Osmosis and Diffusion Lab

Osmosis and Diffusion Lab

Purpose | Hypothesis | Predictions | Materials | Procedure | Observations  Conclusion | Error Analysis

Purpose:

To simulate and then observe the diffusion of solutes and the osmosis of water through a semipermeable membrane so that we can observe these two phenomenons in a different circumstance (other than the egg lab).

Hypothesis: 

This lab will show diffusion because the large carbohydrates will not be able to pass through the semipermeable membrane simulated by the tubing, but the small molecules of water and iodine will be able to.

Predictions:  

The dialysis tubing is acting as a semipermeable membrane, so we believe that the iodine will diffuse through the tubing and reach an equilibrium on the outside and inside of the tubing.  This will be clear by the pockets of starch that turn black on the inside of the tubing.  Because the sugar and starch have larger molecules, they will be too large to fit through the tubing and will therefore remain only on the inside of the tubing.  Since the sugar and starch will only be on the inside of the tubing, this will leave a higher solute concentration on the inside of the tubing, and water will osmosize from outside to inside the tubing, from the hypotonic to the hypertonic solution. 

Materials: 

 

  • Hot plate
  • Two 1000 mL beakers
  • One approx. 500 mL beaker
  • Two 250 mL beakers
  • 100 mL Graduated Cylinder
  • 10 mL Graduated cylinder
  • Filter paper (Coffee Filter)
  • 150 g Table Sugar
  • 10 g Corn Starch
  • Dialysis tubing
  • String
  • Magnetic stir bar
  • Distilled water
  • Iodine
  • Benedict's Reagent
  • Scissors
  • Balance scale

Procedure:

  • Measure out 10 g of starch on balance scale and add to 990 mL of water to create a 1% solution.
  • Measure out 150 g of sugar (sucrose) to 850 mL of water to create a 15% solution.
  • Place both solutions on hot plates add a magnetic stirring rod to stir fully for 8-10 minutes.
  • Cut 12 cm of dialysis tubing, fold and tie at bottom.
  • Add 15 mL of each solution to dialysis tubing, and fold and tie top.
  • Weigh and observe dialysis tubing.
  • Add water to a beaker until dialysis tubing is completely submerged, for us, 240 mL.
  • Add 12 drops of iodine to beaker.
  • Cover beaker with plastic bag and tie off top.
  • Observe and weigh tubing after 15 minutes.
  • Observe and weigh tubing after one day.
  • Cut tubing and take pH level.

 

Observations: 

As the magnetic stir rod stirred the two solutions, the sucrose solution stayed a transparent color while the starch became an opaque white.  The sucrose had become completely dissolved while the starch had formed a precipitate, which eventually settled at the bottom.  When combined in the dialysis tubing, the new solution was still a murky white, but seemed to be of a slightly lighter color than the starch alone.  The dialysis tubing almost immediately became damp, even to the touch on the outside.  The dialysis tubing, when tied, was almost full with solution, and afraid that a lot of water would osmosize into the tubing and cause it to burst, we cut new tubing that was 18 cm long instead.  We restirred the solutions and added them to the tubing again, however we mixed them this time in the graduated cylinder rather than inside the tubing.  This time, there was at least 2 cm at the top of the tubing above the fluid level.  Again, the formerly crinkly dialysis tubing became damp to the touch.  The starch seemed to have settled at the bottom of the tubing.  We weighed the tubing, which weighed 29.3 grams, and placed in a smaller beaker.  We filled the beaker with 240 mL of distilled water, almost completely submerging the dialysis tubing and completely submerging the part of the tubing that contained the sugar/starch solution.  We added 12 drops of iodine to the solution, and the water turned a light brown color.  Other groups had much darker water, and we have no explanation as to the apparent fact that there seemed to be less iodine in our container than everybody else's did.  We covered the beaker with a plastic bag and tied a string around the bag, but we were not able to tie the string around tight enough to keep all air out.  We also took the pH of the solution that we put inside the dialysis tubing and the distilled water tinted with iodine inside the beaker.  Both had a pH level of 6, but the scale was from 0-13 rather than the usual 1-14, meaning that the levels were neutral.

After 15 minutes:   There was a little bit of a purple-black concentration at the top where starch had clearly settled and reacted with the iodine, which had already begun to diffuse into the tubing. The tubing had already turned a slightly brownish color like the outside water, also illustrating the fact that iodine had diffused inside.  There was a solid purple concentration with granules of starch at the bottom of the tubing, much thicker than the one at the top.  The beaker itself and its contents had not apparently changed.  A little water sloshed out of the beaker when we returned it to its resting spot, away from any sun or room light.  Other groups with more apparent iodine in their tubing had a greater concentration and darker color of starch reacting with iodine inside their tubing, but had other similar results.

Day 2:   Inside the tubing, the bottom was thick and black, from an even greater amount of iodine that had diffused in overnight reacting with starch that had almost totally settled at the bottom.  Small concentrations of starch, leucoplasts, that were stuck to the sides of the tubing had become a lighter purple.  The color of the two solutions showed that iodine had reached an equilibrium on the inside and outside of the tubing.  When we shook the dialysis tubing, the solution became a consistent dark purple.  A lack of black concentrations of iodine reacting with starch outside the tubing meant that no starch had diffused to the outside.  Possibly the most surprising result we received was that very little water actually osmosized into the tubing, where there was clearly a higher solute concentration.  There was no visible difference in the fluid level and the tubing now weighed 29.5 g.  However, the iodine was decidedly more pronounced inside the tubing, but the water inside the beaker was a lighter colorWe broke the tubing and took the pH reading on both the inside and outside of the tubing.  Both were still 6, neutral on the 0-13 scale.  We tested for the sugar by adding solution from both the fluid in the beaker and fluid from the dialysis tubing to test tubes containing Benedicts' reagent.  As a control group, we did the same with distilled water.  We heated for five minutes by putting all three tubes in a beaker containing water on a hotplate as instructed, and found no change in color that would show the presence of sugar.  We added hydrochloric acid to each solution to break the disaccharide's, sucrose's, bonds, and therefore produce two monosaccharides, each with free aldehyde or ketone group that could react with the Benedict's reagent, but still saw very little change.  After two minutes, and still after 45 minutes, the solution from the dialysis tubing had turned slightly green, identifying a small amount of sugar, and the other two solutions had not changed colors, showing that they contained no sugar.  This meant that the sugar had not diffused through the tubing either, like the starch.

 

Day

Weight of Dialysis Tubing (g)

Original

29.3

After 1 day

29.5

The weight of the dialysis tubing grew in very little, only .6%, between the two days.  As explained below, this was a surprising result.

 

Conclusions:  

Again, our results supported our predictions in this experiment.  The iodine diffused through the tubing and reached an equilibrium, but the molecules of sugar and starch were too large to diffuse through the tubing.  The tubing was supposed to create a plasma membrane, with the internal cell being the sugar/starch solution and the water with iodine being the extracellular matrix.  However, this was slightly unrealistic.  Unlike the egg lab, the tubing was nonliving, so it only discriminated passage according to size, not to polarity of the molecules.  Also, it had no proteins to allow passage of other molecules that could not normally pass through the membrane via diffusion.  In a plasma membrane, proteins facilitate diffusion of polar molecules that can not pass through the phospholipid bilayer themselves, and proteins embedded in the membrane are used for active transport, the moving of materials against their concentration gradient, also.  Also not available to the dialysis tubing are the processes of  endocytosis and exocytosis, the way that a plasma membrane allows passage of large molecules.  In exocytosis, macromolecules are enclosed in a vesicle that fuses with the membrane and spills its contents into the outside of the cell.  In endocytosis, the cell does the opposite; it takes in large macromolecules and transports them into the cell inside vesicles formed from the plasma membrane.

 However, our dialysis tubing had none of these options.  Its selective permeability was based solely on size.  Iodine, which is a fluid and has very small molecules, was able to diffuse through the dialysis tubing and reach an equilibrium on the inside and outside of the tubing.  However, sucrose and starch are macromolecules, so they were too large to diffuse through the tubing and enter the "extracellular matrix."  Sucrose is a disaccharide made of a glucose molecule linked to a fructose molecule.  Starch, a polysaccharide, is made of glucose molecule formed in rings.  Compared to H2O, with three atoms, and O2, with two atoms, sucrose, C12H22O11 is very large, and starch is much larger still.  We were able to tell that no sugar and starch diffused through the dialysis tubing because there was no interaction between the iodine and starch outside of the tubing, and the Benedicts test of the outside area came up negative, while the test of the tubing fluid came up slightly positive.  All of the starch with iodine settled at the bottom or on the side of the dialysis tubing.  This was probably because the iodine made the starch nonpolar, so it was repelled by the water and settled at the side.  The Benedicts test did not originally work because sucrose is made of two monosaccharides with their oxygen atoms of the ketone or aldehyde group bonded to the next molecule through dehydration synthesis.  It is these oxygen atoms, double bonded to the carbon chain in monosaccharides, that the Benedict's reagent reacts with to change colors.  The hydrochloric acid broke the disaccharide into monosaccharides and allowed the double bonded oxygen to react.  We do not know why the test still showed only a trace of sugars while the solution was 7.5% sucrose.  However, we know that this experiment showed the selective permeability based on size of a typical cell membrane.

 Our most surprising result was that very little water osmosized into the dialysis tubing.  When making our predictions, we assumed that a lot of water would move into the tubing because the concentration of solutes was greater.  Since there were no solutes in the distilled water, and the water from the inside contained 8% polysaccharides, we believed that water would bind to the solutes because of its polarity.  Because there would then be less free water molecules inside the tubing, molecules from the outside would diffuse into the tubing until they had totally accounted for the difference in solutes.  We predicted that the inside solution would be hypertonic, and the outside solution hypotonic.  However, the fact that the liquid inside the tubing grew very little in size and weight overnight seems to suggest that we were incorrect in our prediction.  This mistake in our guesses may have been because the iodine actually had a high solute concentration, but this is unlikely because it passed through the tubing so easily. However, the tubing did not account for polarity.  The iodine may have been more polar than the carbohydrates in the tubing solution, and therefore would have caused more water molecules to be attracted to it.  This would have stopped osmosis for some time, until the iodine had reached its equilibrium.  If this did not happen until late in the night, it might have accounted for the small amount of water that osmosized in. 

This is a small posibility because the amount of iodine we used was so small, less than one percent.  When repeating this experiment, it would be helpful to know at what time the iodine did reach its equilibrium and what was the weight of the dialysis tubing at that time.  We can safely say that the two solutions were approximately isotonic to each other, causing so little water to osmosize in either direction.  There are actually two possible reasons that so little water moved through the membrane.  One is that, as stated above, the internal solution had only an 8% solute concentration, so the solutions were approximately isotonic.  However, the amount of water that moved inside would have probably accounted for more than a .2 g growth.  The dialysis tubing, which became quite porous overnight, quite possibly could have had liquid fall out of it in the short time taken to put it on the scale.  We saw water dripping out, but we originally thought that this was from the outside, and not the inside.  This mistake may have caused the only small amount of weight gain.

 When conducting this experiment, we were able to observe what we had been studying in the book.  We were able to make more accurate predictions and conclusions in this lab than in our previous lab involving osmosis, the egg lab.  We were correct in our predictions, as we knew that we would be.  This lab taught us about the processes of diffusion and osmosis, but it also taught us experimental techniques.  We learned to carefully measure and mix solutions, and to correctly perform tests for sugars and carbohydrates.  As with the other two labs in this group, using our knowledge of the book we were able to carefully observe processes that we knew took place.

 

Error Analysis: 

Again, we made very few errors in an experiment where our results seemed to support our predictions.  We realize that we made at least one error in handling the beaker, in which a little water spilled out, and it is possible that we may have made others.  Also, the fact that our Benedict's test showed up only slightly positive may mean that we made an error in this test also.  The color of the Benedict's solution was a bluish-green rather than the bright green or bright red that usually indicates the presence of sugar.  Also, the fact that our tubing was not completely submerged may have influenced the amount of water that was able to enter through osmosis.  Our measuring of this water, as stated above, could have also been faulty due to the fact that some water dripped out of the tubing while we handled it and moved it to the balance.  We believe that we tied the tubing well so that fluids could only pass through the sides, but this could have been a mistake as well.  However, this was also a simple experiment in which we seemed to make very few mistakes because our predictions were for the most part supported.

 

 

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