Transcript AP Biology Lab #1 Osmosis and Diffusion

AP Biology Lab #1
Osmosis and Diffusion
Mrs. Smith
Briar Woods High School
Percent Change in Mass of Dialysis Bags
Gr 1
.0M
.2M
.4M
.6M
.8M
1.0M
Gr 2
Gr 3
Gr4
Gr 5
Gr 6
Gr 7
Total
Average
Percent Change in Mass of Potato Cores
Gr 1
.0M
.2M
.4M
.6M
.8M
1.0M
Gr 2
Gr 3
Gr4
Gr 5
Gr 6
Gr 7
Total
Average
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)