1. AP Biology Lab:
Diffusion & Osmosis

2. Objectives of the lab:
Investigate the relationship among surface area, volume, and the rate of diffusion
Investigate osmosis in plant cells
Design an experiment to measure water potential in plant cells
Analyze the data collected in the experiments and make predictions about molecular movement through cellular membranes
Work collaboratively to design experiments and analyze results
Connect the concepts of diffusion and osmosis to the cell structure and function

3. Format of the lab: 3 sections: To be completed today in periods 8 & 9:
1. Surface area and Cell Size
2. Modeling Diffusion & Osmosis
3. Observing Osmosis in Living Cells
To be completed today in periods 8 & 9:
Sections 1 & 2
Possible introduction to section 3 if time permits
To be completed tomorrow in periods 8 & 9:
Introduction to water potential
Section 3:
Observing osmosis in anacharis (elodea) leaves
Designing your own experiment to test for osmosis in living cells
Performing your designed experiment & analysis of results

4. Pre-lab questions for section 1:
1. Why are cells small?
2. How does the size of a cell affect diffusion rate?

5. Surface to Volume Ratio of Cells
Animation

6. Pre-lab questions for section 2:
3. What materials are required to diffuse through a cell’s membrane? What factors determine the material’s ability to diffuse?
 4. How would you determine whether or not an aqueous substance was able to diffuse in or out of a cell?

7. Diffusion
Animation:

8. Pre-lab questions for section 3:
5. What would happen if you applied saltwater to a plant’s roots? Be specific with regard to cell structure.
6. Will water move into or out of a plant cell if the cell has a higher water potential than the surrounding environment?
7. How does a plant cell control its internal (turgor) pressure?

9. Osmosis Animation Topics covered: Polarity of water Hydration shells
Solute concentrations
Hypo/Iso/Hypertonic Solutions
Animation

10. Water Potential
Physical property of water that determines the direction that water will flow
In the world of water potential, 0 is high!
Usually a negative number
Occasionally a positive number
Determined by the solute concentration and pressure

11. Water Potential
Water potential = Solute Potential + Pressure Potential
Equation: Ψ = Ψs + Ψp
Measured in bars = a metric measure of pressure

12. Ψ = Ψs + Ψp Ψs = the solute (or osmotic) potential Ψs = -iCRT
I = ionization constant (number of particles formed)
Glucose (1)
NaCl (2)
CaCl2 (3)

13. Ψ = Ψs + Ψp Ψs = the solute (or osmotic) potential Ψs = -iCRT
I = ionization constant (number of particles formed)
C = osmotic molar concentration of the solute
R = pressure constant (handbook value R= liter bars/mole 0K
T = temperature in 0K ( C) of solution

14. Water Potential Principles
Water moves from high water potential to low water potential
Water potential = tendency of water to move from high free energy to lower free energy
Distilled water in an open beaker has a water potential of 0 (zero)

15. Water Potential Principles
Addition of solute decreases water potential
Addition of pressure increases water potential
In cells, water moves by osmosis to areas where water potential is lower
A hypertonic solution has lower water potential
A hypotonic solution has higher water potential

16. Concept of Water Potential
Animation water uptake