AP Biology Lab: Diffusion & Osmosis

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

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

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

Surface to Volume Ratio of Cells Animation

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?

Diffusion Animation:

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?

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

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

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

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

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=0.0831 liter bars/mole 0K T = temperature in 0K (273 + 0 C) of solution

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)

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

Concept of Water Potential Animation water uptake