Osmosis

Methods and Materials: To test the hypothesis that the rate of osmosis is positively related to the difference in the osmolarities of the solutions on the two sides of the membrane, we needed to fill up one side of the membrane with a liquid. First, we filled three culture dishes about half full of ordinary tap water which has an osmolarity of zero. We then labeled each dish; the first was labeled 0%, the second was labeled 20%, and the third one was labeled 60%, to organize the experiment into three osmolarities. Next, we needed to fill our membrane with liquids of different osmolarities. We used three 20cm strips of dialysis tubing to simulate a semi-permeable membrane of a cell because the tubing allows water to pass through, but nothing else. We first had to soak the three strips of tubing in water for a few seconds, and once removed from the water open them up. This was accomplished by gently rolling the end of the tubing between your index finger and thumb. At the opposite end, we applied a dialysis clamp to seal the “cell” end by folding about half a centimeter down to ensure a complete seal. We repeated this process until we had three empty, open cells. To the first cell we added 10 ml of tap water and sealed it by folding the top end down about half a centimeter and we applied a clamp after getting all the air out of the tube. We then weighted and recorded its starting weight. We repeated this process for the other two cells but added 10ml of 20% sucrose solution to one, and 10ml of 60% sucrose solution. With all three cells filled, weighed, and ready, we dropped them into their labeled dish simultaneously. After allowing 45 minutes for osmosis to run its course, we removed the cells from their dish; we blotted them dry and recorded its final weight, being sure to do only one at a time to avoid confusing the cells. Finally we calculated the weight gain by each cell during the experiment and calculated the mean weight change from four repetitions.

Results: The results of this experiment are pretty straight forward and easy to see. (See table 1.1 and Graph 1.1) As the difference in osmolarity increased between the solutions, the resulting weight gain also increased, indicating that osmosis was occurring at a faster rate. The cell at 0% difference gained barely any weight because osmosis didn’t really need to occur because there was no higher or lower concentration for the water to flow into. The cell at a 20% difference in osmolarity increased a fair amount of weight from the water seeping into the cell. The third cell at 60% difference experienced a good amount of weight gain due to water flooding in from its high concentration outside the cell to the low concentration inside the cell. (See table 1.1) Therefore it’s easy to conclude that the hypothesis that the rate of osmosis is positively related to the difference in the osmolarity of the solutions on the two sides of the membrane is indeed correct.

Table 1.1 Mean Weight Change at Different Solution Concentrations

Solution % Weight Gain (g)

0% .6g

20% 2.7g

60% 5.7g

Table 1.1 legend: This table shows what the average weight change is per solution out of four repetitions. The solution represents the difference in osmolarity compared to the 0% water solution the cells were emerged in. The more