Name: ____________________________________ Period: _______  Date: ______

 

General Biology

Diffusion Across the Cell Membrane

 

Background:

Diffusion is the movement of substances from areas of high concentration to areas of low concentration.  This is one method for transporting substances across the cell membrane.  The ability of substances to cross the cell membrane by simple diffusion is primarily determined by their size and polarity.  In this lab we will be focusing on size.  Glucose is a monosaccharide which the monomer units for the polysaccharide starch.  The cell membrane will be represented by dialysis tubing.  This mimics the cell membrane because it is selectively permeable.  The substances you will be testing are glucose and starch.  You will be using two indicators to check for the presence of starch or glucose.  Iodine is normally a yellow/brown color.  This indicator turns purple/blue in the presence of starch.  Benedicts solution is an indicator that will change from blue to green/yellow/orange in the presence of a simple sugar such as glucose. 

 

Purpose:  The purpose of this lab is to investigate the effect of a molecule’s size on its ability to diffuse across the cell membrane.

 

Hypothesis: Write a hypothesis in the form of an If…then statement about the ability of starch and glucose to diffuse across the dialysis tubing.

 

 

 

 

 

 

Materials

2 pieces of dialysis tubing                                           4 pieces of dental floss

Glucose solution                                                         Starch solution

Iodine                                                                          Benedicts solution

Two beakers                                                                Hot plate with hot water bath

Pipette                                                                         Test tube

 

Directions: Side A of lab station

  1. Obtain one piece of cellulose tubing.  Tie a knot one cm from one end.  The knot must be very tight.
  2. Open the other end of the tubing by rubbing it between your thumb and forefinger.  If it has already dried out, dip back in some water and try again.
  3. Pour starch solution into the bag.  Leave enough room to tie off the open end. 
  4. Knot off the open end.  Again, tie very tightly.
  5. Rinse the bag off with running water to remove any excess starch from the outside of the bag. 
  6. Fill a beaker 1/4 full with water.  Add enough iodine to turn the water yellow.
  7. Place the cellulose tubing in the beaker.  Be sure that the ends of the tubing are up toward the ceiling and not in the water; just the center of the tubing should be immersed in the water.  Wait 30 minutes.
  8. After 30 minutes, observe any changes in color you see in the cellulose tubing and beaker water.  Record these in Data Table A.
  9. Record whether starch is present or absent in Data table B.  (See background info for explanation.)
  10. Throw the dialysis tubing in the garbage.  Empty and rinse out your beaker.  Dry your beaker with a paper towel and throw the paper towel in the trash. 

 

Directions: Side B of lab station

  1. Obtain one piece of cellulose tubing.  Tie a knot one cm from one end.  The knot must be very tight.
  2. Open the other end of the tubing by rubbing it between your thumb and forefinger.  If it has already dried out, dip back in some water and try again.
  3. Pour glucose solution into the bag.  Leave enough room to tie off the open end. 
  4. Knot off the open end.  Again, tie very tightly.
  5. Rinse the bag off with running water to remove any excess glucose from the outside of the bag. 
  6. Fill a beaker ¼ full with water.  Place the cellulose tubing in the beaker.  Be sure that the ends of the tubing are up toward the ceiling and not in the water; just the center of the tubing should be immersed in the water.  Wait 30 minutes. Before returning to your seat, take the beaker with distilled water ONLY and put it on the hot plate.  Turn on the hot plate to #5.  Have a seat.
  7. After 30 minutes, obtain ten drops of the solution in beaker B using a plastic pipette and place in a test tube.  Place ten drops of benedicts solution inside the same test tube. 
  8. Place the test tube in the hot water bath for 5 minutes.
  9. Record whether glucose is present or absent in the beaker water in Data table B.  (See background info for explanation.)
  10. Throw the dialysis tubing in the garbage.  Empty and rinse out beaker B and the test tube.  Dry beaker B.  Leave the water in the beaker used for the hot water bath. Turn off the hot plate and unplug it.

 

Data Table A: Diffusion test for the presence of starch

 

Color of Solution

Presence of starch (positive or negative)

 Inside the cellulose tubing.

black/purple

 

positive

Inside the beaker.

yellow/brown

 

negative

 

Data Table B: Diffusion test for the presence of glucose

 

Color of Benedicts Test

Presence of Glucose (positive or negative)

Liquid inside the beaker

orange

 

positive

Questions:

 

  1. The cellulose tubing represented which part of an actual cell?

 

 

  1. What diffused across the cellulose tubing in beaker A, the starch or the water with iodine? 

 

 

  1. Explain your answer to question #2.  How do you know which substance diffused across the cell membrane? 

 

 

 

  1. Did the glucose in beaker B diffuse out of the cellulose tubing and into the beaker water?  (Yes or No) 

 

  1. Support your answer to question #4 with evidence from your data tables.

 

 

 

  1. What does selective permeability mean? 

 

 

 

  1. Is cellulose tubing a selectively permeable membrane? 

 

 

  1. Which type of molecule is larger (a polymer), glucose or starch?

 

  1. How could you tell from the experiment which one is larger?  Hint: use your data table to help answer this question.

 

 

 

 

  1. Write a one sentence explanation about the relationship between the size of a molecule and its ability to diffuse across the cell membrane.

 

 

 

 

 

 

  1. Why is it important for a cell to be selectively permeable?