BL - 11                                                            NAME                                                                       

 

DIFFUSION THROUGH A MEMBRANE

 

BACKGROUND

 

The cell membrane acts as a barrier to some substances while allowing other substances to enter or leave the cell. When a membrane allows a particular substance to pass through it, it is said to be permeable  to that substance. If a membrane allows the passage of some molecules or ions but blocks others, the membranes is selectively permeable. Cell membranes of living organism are selectively permeable. The cell membrane acts as a selective barrier between the internal  and external environments of the cell. The permeability of the cell membrane can change in response to changes in the cellís external and internal environments. Some substances, such as water, oxygen, and carbon dioxide, can pass freely through a cell membrane. Other substances have a more limited access to the cell.

 

In this activity, a non living cellophane membrane will be used as a model for a living cell membrane. Cellophane membranes are selective, but unlike living membranes, their selectivity does not vary. Water and other small molecules can pass through, but large molecules are blocked. The diffusion of molecules through the membrane is caused by random molecular movements and collisions.

 

OBJECTIVES

 

In this activity you will:

   1.      Learn simple tests for starch and sugar.

   2.      Observe the results of diffusion through a cellophane membrane.

   3.      Determine the permeability of a non living membrane for glucose, iodine, starch.

 

MATERIALS

 

Plastic Bag

Starch solution

Saturated glucose solution

Lugolís iodine solution

Clinitest tape

25mL graduated cylinder

300mL beakers

test tape

glass rod

pipette

string

 

PROCEDURES AND OBSERVATIONS

 

   1.      Fill the bag with approximately 20ml starch solution. Then add 10mL of the glucose solution. Tie the top of the bag tightly with string. Rinse the tubing with running water to remove any spillage from the outside.

 

   2.      Mix 15mL of Lugolís iodine solution into a 300mL beaker half filled with tap water. Place the cellophane dialysis tubing in the beaker and set it aside for about 20 minutes.

 


 

   3.      Add one pipette-full of glucose solution to a test tube. Rinse the pipette. Add one pipette-full of Lugol's iodine solution to the test tube, and stir. Record any color change you observe. If there is no reaction, write "NR." After rinsing the test tube and pipette thoroughly, repeat the procedure twice, testing Lugol's iodine solution with starch and tap water. Now place one drop of glucose solution on a piece of Clinitest tape, and record the result. Repeat the procedure with starch solution and tap water.

 

   4.      Observe the colors of the solutions in the cellophane dialysis tubing and in the beaker.

            a.   What color changes do you observe?                                                                                 

 

   5.      With a pipette, take several drops of solution from the bottom of the beaker. Apply the drops of a piece of Clinitest tape. Wait 30 seconds.

            b.   What do you observe?                                                                                                       

 

 

 

ANALYSIS AND INTERPRETATIONS

 

   1.      What do Lugol's iodine solution and Clinitest tape test for?

                                                                                                                                                           

   2.      What caused the color change observed in Step 4?

                                                                                                                                                           

   3.      Was there any evidence of starch in the solution in the beaker? How could you tell?

                                                                                                                                                           

   4.      Given the results in Step 5, what substance was present in the solution in the beaker?

                                                                                                                                                           

   5.      Which substances diffused through the bag? In which direction?