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:
• 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
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