Osmosis and Diffusion Lab

The purpose of this lab was to observe the rate of osmosis and diffusion, as well as the effect of molecular size of the particles on this rate. Part I of the lab was a demonstration of osmosis and diffusion, that dealt with raisins in different liquid environments, each with a different concentration of sugar. Part IV of the lab was using the same idea as the demonstration, by putting objects in different concentrations of a substance; in this case elodea leaves in salt water. In both cases, the objects in a greater concentration of the substance were stripped of their water. However, where there was a little or no concentration of sugar or salt, the objects did not lose their water, and in the case of the raisin, became saturated with excess water.

Part II of the lab dealt with the rate of osmosis, and how molecular size was a factor on it. The purpose was to see how easily molecules of smaller diameter, which were starch molecules, would pass through an artificial membrane, as opposed to how difficult it would be to pass through in the case of larger molecules, which were IKI molecules. Results were that the smaller substance passed though the membrane easily and rather quickly, and the larger substance did not penetrate the membrane whatsoever. Part III of the lab was to view the effect of molecular size on the rate of diffusion. Two substances, both being of different molecular size were placed in the same environment. Results were that the substance with the smaller molecules, diffused farther away from where it was placed than did the substance of greater molecular size in the same amount of time.

The title of this lab is ‘Osmosis and Diffusion.’ Both these terms deal with the transfer of a substance from one place to another, depending on where there is more space. Diffusion is the process by which molecules of a substance move from areas of higher concentration of that substance to areas of lower concentration. Diffusion can be the transfer of anything anywhere. However, that is not true for osmosis. Osmosis is diffusion, but a specific type of diffusion. Osmosis is only the diffusion of water molecules through a selectively permeable membrane. The comparison of the concentration of a substance on one side of the membrane as opposed to the other is called the concentration gradient. If an object has a lower concentration of a substance than does it’s surroundings, then the object is called hypotonic.

If it has the same concentration of a substance as it’s environment, then it is isotonic. If the object has a higher concentration of a substance than does it’s environment, then it is hypertonic. Does the difference in the concentration gradient effect the rate of diffusion or osmosis? Are osmosis or diffusion also affected by certain aspects; such as molecular size or weight? The answers to these questions were tested and answered during the different phases of this four part lab.

Part I of the lab was the demonstration. Three raisins of roughly the same size were taken from a box of raisins, and each were placed in a different environment. The control was the one of the raisins in an empty container. One of the raisins was placed in a container filled with distilled water (0% sugar concentration), and the other raisin was placed in a container filled with karo syrup, which is a super saturated sugar solution. The independent variables here are the water and the karo syrup, while the independent variable is the rate of change in the size of the raisin due to the direction the water will move; either into or out of the raisin itself. If there is a movement in water at all, then osmosis will have taken place, since osmosis is the transfer of water across a membrane; in which this case it is the raisin’s outer skin. The hypothesis was that if there is a greater concentration of sugar outside of the raisin, osmotic pressure will cause the raisin to become dehydrated, and if there is less sugar outside the raisin, the raisin will take in water. The raisins were left to sit for two days and later were observed.

Part II of the lab involved using different substances on opposite sides of a membrane, and observing which way the water would diffuse. The substances used were IKI and Starch. The procedure went as follows. Two beakers, one labeled ‘IKI’, the other ‘Starch,’ were filled accordingly with the same amount, 40 ml, of each liquid. Then, two small pipettes were also labeled ‘IKI’ and ‘Starch,’ and were filled about halfway accordingly. Then, a small piece of dialysis membrane was secured with a rubber band to the open side of each pipette, and each test tube was placed in the beakers, dialysis membrane down. The IKI test tube was placed in the Starch beaker and the Starch test tube was placed in the IKI beaker. Here, the independent variables were the different substances themselves, and the dependent variables were any change in color or height of the substances in the test tubes and beakers. The hypothesis was that if there was a difference in molecular size between the starch and IKI, then the substance with the smaller molecular size would move across the dialysis tubing in both of the setups. The setups were left to sit for 30 minutes and later observed.

Part III of this lab had to do with the effect of different substances of different molecular weight on the rate of diffusion throughout a gel like surrounding. The two different substances used were methylene blue, who’s molecular weight is 357, and potassium permangenate, who’s molecular weight is 158. These two substances were placed in a petri dish filled with solid agar. The procedure began with the removal of a small round piece of agar from each of the two petri dishes with the use of a straw. Then, the holes were filled with one drop of each the methylene blue and potassium permangenate. The independent variables were the different substances, and the dependent variables were the rate and distance of which the substances diffused into the surrounding agar. The hypothesis was that the potassium permangenate would diffuse further than the methylene blue because of it’s smaller molecular size. Both of the plates were left to sit over night and were later observed.

The final part of this lab was observing osmosis under real life conditions with actual cells. Five different Elodea leaves were placed on microscope slides, and added to each one was a different concentration of salt water. The weakest being 0%, and the highest being 5%. Each leaf had a 1% higher concentration of salt water added than did the previous one. The independent variables here were the different concentrations of salt water, and the dependent variables were the size of the cell membrane, affected by any osmosis that may have taken place. The hypothesis was that as the concentration of the salt solution increased, the size of the cell membrane would decrease because of osmotic pressure pulling the water outside of the cell to an area with less water concentration. After the salt water had been added to the leaves, they were immediately viewed under the microscope, and the results were recorded.

1. When the raisin was placed in distilled water, the outer covering became stretched, as the raisin’s diameter increased.

2. The Karo syrup did not cause a dramatic change in the raisin’s outer covering because the concentration gradient between the syrup and the raisin was small, thus, not much osmosis took place.

3. I think that a raisin contains fructose, because fructose is the type of sugar found in fruits.

4. Compared to the sugar concentration in the raisin, the karo syrup contains a greater concentration of sugar.

Part II Data Questions

1. It grew to twise it’s size.

2. Starch is a large molecule compared to other biomolecules, because it is a complicated polymer. Starch cannot cross the dialysis tubing due to its large size.

3. Both pipettes showed a change in height of their liquids. The pipette filled with starch gained height while the pipette with IKI lost height The reason why this could be is that the smaller molecules in IKI travel through the membrane, while the starch molecules are unable. Also, osmotic pressure will cause the IKI to cross the membrane where there is less water due to the density of starch.

Part III Data Questions

1. The agar plate had two spots that had spread out from the tow original holes. The methylene blue diffused 10 mm into the agar over night, while the potassium permangenate diffused only 4 mm into the agar over night.

2. Based on this activity, we can say that as the molecular weight of a substance decreases, the rate at which it diffuses increases.

3. Rate of diffusion can have great effects on living organisms. For example, a small virus may spread quicker throughout a humans body while a larger virus may spread slower. Smaller viruses my then be more deadly and leave a person with less time to help cure it.

Part IV Data Questions

1. As the concentration of the salt in the external environment increased, the internal structures of the elodea cells became crushed and drained of their water; the cell membrane being the most obvious.

2. Concentration gradient is the comparison of the concentration of a substance on one side of a membrane, as opposed to the concentration on the other side.

3. The cell never was hypotonic in relation to its environment, because the cell membrane never burst through the cell wall.

The cell was hypertonic in relation to its environment when the water being added had a 2% or more salt concentration. The cell membrane then began to start falling in on itself.

4. We observed osmosis in part IV because the elodea cell was losing water through its membrane.

This lab was able to demonstrate both diffusion and osmosis, and some aspects that may have an effect on either; like molecular size or weight. In part I, the raisin placed in the karo syrup shrank just barely, while the raisin in the distilled water grew to almost twice its original size. A transfer of water was the reason for these changes in both cases. This proves the hypothesis correct, and demonstrates osmosis. Since it is known osmotic pressure will cause water to cross a membrane to where there is less water, it can be said that karo syrup has more fructose than does a raisin, since water was sucked from the raisin and transported to the karo syrup. But, the raisin was the one doing the sucking when it was in the water, since raisins are dehydrated to begin with. There was more of an increase with the raisin in the water than there was a decrease with the raisin in the syrup, since the concentration gradient was so much greater for the raisin in the water.

That shows how there will be a greater change when the concentration gradient is great. It can be said that the raisin in the water is hypotonic compared to the water since it has less water, and the raisin is hypertonic compared to the syrup since it has more water. In part II, the IKI in the pipette dropped 1.5 ml, and the starch in the pipette rose 1 ml. Both the starch in the pipette and the beaker became pink, because osmotic pressure brought the IKI through the membrane and into the starch. In part III the potassium permangenate diffused 10 mm into the agar, while the methylene blue diffused 4 mm. This proved the hypothesis because the potassium diffused further, and it had less molecular weight than the methylene blue. A statistical analysis was done with the null hypothesis being that molecular weight does not affect the rate of diffusion.

The null hypothesis for all t tests were that different molecular weights (357, 158) would not effect the rate at which the substances diffused. The confidence level of 0.05 was chosen. The null hypothesis was rejected when comparing the diffusion rate of 357 to 158 (t-test

Finally, part VI of the lab showed the elodea cell’s reaction with salt water. As the salt concentration increased, the size of the cell membrane decreased. Osmotic pressure caused the water within the cell to be sucked out into its environment since there was salt in place of water. At 0% salt concentration, the cell was isotonic to its surroundings. It was still (appeared to be) isotonic at 1%. However, beyond that, the cell became hypotonic with its environment.

Most of this lab was done with ease, and the hypothesis were supported by the results. However, in part two, to much starch and IKI were put into the pipettes. Thus, gravity overcame the osmotic pressure, and ruined the experiment. So, instead of using these results, results were borrowed from a same setup, that worked out correctly. This lab was pretty thorough, but still, more substances could have been experimented with, and would have given a better understanding of how osmosis and diffusion work.