International A Level Biology / Membranes and transport
Osmosis and water potential revision notes
Edexcel International A Level Biology (YBI11), chapter 4: Membranes and transport, specification point 2.4. Free lesson.
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Water potential, ψ, describes how readily water moves out of a solution. It helps you explain osmosis and predict how animal and plant cells change in different solutions.
What water potential means
Water potential is a measure of how readily water molecules move out of a solution. It is written as ψ and measured in kilopascals (kPa). Pure water has the highest water potential, 0 kPa. Dissolving a solute lowers the value, so every solution has a negative water potential. The more solute there is, the more negative ψ becomes. A value of −200 kPa is higher than −800 kPa because it is less negative.
Why solutes lower water potential
Water molecules are attracted to dissolved solutes, such as salt ions or sucrose molecules. Some water molecules cluster around the solute and are not free to move. Only free water molecules can cross a membrane, so fewer free molecules means a lower water potential. In a mixture, the total of all dissolved solutes affects ψ. More dissolved solute means fewer free water molecules and a more negative ψ.
How osmosis moves water
A partially permeable membrane lets water molecules through but not solute molecules. Water molecules move randomly in both directions, but more move from the side with higher ψ to the side with lower ψ. This net movement is osmosis.
How animal and plant cells respond
The effect on a cell depends on whether the solution outside has a higher, the same, or a lower water potential than the cell. Animal cells have no cell wall, so they can burst as water enters. Plant cells have a cellulose cell wall that resists expansion. For a red blood cell, a higher ψ outside makes water enter, so it swells and may burst. At the same ψ, there is no net water movement and the cell stays normal. A lower ψ outside makes water leave, so the cell shrinks. For a plant cell, a higher ψ outside makes water enter and the cell becomes turgid. At a lower ψ outside, water…
Apply water potential to a plant cell
Compare the two values before deciding which way water moves. Then describe the movement and the cell's response.