AP Biology Lab #1 Osmosis and Diffusion Mrs. Smith Briar Woods High School

.0M .2M .4M .8M 1.0M Percent Change in Mass of Dialysis Bags Gr 1 Gr 2 Total Average .0M .2M .4M .6M .8M 1.0M

.0M .2M .4M .8M 1.0M Percent Change in Mass of Potato Cores Gr 1 Gr 2 Total Average .0M .2M .4M .6M .8M 1.0M

Water Potential Botanist use the term water potential when predicting the movement of water into or out of plant cells. Water potential is affected by two factors: pressure and the amount of solute. If a plant cell is placed in distilled water, water will enter the cell and the cell contents will expand. However, the elastic cell wall exerts a back pressure, which will limit the net gain of water.

Water Potential Water potential is calculated using the following formula: Water potential = pressure potential + solute potential Pressure potential : In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake. Solute potential : The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also. In sum, as solute is added, the water potential of a solution drops, and water will tend to move into the solution. In this laboratory we use bars as the unit of measure for water potential; 1 bar = approximately 1 atmosphere.

Water Potential The water potential of pure water in an open container is zero because there is no solute and the pressure in the container is zero. Adding solute lowers the water potential. When a solution is enclosed by a rigid cell wall, the movement of water into the cell will exert pressure on the cell wall. This increase in pressure within the cell will raise the water potential.

Calculation of Water Potential i= Ionization constant ( for sucrose it is 1) C= Molar concentration of potato ( use graph to find this value) R= Pressure constant (R= .0831 liter bars/moleK) T= Temperature K ( 273 + C of solution) (22C)