All cells are surrounded by a phospholipid bilayer membrane. The function of the phospholipid bilayer is to act as a barrier between the living cell and the environment. The phospholipid bilayer also regulate the passage of solutes into and out of the cell. The phospholipid bilayer membrane can differentiate which solutes to move in or not. One process by which solutes enter cells is by diffusion, which is the movement of solutes from high concentration to low concentration. Another process is osmosis, which is when water molecules diffuse across a cell membrane from regions of low solute concentration to high solute concentration.

Diffusion

Diffusion is defined as the movement of solute molecules from a region of high concentration to low concentration. To learn about some of the factors that affect the rate of diffusion, we will measure the distances that two colored solutes move through a gelatin agar medium. The gelatin medium is composed of agar and water; and mimics an aqueous system. The experimental plan is that we will add drops of different solutes into small test tubes containing agar. The tubes will be incubated under different environmental conditions for about 1 hour. We will compare the relative diffusion rates of the solutes by measuring the distances traveled by the solutes in each test tubes. The result is that longer distances traveled by the solutes indicate a higher diffusion rate and shorter distances traveled indicate a slower diffusion rate.

The solutes that we will used are: KMnO4 ( potassium permanganate, molecular mass = 158 grams/mole) and a dye called Aniline Blue ( molecular mass = 738 grams/mole).
The experiment will address the following questions:

1. How will increasing temperature affect diffusion rate?
2. How will increasing solute molecular weight affect diffusion rate?
3. How will increasing solute concentration affect diffusion rate?

Procedure:

1. Take 6 agar-filled test tube and label 1- 6.
2. Add the different solutes into the test tube 1- 6 using table 1 as a guide.

Table 1

Tube # Solute Incubation Distance Moved (mm)
1 0.1 M KMnO4 4C 4
2 0.1 M KMnO4 room temp 7
3 0.1 M KMnO4 35C 12
4 0.1 M An. Blue room temp 6
5 0.02 M An. Blue room temp 8
6 0.01 M An. Blue room temp 9

Osmosis is when water molecules diffuse across a cell membranes from regions of low solute concentration to high solute concentration. Water molecule are in constant motion, and the rate of water is dependent on the temperature. Many cells embark on a situation in which the total concentration of solutes is different on the inside and on the outside of the cell. When this occurs, water will move into or out of the cell depending on the relative concenration of solutes on either side of the membrane. The solution on the side of the membrane where solute concentration is less than that of the other side is referred to as being hypotonic. The solution on the side of the membrane where solute concentrations is greater than that of the other side is referred to as being hypertonic. When the solute concentration is the same on both sides of the membrane, the solution is said to be isotonic.

To learn about osmosis, we will use a synthetic permeable membrane called a dialysis tubing to create "artificial cells". We will fill these dialysis bags ( artificial cells) with solutions of various concentration and place them in beakers with solutions of varying concentration.

Procedure:

1. Take 4 beakers label each with number 1, 2, 3, or 4.
2. Fill these beakers with bathing solutions that correspond with numbers on beakers. Volume here should be 200 ml
3. Obtain 4 strips of dialysis tubing and soak them in distilled water.
4. Use table 2 to create the dialysis bags with appropriate contents listed in the table.
5. After weighing and recording the weights of the bags, place them into the beaker with the number corresponding to the bag #.
6. Incubate the bags for at least one hour. At the end of the incubation time, blot the excess solution to the surface of each bag, and weigh and record the weights.

Table 2

Bag or Bag Content Bathing Beginning Ending
Beaker # (approx. 10 ml) Solutions Weight (g.) Weight (g.)
1 water 40% sucrose 10 6
2 water water 10 10
3 20% sucrose water 10 15
4 40% sucrose water 10 20