D I F F U S I O N & O S M O S I S

AP BIOLOGY LAB:

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?

A N I M AT I O N

SURFACE TO VOLUME RATIO OF CELLS

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?

A N I M AT I O N:

DIFFUSION

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?

A N I M AT I O N

OSMOSIS

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

A N I M AT I O N WAT E R U P TA K E

CONCEPT OF WATER POTENTIAL