Every Rose

Osmosis and Diffusion Lab

Purpose: The purpose of this experiment was to show that osmosis will occur when water is on the outside of a selectively permeable membrane encasing a hypotonic solution. 

Abstract:  This experiment studies the process of osmosis in hypotonic, hypertonic, and isotonic solutions.  Sucrose solutions are used in a selectively permeable membrane to show that water will come into a cell that has a higher concentration of sucrose than water.  Therefore the rate of the osmosis and diffusion is proportional to the Molarity of the solution.   

Introduction: Osmosis and diffusion are two processes that occur constantly among cells.  When the concentration of a solution differs inside and outside of a cell the solution will diffuse through until it reaches equilibrium. Osmosis occurs in the cell’s permeable membrane.  In this lab, dialysis tubing is used to represent the selectively permeable membrane in a cell. Dialysis tubing is made from cellulose or cellophane to allow water or smaller molecules diffuse through it.   In selectively permeable membranes, solutions travel across their concentration gradients by osmosis. During osmosis, there are three diverse types of solutions that move across the gradient in different ways.  In a hypertonic solution there is a higher concentration of the solute than in the other solution.  Therefore, when the cell is put into a hypertonic solution, the water diffuses out of the cell which causes the cell to shrivel.  In a hypotonic solution, the solution has a low concentration of the solute, therefore when a cell is added to this solution water diffuses into the cell until it eventually lyses because it is over filled with water.   When a solution is said to isotonic it means that there is the same concentration between it and the cell.  Here water flows through without changing the mass of the cell.   It is vital that osmosis can occur in these situations because without it cells would die because of differing pressure from outside and inside.             

The measurement that predicts the movement of the water is ψ, or psi.  This is the measure of the water potential inside an individual cell. Ψ can be either positive or negative depending on the solution the cell is in.  Ψ is more negative when the solution is hypertonic, so the water is moving out of the cell more quickly than it is coming in.  When Ψ is more positive the solution is hypotonic and therefore the water is moving into the cell eventually causing it to explode from expansion.             

This experiment is proving that in a selectively permeable membrane enclosing a sucrose solution, water will diffuse into it.  This is because there is a low concentration of water inside the solution, so what moves in.  Proving that inside the dialysis bag is a hypotonic solution.   

Materials and Methods: 

Procedure 1A:

Take a 30-cm piece of dialysis tubing that has been soaking in water.  Tie off one end to form the bag.  Test the 15% glucose/1% starch solution for glucose.  Record the results.  Put 15 mL of the 15% glucose/1%starch solution in the bag and tie off the other end.  Record color.  Fill a 250 mL beaker with distilled water.  Add 4 mL of Lugol’s solution to the distilled water and record color.  Test the solution for glucose and record the results.  Next put the bag in the beaker with the solution.  Let this stand for 30 minutes or color change.  Record final colors in both bag and solution.  Test the beaker and the bag for the presents of glucose and record in data table. 

Procedure 1B:           

To start the experiment take 30-cm strips of precut dialysis tubing and make them into bags by tying knots in the end, and pour about 15-25 mL; though the amount of the solution won’t affect the outcome, of the following solutions into separate bags: a.) tap water, b.) 0.2 M sucrose, c.) 0.4 M sucrose, d.) 0.6 M sucrose, e.) 0.8 M sucrose, f.) 1.0 M sucrose. Then remove all the air from the bag and tie another knot at the end, but leave room for expansion of the contents.   Make sure to rinse each bag of possible excess sucrose.  Record the mass of each bag expressed in grams.  Fill 250- mL beakers with tap water and put one dialysis bag in each one.

Let them stand for 30 minutes before drying them and taking a second measurement. Take data and record it in a chart that includes the initial mass, final mass, mass difference, and Percent change of each solution.  Also record the percent changes from the rest of the experiments. 

Procedure 1C:

Pour 100 mL of a solution into a 250 mL beaker and cut 3 cm. thick slices of potatoes.  Use a cork borer to make four potato cylinders, not including potato skins.  Use four potatoes per beaker.  Keep potatoes covered until they are weighed to keep air from touching them.  Record the mass of potatoes.  Put the potatoes into the sucrose solution.  Cover the solution and let sit overnight.             

Following day, remove potatoes from the solution, blot excess solution, and weigh.  Record the final mass and the class data in table.  Graph all results. 

Results: The following results were recorded both in class and later on in a computer program. 

1A. Table 1.1

		Solution color		Presence of glucose
	Initial Contents	Initial	Final	initial	final
Bag	15% glucose and 1%starch	clear	blue	present	present
Beaker	H2O and IKI	amber, clear	amber, clear	not	present

1B. Table 1.2

Contents in Dialysis Bag	Initial Mass	Final Mass	Mass Difference	Percent Change in Mass
a. 0.0 M distilled Water	6.1 g	6.0 g	-0.1 g	-1.60%
b. 0.2 M Sucrose	9.8 g	10.0 g	0.2 g	2.00%
c. 0.4 M Sucrose	8.8 g	9.3 g	0.5 g	5.00%
d. 0.6 M Sucrose	9.5 g	10.5 g	1.0g	10.50%
e. 0.8 M Sucrose	10.4 g	11.6 g	1.2 g	11.50%
f. 1.0 M Sucrose	6.0 g	6.3 g	0.3 g	5%

Percent Change Group 2

       The previous results show the results of a single experiment.  It shows the differences in mass between the initial and final masses of the dialysis bags and the percent of the change representative of these differences. 

1B. Table 1.3

Contents in bag	Group 1	Group 2	Group 3	Class Average
a. 0.0 M distilled Water	0.90%	-1.60%	1.75%	0.35%
b. 0.2 M Sucrose	5.83%	2.00%	3.50%	3.78%
c. 0.4 M Sucrose	6.78%	5.00%	8.60%	6.79%
d. 0.6 M Sucrose	9.32%	10.50%	11.10%	7.97%
e. 0.8 M Sucrose	14.55%	11.50%	12.20%	12.75%
f. 1.0 M Sucrose	22.86%	5%	23.60%	17.53%

       The previous results show three experiment’s percent changes. The navy line represents group 1’s percent change, group two’s in pink, group three in yellow, and aqua represents the class average.   

1C. Table 1.5

	Potato Initial Mass	Potato Final Mass	Class Average Change in Percent
a. 0.0 M distilled Water	1.50 g	2.00 g	0.50%
b. 0.2 M Sucrose	1.80 g	1.90 g	0.10%
c. 0.4 M Sucrose	1.80 g	1.70 g	-0.10%
d. 0.6 M Sucrose	1.80 g	1.40 g	-0.40%
e. 0.8 M Sucrose	1.50 g	1.10 g	-0.40%
f. 1.0 M Sucrose	2.30 g	1.50 g	-0.80%

       The previous results show the average percent change in potato mass.   

Discussion:  In Procedure 1A the hypothesis was proved correct that a dialysis bag’s selectively permeable membrane allowed glucose to move through it, but not iodine.             

In procedure 1B the results also proved the hypothesis correct.  The solutions in the dialysis bags were more concentrated of sucrose than of water, therefore the water from the beaker traveled into the bag through osmosis.  In this group 2’s experiment, the trend that was present in the rest of the group’s data did not match up.  The percent change for the 1.0 M sucrose Solution there was not as drastic increase in mass.  The error here could have been caused by time limitation.  The 1.0 M solution was the last bag to be put into the water filled beaker, and also the first to be taken out and weighed.  Therefore this solution did not have as much time as the others to perform osmosis through the membrane.  It was also odd that the results show that water diffused into the membrane, when there was the same concentration inside and outside the cell.  Normally it would be that the water wouldn’t move because it is an isotonic solution in that case.  The error here could have been caused by a number of things, maybe a trace substance in either tap water because the water in the beaker was taken from a different sink than the dialysis bag.  In the potato experiment, the hypothesis was that water will diffuse out of the potato and into the surrounding solution because the potato’s concentration is higher.  This was proved in the results because the final mass of the potatoes had decreased because of water loss.  In the potato lab charts it is shown because the percent change decreases in the class average. 

Conclusion:  The experiment showed that as a solution’s molarity increases, the rate of osmosis increases proportionally.  

References: 

Cambell, Reece.  2005.  Biology, 7^th Edition.  San Francisco, CA: Pearson Benjamin Cummings. 

College Board.  2001. Biology Lab Manual.  College Board Entrance Examination.