Water potential

solvation

• solvents: liquids that can dissolve other substances to make solutions

• solutes: dissolved substances in solutions

• solvation: process of dissolving. water dissolves many different types of molecules by forming H Bonds with them. dissolves ions because poles of water molecule attracted to charged substances

types of solutions

• hypertonic: higher solute conc

• isotonic: same solute conc

• hypotonic: lower solute conc

• water moves from low to high solute concentration, up the solute conc gradient

movement of water

• diffusion of water molecules from a less concentration solution to a more concentrated solution across a partially permeable membrane

• in isotonic environment, there is dynamic equilibrium, water molecules move in and out of cell equally

water movement with no cell wall

• hypotonic: cell gains water by osmosis, has no cell wall to create turgor pressure so might burst

• hypertonic: cell loses water by osmosis, cell becomes crenated

water movement with cell wall

• hypotonic: increase in vol of cytoplasm, turgor pressure develops

• hypertonic: decrease in vol of cytoplasm, gap between cell wall and plasma membrane, cell is plasmolysed

isotonic fluids in medicine

• intravenous fluids must be isotonic to avoid osmotic damage to blood cells

• tissues/organs for transplantation must be bathed in isotonic solution to prevent osmotic damage

water potential

• measure of potential energy per unit volume

• impossible to measure absolute quantity of potential energy, values relative to pure water at 20 c and atmospheric pressure

• in kPa

water potential movement

• water moves from high to lower water potential because this minimises its potential energy

  • solutions w/ high water potential contain water molecules w/ greater potential energy for movement, so greater tendency to move

solute potential and pressure potential

• ψw = ψs + ψp

• solute potential:

  • maximum of zero

  • more solutes dissolved, more negative

• pressure potential

  • hydrostatic pressure of water

  • usually positive as cytoplasm exerts pressure on cell wall - turgor pressure

  • negative pressure potential can occur in xylem

water movement in plant tissue

• hypotonic:

  • solute potential more negative in plant tissue than in solution, so water moves from solution to tissue

  • pressure potential increases as cytoplasm presses against cell wall

  • water potential eventually reaches equality inside and outside of cell, cell becomes turgid

• hypetonic:

  • solute potential more negative in solution, so water moves from tissue to solution

  • as water leaves, pressure potential decreases, if becomes 0 cell becomes flaccid

  • volume of cytoplasm decreases and plasma membrane pulls away from cell wall so plasmolysis occurs