Cell Membrane Diffusion Lab

When you have completed this laboratory you should be able to:

1. Define the terms diffusion, concentration gradient, active transport, net diffusion, dialysis, and facilitated diffusion.

2. Demonstrate the functioning of a semi-permeable membrane.


Diffusion is the movement of a substance from an area of high concentration to an area of lower concentration. The term solute refers to molecules dissolved in a solvent. In salt water, water is the solvent (dissolves substances) and Sodium Chloride (salt) is the solute (substances dissolved in water). In this experiment, glucose and starch molecules will be the solutes, and they will be dissolved in the solvent water.

Diffusion moves solute through a solvent until the solute is equally distributed. This passive mechanism is driven by differences in concentration, or molecular movement from areas of higher concentration to lower concentration. Diffusion occurs naturally within the atmosphere, lakes and ponds, and across cell membranes. Diffusion is different from another way organisms move materials. This other way is called active transport. It is the movement of a substance through a membrane against, or in opposition to, the concentration gradient. In active transport the organism actively seeks and moves the substance it needs across the cell membrane.

Net Diffusion

Net diffusion is the movement of more particles of a substance in one direction than in the opposite direction. The net diffusion of any substance occurs down its own concentration gradient. Therefore, the net diffusion of solute particles occurs from more concentrated to less concentrated areas, and the net diffusion of water molecules, in contrast, occurs from the more dilute (more water) to the less dilute (less water).


Dialysis is diffusion under certain conditions. It takes place when a solution contains both crystalloids, or solute particles whose diameters are very small, such as glucose and oxygen, and colloids, or solute particles whose diameters are relatively large, such as starch and proteins. When this solution is separated from plain water by a membrane permeable to small diameter crystalloids, but impermeable to larger diameter colloids, the crystalloids diffuse through the membrane and the colloids remain behind. Therefore, dialysis can be defined as a form of diffusion which separates crystalloids from colloids. Human kidney dialysis machines operate upon this principle. This laboratory actually is a demonstration of this special type of diffusion.

Facilitated Diffusion

Facilitated diffusion resembles both ordinary diffusion and active transport. It is a passive process in that a substance moves down its own concentration gradient. However, like active transport, facilitated diffusion also is assisted by an outside force. The active agent is a carrier molecule, such as a specific protein on the outer surface of a cell membrane, which binds to the substance to be carried through the membrane. The bound carrier molecule rotates within the membrane and thereby carries the substance rapidly from one side of the membrane to the other. There it dissociates from the substance, releasing it into the cytoplasm of the cell. The carrier molecule has thereby facilitated, or helped the substance through the cell membrane.

This laboratory is designed to help you understand how materials move in and out of cells. You will construct a model of a cell membrane in order to study the movement of water, glucose, and starch through cell membranes.



2 500 ml. beakers
pen or pencil
Pasteur Pippette
2 test tubes


starch suspension
glucose solution
iodine solution
Benedict's solution
distilled water
Hot Plate
dialysis tubing

Procedure: Hour 1


A. Soak a 20 cm. section of cellophane dialysis tubing in water for five minutes. Then gently rub the ends between your thumb and index finger until the ends separate. Carefully push a pen/pencil/pasteur pipette through the opening in order to hold the tube open. Twist one end of the tubing and tie it tightly with string. Remove the glass rod and fill this dialysis bag approximately half with starch suspension and half with sugar solution. Tie the top of the bag with string, leaving a loose piece of string 10 to 15 cm. long. Thoroughly rinse the bag with water from the sink (make sure no water enter an open end of the bag.

1.Why is it important to rinse the bag with water

B. Place the bag in a beaker of distilled water. Leave the loose piece of string outside the beaker so that you may later use it to remove the bag from the water. Wait fifteen to twenty minutes, performing step C during the interim.

C. Put on your goggles and apron. Set up a boiling water bath by placing a beaker 1/3 full of water on a hot plate set on high.

D. After fifteen to twenty minutes remove a dropperful of water from the beaker in which you have been soaking the dialysis bag filled with starch suspension and sugar solution. Place this water in a test tube and add 10 drops of Benedict's solution. Heat the test tube in the boiling water bath for five to ten minutes.

2.What are you testing for (refer to our organic chemistry lab)?

3.Initially what is the color of the water and Benedict's solution mixture in the test tube?

4.What is the color of the water and Benedict's solution mixture after being heated in the boiling water bath?

5.Explain what your answers to questions three and four indicate.

E. Remove another dropperful of water from the beaker in which you have been soaking the dialysis bag filled with starch suspension and sugar solution. Place this water in a test tube, and add 3 drops of iodine solution. Observe the color of the water after the iodine has been added.

6.What changes, if any, do you observe?

7. If a change has occurred, what does this indicate (refer to organic chemistry lab)?

F. Put ten drops of iodine into the beaker immediately next to the dialysis tubing. Wait five minutes and then look closely at the contents of the dialysis bag.

8. What changes, if any, do you observe?

9. If a change has occurred, what does this indicate (refer to organic chemistry lab)?

10.Compare the size of a glucose molecule and a starch molecule (refer to exercise 4). How does this size difference affect the results you encountered in this laboratory?

11. What evidence, if any, did you find which indicates that the cellophane dialysis bag was semi-permeable?

12. Did diffusion take place in this laboratory? If so, what type of diffusion?


Define the following terms. You will be held accountable for the concepts prior to taking the unit review:		

concentration gradient
semi-permeable membrane
facilitated diffusion
net diffusion
active transport