D2.3  Water Potential

Theme:  Continuity and Change

Continuity of life depends on regulated water and solute concentrations inside a cell.

• Continuity of the water chain is maintained by the hydrogen bonding between water molecules (cohesion) and between water and the xylem cell walls (adhesion). 
• There is a consistent water potential gradient from the soil, through the plant and into the atmosphere. For a plant to function, the water potential must remain lower in the leaves than in the roots, and lower in the roots than in the soil. This steady gradient ensures a predictable, continuous flow of water.
• A healthy plant cell maintains a turgid state, with intracellular water exerting pressure against the cell wall to keep the plant upright.  The turgid state is maintained by a high solute concentration in the vacuole, so that water consistently enters the cell via osmosis.
• For an animal cell to maintain its structure, it must be bathed in an isotonic fluid. This ensures that the water potential inside the cell and outside the cell is equal, resulting in no net movement of water.
  
  

The movement of solutes alter the energy state of water, forcing it to move. 

• Change in the solute potential occurs when solutes (like sugars or ions) are added to a system. This decreases the free energy of the water molecules, lowering the water potential and leading to osmosis (movement of water across the membrane into areas of higher solute concentration).
• As water enters a plant cell, the physical pressure against the wall increases. This change in pressure potential eventually balances the solute potential, reaching an equilibrium where net water movement stops. This dynamic shift is what prevents plant cells from bursting.
• Without a cell wall to provide a "pressure potential," any change in the external water potential leads to immediate and often fatal physical changes in the animal cell.
  

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?

​
​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?

Key Terms to Know: all are higher level only

Contractile Vacuole
Crenation
Hydrogen Bond
Hypertonic
Hypotonic
Intravenous Fluid
Ion

Isotonic
Kilopascals (Kpa)*
Osmosis
Plasmolysis
Polar
Potential Energy*
Pressure Potential*

Solute
Solute Potential*
Solvent
Turgor Pressure
Water Potential*
Xylem Vessel*

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.​