Diffusion & Osmosis (Lab 1) Review
 

Some Key Review Concepts:

Diffusion

Molecules, which are in constant motion, tend to move from regions of higher concentrations to lesser concentrations. Diffusion is defined as the net movement of molecules down their concentration gradient. 

Osmosis

Osmosis is the passive transport (diffusion) of water. In osmosis, water moves through a semi-permeable membrane from a region of higher concentration to a region of lower concentration.

Solutions

The terms hypotonic, hypertonic, and isotonic are used to compare solutions relative to their solute concentrations. The hypotonic side is the side with the larger water percentage and a lower solute concentration. The hypertonic side is the side with the smaller water percentage and a higher solute concentration.  It is isotonic when both sides have equal concentrations of solute and water percentages.  

Lab Design

Exercise 1: Diffusion

Fill a dialysis bag with a sugar/starch solution and immerse the bag in a dilute iodine solution. Water, sugar, starch, and iodine molecules will all be in motion, and each molecule will move to a region of its lower concentration, unless the molecule is too large to pass through the membrane. Your task is to determine relative size of the various molecules and gather evidence of molecular movement.

Note: When iodine comes in contact with starch, it changes from an orange-brown color to blue-black.

Exercise 2: Osmosis

Investigate the relationship between solute concentration and water movement by filling six different dialysis bags with increasing concentrations of sucrose and placing the bags into distilled water. After the time for the experiment has elapsed, you compare the initial weight of each bag with its final weight, calculate the percent change in mass, pool your data with that of your classmates, and graph your results.

Exercise 3: Water Potential & Potato Core

The exercise is similar to Exercise 2, except that you use cores from potatoes instead of dialysis bags. Submerge the cores in solutions of varying sucrose concentrations. When calculating the percent change in mass, you will notice some of the cores will have gained weight while others will have lost weight, depending on the movement of water. You then graph this data and determine which concentration of the sucrose solution is in equilibrium with the cores. Since you know that the pressure potential of the surrounding solution in an open beaker is zero, you can now calculate the water potential.

Exercise 4: Water Potential

Use the value for the molar concentration of the potato cores that you obtain in Exercise 3 to determine the water potential for the potato cells.

Exercise 5: Plasmolysis

Watch the effect of placing a living cell into a solution that has a lower or higher concentration of water than the cell.

Analysis of Results

Solute potential () = –iCRT

i= The number of particles the molecule will make in water; for NaCl this would be 2; for sucrose or glucose, this number is 1

C= Molar concentration (from your experimental data)

R= Pressure constant = 0.0831 liter bar/mole K

T= Temperature in degrees Kelvin = 273 + °C of solution