AP Lab 1

1C-1E

Osmosis and Water Potential

What is water potential?

• A measure of how likely water is move away from an area
• The higher the water potential the more likely water is going to _____
• Water potential is represented by:

ψ = ψ_s + ψ_p

• ψ_s - solute potential
• ψ_p - pressure potential

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• The net movement of water will always be from an area of higher water potential to an area with lower potential.

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• Water will move from the higher cell potential to the lower cell potential.

Solute Potential (ψ_s )

• The contribution of solutes to the water potential of a substance.
• Can be 0 (if there are no solutes in solution, i.e. pure water) or -
• The addition of solutes lowers the water potential (becomes negative)
• Let’s review how solutes affect the movement of water…

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Which way will the water move?

WHY?

So, why the need for a membrane?

Water Balance in Cells

Solute Potential (cont.)

• What is the solute potential of pure water?
• 0
• If we add solutes to pure water is water more or less likely to leave?
• So what happens to the solute potential as solutes are added?
• It becomes more and more negative.

Pressure Potential (ψ_p )

• The pressure potential represents the pressure component that makes water likely to move.
• As water moves into a cell, it exerts an outward pressure
• What cell part would be responsible for pressure potential?
• Cell wall

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Flaccid = lacking water/firmness

Turgid = swollen, full of water

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If enough solute is added to the water outside of cells, water will leave the cells, moving from an area with higher cell potential to an area with lower cell potential. A continued loss causes the cell membrane to shrink away from the cell wall (plasmolysis) - see 1E of lab.