Lab 5 An Investigation of Osmosis in Living Cells and Tissues We’ve established the molarity of my different solutions What else have we done? We’ve established the tonicity of the potato cells, or solution concentration at which the cells are hyper, iso and hypotonic Tonicity can be important in any number of areas where water movement across a membrane occurs

W What is the isotonic concentration? Where are the hypertonic solutions? To the right of the isotonic point. Where are the hypotonic solutions?

Tonicity is the relative concentration of solutes dissolved in solution which determine the direction and extent of osmosis Students should understand conditions of tonicity: hypertonic, isotonic, hypotonic Students should understand the different responses of cells with cell walls versus cells without cell walls in solutions of different tonicity Refers to the solute concentration of the aqueous environment of the cell Indirectly tells us about water concentration “Hyper” – high solute/low water outside, so water flows out of the cell

What about an absolute value of tonicity? Since tonicity can be determined by a number of dissolved solutes (for example, in the cell) is there a value that can compare solutions comprised of differing solutes? A way to say “This is the value that is isotonic to this cell type” “This is the value that is isotonic to these plant root cells” “IV fluids need to be this tonicity so as to not damage blood cells”

Water potential - a value that describes the tendency of water to move across a membrane into or out of a solution A measure of the relative energy (in the tendency of water to move) in solutions separated by a permeable membrane Determined by the relative solute concentrations Are there other factors that influence the movement of water across a membrane?

Water potential  a value that describes the tendency of water to move across a membrane into or out of a solution …is the free energy per mole of water

Water potential  …is calculated from two major components: (1) the solute potential (ψS), which is dependent on solute concentration (2) the pressure potential (ψP), which results from the exertion of pressure — either positive or negative (tension) — on a solution   ψ = ψP + ψS Water Potential = Pressure Potential + Solute Potential In biological systems PP is typically equal on either side of the membrane, so PP is often 0 (pressure on either side cancels each other out) So ψ = ψS How do we calculate solute potential?

The water potential of pure water in an open beaker is zero (ψ = 0) because both the solute and pressure potentials are zero (ψS = 0; ψP = 0). The addition of solute to the water lowers the solute potential and therefore decreases the water potential. This means that a solution has a negative water potential because of the solute. The more solute, the lower the water potential. The solute potential (ψS) = – iCRT

The solute potential (ψS) = – iCRT i = the ionization constant C = the molar concentration R = the pressure constant (R = 0.0831 liter bars/mole-K) T = the temperature in K (273 + °C) Units = bars or megapascals (MPa) i = ionization constant Sucrose = 1 NaCl = 2 CaCl2 = 3

Practice Problems What is the water potential of the solution…. A 0 Practice Problems What is the water potential of the solution…? A 0.15 M solution of sucrose at atmospheric pressure and 25°C. A 0.15 M solution of NaCl at atmospheric pressure and 25°C. The water potential in root tissue was found to be -3.3 bars. If you place the root tissue in a 0.1 M solution of sucrose at 20°C in an open beaker, what is the water potential of the solution, and in which direction would the net flow of water be?

How would we determine the water potential of potato cells, represented by cubes made from potatoes? What is “C”?