6.3 - Water potential

Water potential

potential energy of water per unit volume (unit = kPa)

Values of water potential

impossible to measure absolute quantity so values are relative to pure water (0 kPa) at atmospheric pressure and 20°C

Factors affecting water potential

concentration of solute molecules and pressure of H2O molecules in the solution

Water potential formula

water potential (ψw) = solute potential (ψs) + pressure potential (ψp)

Describe the solute potential

goes from 0 downwards and the more solute is present the lower the solute potential, since water molecules make hydrogen bonds and surround solute molecules leaving fewer molecules free to move

Describe the pressure potential and how it is affected by extracellular water potential

generally positive inside cells, if placed in a solution with higher/lower water potential then water will move out/in respectively affecting the pressure potential until equilibrium is reached

Example of negative pressure potential

transpiration stream: in the xylem which pulls up the water via capillary action

Hypotonic solution

higher water potential in the solution compared to the cell (water moves into the cell, animal cells will often lyse bcs no cell wall, plant cells will fill up and exert turgor pressure on the cell wall)

Isotonic solution

same osmotic concentration so same water potential (no net movement of water, used in hospitals for dehydrated people and storage of organs for transplantation)

Hypertonic solution

lower water potential in the solution (water moves out of the cell, animal cells will shrink via crenation, plant cells will become flaccid or even plasmolysed as the plasma membrane pulls away from the cell wall)