Diffusion and Osmosis of Solutes and Water Across a Membrane

Abstract

This experiment gave a visual understanding of osmosis and diffusion. The first experiment proved that solutes would move down a concentration gradient if permeable to the selective membrane. The second experiment proved different solute concentrations affect the movement of water, depending on the solute concentration inside the cell. The purpose of this lab was to look for different solutes that can cross an artificial membrane and to observe the effect of different concentrations of sucrose on the mass of a potato cell.

Results for Part One suggested that the molecular weight of albumin and starch was too large to pass through the dialysis tube, but glucose and sodium sulfate molecules were small enough to pass through the dialysis tube. Also, a decrease in water weight occurred due the dialysis tube being placed in a hypertonic solution. Results for Par Two showed the potato cell having a molar concentration of 0. 2734, which caused sucrose concentrations above 0. 2 M to have a decrease in mass. Inversely, sucrose concentrations below 0. 2 M caused an increase in mass.

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Diffusion is the random movement of molecules spreading evenly into available space (Cain, Jackson, Minorsky, Reece, & Urry, 2011). Movement of water also follows a similar concept, however, water can act as a shield for solutes and become unavailable to diffuse while in other cases water is free and will move to an area of low solute concentration to an area of high solute concentration: this processes is better known as osmosis (Keith, Messing, Schmitt, & Feingold, 2010). Osmosis and diffusion can occur along a permeable membrane or selective membrane.

A cell with a selective membrane allows small molecules and ions to pass through but excludes others; also, substances that are able to pass through the membrane do so at different rates. On the other hand, permeable membranes allow nonpolar molecules, such as hydrophobic molecules (water fearing), to dissolve in the lipid bilayer, which allows the molecule to easily cross the membrane. However, molecules such as glucose can pass through the lipid bilayer, but not as rapidly as nonpolar molecule (Cain et al. , 2011).

Understanding the concept of osmosis helps explain why lakes cannot have an increase in salinity. If saltiness of a lake increases, species living in the lake could die. This occurs when the lake water becomes hypertonic solution, which causes the animal cells to lose an excessive amount of water forcing the cell to shrivel up and die (Cain et al. , 2011). On the contrary, understanding the concept of diffusion can help explain why after spraying perfume in one area of the room, then after several minutes, the perfume is smelled throughout the room.

This is because particles of the perfume move randomly and eventually spread out evenly throughout the room. Moreover, in the experiment performed, diffusion and osmosis was observed using artificial systems (plastic membranes) and potato cells. The null hypothesis for Part One of the experiment is that the concentration gradient has no effect on the weight of the dialysis tube. The alternate hypothesis is that the weight of the dialysis tube will be affected by the concentration gradient.

The null hypothesis for Part Two of the experiment is that the increase of sucrose concentration has no effect on the mass of the potato cell. The alternate hypothesis is that the difference in sucrose concentration will affect the mass of the potato cell. This experiment tests all hypotheses and helps to explain the concepts of diffusion and osmosis.

Materials and Methods

Part One:

Gloves were used to obtain a 20 cm section of dialysis tube that had soaked in a beaker of distilled water prior to the experiment. The dialysis tube was cleaned with distilled water and then tied off to form a pouch.

Once the pouch was formed, 3 mL of starch and sodium sulfate solution was placed inside the tube, and then tied off and weighed. The weight obtained was recorded as initial weight. While weighing the dialysis tube with the solution of starch and sodium sulfate, eight test tubes were obtained and solution of starch/sodium sulfate was added to two test tubes labeled bag start (Keith et al. , 2010). After weighing dialysis tubing of starch/sodium sulfate and adding the solution to two test tubes, the tubing was placed in a beaker containing a solution of albumin and glucose.

Next, 1. 0 mL of albumin and glucose were then placed in two test tubes labeled solution start. The tubing in the albumin/glucose solution was kept inside the solution for 75 minutes. Every 15 minutes the solution and tube was mixed (Keith et al. , 2010). At the end of the 75 minutes, two 1. 0 mL samples of the albumin/glucose solution from the beaker were added to two test tubes labeled solution end. Then, the dialysis tube was removed from the beaker and rinsed off with distilled water. Once the tubing was rinsed and blotted dry the final water weight was recorded.

After measuring the final water weight, the contents in the tubing was dumped into a beaker and 1. 0 mL of starch/sodium sulfate solution was added to two test tubes labeled bag end (Keith et al. , 2010). In order to test for glucose, a glucose dip and read strip was placed in the first set of test tubes that were labeled bag start, solution start, bag end, and solution end. Then, a protein dip-and-read strip was placed in the same set of test tubes and the results were recorded from both glucose and protein strips.

After testing for protein, solution and bag samples were tested for sodium sulfate. To test for sodium sulfate, three drops of 2% barium chloride were added to the second set of test tubes labeled bag start, solution start, bag end, and solution end. The results were observed and then recorded (Keith et al. , 2010). To see if starch was present in the solutions, iodine solution was added to the first set of test tubes that were used for the glucose and protein strips. Results were then observed and recorded (Keith et al. , 2010).

Part two:

Seven beakers were obtained. 0 mL of solution labeled 0. 2 M sucrose, 0. 4 M sucrose, 0. 6 M sucrose, 0. 8 M sucrose, 1 M sucrose, distilled water, and unknown were each placed in different beakers. Then, a potato was sliced into 28 cylinders using a cork borer. The cylinders were separated into seven groups of four and then placed under a paper towel until the group was ready to weigh the potato cylinders. Once prepared to weigh the cylinders, the weight of each group of cylinders was recorded. Four cylinders were placed into each beaker and sat for an hour (Keith et al. , 2010).

After the beakers sat for an hour at room temperature, the potato cylinders were then removed and blotted dry. Final weight was recorded for each group of potato cylinders. To calculate the percentage change, the following equation was used (Keith et al. , 2010): Percent change =Ending mass-Starting massStarting mass? 100% Results: Part One Results for the first experiment revealed certain molecular weights were unable to pass through the selectively permeable membrane. Table 1 shows that albumin (protein) and starch were unable to pass through the selective membrane.

Albumin’s molecular weight was approximately 64,000 Daltons and starch’s molecular weight was greater than 100,000 Daltons; these results were already known. Initially, glucose was present outside of the dialysis tube but in final results glucose was found in the final bag and final solution. On the contrary, sodium sulfate was initially present inside the dialysis tube but in the final results only, sulfate ion was found in the final solution. The last result was the change in water weight. Water had decreased from the initial weight.

In the second experiment results showed different concentrations of sucrose affected the potatoes’ mass. In the beakers containing 0. 0 M (distilled water) and 0. 2 M concentration of sucrose resulted in water entering the potato cell, which caused the cell to increase in mass.

The beakers containing 0. 4 M, 0. 6 M, 0. 8 M, and 1 M of sucrose concentration had the opposite effect on the potatoes mass. Therefore, the higher the concentration was the greater amount of water left, causing the cell to decrease in mass. Also, the unknown concentration was found to be 0. 5 M of sucrose, which caused the mass in the potato cells to decrease as a result of water leaving the cell.  The molecular weight was found by looking for the x-intercept on the graph below

Discussion

Part One of the experiment indicated that the dialysis tube was selectively permeable and only molecular weight fewer than 64,000 Daltons were able to pass through the membrane. This explains why albumin and starch were unable to pass through the membrane because their molecules were too large. Conversely, glucose was able to pass through the selectively permeable membrane due to its relatively small molecular weight.

However, because glucose was present in both the final bag and final solution this meant that glucose had evenly distributed its molecules by complying with the concept of diffusion. Sulfate ions present outside the dialysis tube in the final results show that sulfate ions were also able to diffuse through the selective membrane into the final solution. A decrease in water weight from initial weight shows that the dialysis tube was placed in a hypertonic solution causing more of the inside solution to diffuse to the outside leading to a decrease in the final weight of the bag.

The null hypothesis is rejected in Part One of the experiment because the concentration gradient did affect the weight of the dialysis tube. This is due to the fact that sodium sulfate completely left the bag, thus causing the bag to decrease in weight. The reason why sodium sulfate left is because there was no sodium sulfate in the solution; therefore, molecules went to an area of lower concentration. The alternate hypothesis is not rejected because the concentration gradient did affect the weight of the dialysis tube.

This is proven by a decrease in initial weight due to sodium sulfate leaving the tube. Part Two of the experiment showed that the potato cells had some kind of change in their mass after being placed in different sucrose concentrations. The change in mass occurred because water either left the cell or entered the cell depending on the sucrose concentration. This explains why distilled water had the greatest increase in mass, because water wanted to go to an area (potato cell) of higher concentration from an area of low concentration.

On the other hand, 1 M of sucrose concentration had the greatest decrease in mass because water wanted to leave the cell to move to an area of higher concentration. Therefore, if the concentration was greater than the molar concentration of the potato cell than water left the cell at a faster rate. The null hypothesis for Part Two of the experiment proved to be wrong because an increase in sucrose concentration did have an affect on the change in mass of the potato cell.

Increased sucrose concentration changed the mass of the cell because the concentration was higher than the molar concentration of the potato cell. Thus, the alternate hypothesis is proven correct. The difference in sucrose concentration will affect the mass of the potato cell.

References

1. Cain, M. L. , Jackson, R. B. , Minorsky, P. V. , Reece, J. B. , & Urry, L. A. (2011). Biology (9th Edition ed. ). San Francisco: Pearson Education, Inc.
2. Keith, E. , Messing, C,. Schmitt, E. , Feingold, J. (2010). Laboratory Exercises in Biology (3rd ed. ). Dubuque, IA: Kendall Hunt Publishing Company.

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