10.3 OsmosisOsmosis the movement of water molecules from a solution of lessnegative water potential to a solution of more negative water potentialthrough a semi permeable membrane.Osmosis is the net movement of water from a high water concentration to alower water concentration through a selectively permeable membrane. Inosmosis there is a net movement of molecules from high to low concentration.You will get some molecules moving back across. Water will keep moving until itreaches equilibrium.Water potential (Ψ) is a measure of the kinetic energy of water molecules.Water molecules are constantly moving in a random fashion. Some watermolecules collide with the cell membrane, creating pressure on it known as waterpotentialThe higher their kinetic energy, the more they move and hit the membrane andthe higher their water potential.Solutes restrict the movement of water, so a stronger sugar solution with lotsof solute particles will lower the kinetic energy and hence the water potential ofthe water.Water movement is restricted due to the attractive forces that exist betweenthe water molecules and the solute particles, causing hydration shells.As the solute molecules dissolve it becomes charged and attracts the charges onthe water molecules. Water molecules surround it. The water molecules are nolonger free as the solute molecules tie them up. This reduces the free watermolecules in the solution.
A dilute sugar solution with fewer solute particles will allow the water potentialto be higher as fewer solute particles are present to attract and restrict themovement of water molecules.Pure water has a water potential of 0, measured in KpaAll solutions have a negative water potential because water in any solutionmoves less freely than in pure water.A dilute solution has a less negative water potential than a more concentratedsolution.Osmosis occuringNo osmosis
Dilute sugar solution Concentrated sugar solutionHigh Conc of water Lower concentration of waterFewer solute particles More solute particlesWater more free to move as fewer Water less free to move about assolute molecules more solutes attracting themWater has more kinetic energy Water has less kinetic energy andtherefore more likely to hit the is less likely to hit the membranemembraneWater has a less negative water Water has a more negative waterpotential potential.How free the water molecules are to move depends on 3 things:1. The number of solute particles2. The pressure in the cell3. The temperature – High temp more kinetic energy.
Solute and Pressure potentialThe concentration of solutes inside the cell is called the solute potential (ψs). andit always has a negative value. The pressure exerted on cell contents by the cellsurface membrane or cell wall is called the pressure potential (ψp). This isbecause the membrane or wall exerts a force on the cell contents to increasethe concentration of water molecules in the cell. It squeezes the contents ofthe cell, trying to push the water molecules out.Water potential of a cell = solute potential + pressure potential Ψ Ψs ΨpWhich way will water move by osmosis in these diagramsHow Water Moves Through a MembraneWater primarily passes through protein channels in the membraneCell membranes are not in fact completely impermeable to water. A tiny amountcan pass through a lipid bilayerThe exact reason is not known. It has been suggested that the constantsideways movement of the phospholipids and the flexing of the fatty acid tailscreates temporary holes that the water molecules can slip through.Molecules have to wait in the middle for a gap to appear in the other sideMembranes with a high proportion of cholesterol are less permeable to water.The cholesterol could have a stabilising effect on the phospholipids. Thecholesterol minimises the creation of temporary holes.
Osmosis in Animal Cells3 Terms used when discussing osmosis in animal cellsHypertonic means the solution has a more negative water potential than the cell.Isotonic means the solution has the same water potential as the cellHypotonic means a solution has a less negative water potential than the cellAs animal cells have no cell wall animals need to regulate the amount of water intheir body. In humans the organ that carries out this function is the kidney.