution.
• Define solvation.
• Explain why water is able to dissolve charged and polar molecules.
• Outline the solvation of hydrophilic and hydrophobic substances.​​

D2.3.2— Water movement from less concentrated to more concentrated solutions.

• Define osmolarity, isotonic, hypotonic and hypertonic.
• State the unit for concentration of a solute in a volume of solution.
• Outline the net movement of water between hypotonic, hypertonic and isotonic solutions.

D2.3.3-- Water movement by osmosis into or out of cells.

• Compare the relative permeability of the plasma membrane to water and solutes.
• Define osmosis.
• State that osmosis is a form of passive transport.
• Explain what happens to cells when placed in isotonic, hypotonic and hypertonic solutions.

D2.3.4- Changes due to water movement in plant tissue bathed in hypotonic and those bathed in hypertonic solutions.

• Explain the change in mass and/or volume of plant tissues placed in either hypotonic or hypertonic solutions. 
• Determine the concentration of solutes in a plant tissue given changes in plant tissue mass and/or length when placed in solutions of various tonicities.​

​​​​​​​D2.3.5— Effects of water movement on cells that lack a cell wall. 

• State the effects of hypertonic and hypotonic solutions on cells without a cell wall.
• Explain why tissue fluid in multicellular organisms must be isotonic to the cells of the tissue.
• Outline the role of the contractile vacuole in freshwater unicellular organisms.​

​​​​​​​D2.3.6- Effects of water movement on cells with a cell wall.

• Describe the strength and permeability of a cell wall.
• Explain the effects of hypertonic and hypotonic solutions on cells with a cell wall with specific reference to turgor pressure and plasmolysis.

D2.3.7- Medical applications of isotonic solutions.

• State the effects of isotonic, hypertonic and hypotonic solutions on human cells.
• Outline the use of “normal saline” in medical procedures.

AHL ​​​​​​​D2.3.8- Water potential as the potential energy of water per unit volume.

• Define water potential. 
• State the symbol and unit for water potential.
• State that pure 20° water at standard atmospheric pressure as a water potential of 0kPa.
• Outline factors that contribute to water potential in living systems.

AHL ​​​​​​​D2.3.9- Movement of water from higher to lower water potential.

• Explain the movement of water from higher to lower water potential.

AHL ​​​​​​​D2.3.10- Contributions of solute potential and pressure potential to the water potential of cells with walls.

• Describe the impact of solute potential and pressure potential on the total water potential of cells with walls.  
• Explain why solute potentials can only range from 0kPa downwards.
• State that pressure potentials are generally positive inside cells.
• State a cell type in which the pressure potential is negative. ​

AHL ​​​​​​​D2.3.11- Water potential and water movements in plant tissue.

• Explain the movement of water in plant cells bathed in a hypotonic solution in terms of solute and pressure potentials.
• Explain the movement of water in plant cells bathed in a hypertonic solution in terms of solute and pressure potentials.​