Simulation of osmosis across the cell membrane using dialysis bags models to measure the direction and the rate of osmosis
Osmosis refers to the process by which the cells exchange their water content with their surroundings (Beckett, 1986). The process is passive just like diffusion only that osmosis involves water only. The water molecules move from a where their concentration is high to a where their concentration is low, through a membrane that is semi-permeable. The permeable membrane only allows water molecules are other small molecules to pass while preventing large and other charged molecules from passing. The pressure that is exerted by the molecules of water that are freely moving in a system is known as the water potential (Kent, 2000).
Some of the factors that affect the osmosis rate include the amount of solute in a solution and this affects water potential. The reduced water potential reduces the rate at which osmosis takes place. The other factors that affect osmosis include temperature since increased temperature results in a faster reaction rate. The distance through which the molecules move across determines the rate of osmosis, a long distance for the particles to diffuse lead to lowering the rate of osmosis (Kent, 2000).
In the cells, the universal solvent is water, and the movement of water across a cell membrane is one of the crucial processes that take place in a biological system. Solution that has more solute content than the solution in the cell are said to be hypertonic and the water potential in the cell is greater than that outer side of the cell. This causes the cell to loss its water content to the surroundings. The solutions that have less solute than the solution in the cell are said to be hypotonic, and the surrounding has a greater water potential than the inside of the cell. This causes the cell to gain water from the environment and makes the volume of the cell to increase (Rowland, 1992). The solution that has a similar concentration with the solution that is in the cell is said to be isotonic. The water potential in and out of the cell is the same and thus there is no net water movement (Marvel & Kepler, 2009).
The purpose of this is to make a simulation of osmosis across the cell membrane using dialysis bags as models of cells under different conditions in order to measure the direction and the rate of osmosis. The experiment will seek to answer the question whether solute concentration affects the rate of osmosis.
Methods
The dialysis tubing clamps were taken and paired up using the appropriate letters for each dialysis bag. Five pieces of water soaked dialysis tubing that were 15 cm long were used and one end of each tube was sealed by folding it over and secured using a clamp. The other end of the tube was opened by rolling the end between the thumb and finger. Using a 50 ml graduated cylinder, the tubes were filled with content. Bag A was filled with 25 ml of 1% sucrose concentration, Bag B with 25 ml of 1% sucrose solution, Bag C and D with 25 and 50% of sucrose concentration solution. Bag E was filled with 25 ml of unknown concentration of solute solution. For each of the tubes, the open end was loosely folded and pressed on the sides to push up the fluid removing some of the air. It was ensured that the dialysis bag was not so full that water could not enter into the bag. The end was clamped securely with matching labeled clamps, rinsed under slowly flowing water and the bag was checked for any leaks.
Excess water from the outside was blotted, and the weight of each dialysis bag determined to the nearest 0.1g. The weight was recorded as the initial weight. Bags B, C, D, and E were placed in a beaker, and the beaker was filled with just enough solution of 1% sucrose to cover the bags. The time when the solution was added was recorded. Bag A was placed in a small beaker that was then filled with just enough solution of 50% sucrose to cover the bag. The time when the solution was added was recorded. At an interval of 15 minutes for around one hour, the bags were removed, blotted them dry and their weight taken to the nearest 0.1g. The results were recorded. Handling the bags with care, they were quickly returned to their respective beakers.
After the experiment, the dialysis tubing clamps were rinsed with tap water and returned to their box. The dialysis tubing were also rinsed and placed in a container with water. The beakers were also rinsed using tap water and returned to the working bench.
Results
The total weight change was calculated and used in the plotting of the graph of the total weight against time as shown in Figure 1 below. Other than Bag A, all the other bags gained water content from the solution in the beaker. However, the amount of water gained in the first 15 minutes interval was more than the one gained in the subsequent intervals with the amount reducing even more by the last interval.
Figure 1: A trend of the total weight (grams) against time in minutes for the various dialysis tubing with different sucrose concentration
Discussion
Each of the five dialysis bags that were used in the experiment represents a cell model. Bag A shows a cell that has a content that is hypotonic (1% sucrose concentration) to the environment (50% sucrose concentration). The concentration of the substances that are in the cell is less than that in the environment. This makes the water potential in the cells to be greater than that of the surrounding. This water potential causes water content to be lost from the dialysis tubing into the beaker where the concentration is high. The rate at which Bag B lost its water content was 4.95g/minute.
Bag B represents a cell whose cytosolic concentration is equal to the environment concentration. The solution in the dialysis tubing had a 1% sucrose concentration as well as the environment. In such a set up there is no net water movement between the dialysis tubing and the surrounding solution. Bag B started by gaining water from the solution in the beaker at a rate that reduced gradually and by the 60th minute the some of the water gained was lost to the environment. Overall, Bag B gained water at a rate of -0.675g/minute. The expected result for this Bag was to have a rate of 0.0g/minute since the solution was isotonic to that in the beaker. This may have occurred due to errors in making the concentrations.
Bag C was filled with a sucrose solution at a concentration of 25%, making the dialysis tubing to have a solution to be hypertonic to the solution in the beaker. Bag C represents a cell that is in an environment that has greater solute concentration than that in the cell. The water potential was, therefore, greater outside the dialysis tubing than inside making water to move into the dialysis tubing. The rate increases by the 30th minute and drastically reduced by the 60th minute. By this time, the concentration in and outside the dialysis tubing had almost attained equilibrium. Overall, rate of osmosis in this system was -4.675g/minute. These results appeared just as they were expected to appear.
Bag D had a solution with a 50% sucrose concentration and thus hypertonic to the solution in the beaker. The bag also represents a cell that is in a solution of higher concentration than its content. The dialysis tubing gained water from the surrounding, but by the 60th minute the two solutions had not attained equilibrium. The results occurred as expected as the system was expected to take longer to attain equilibrium than Bag C system. The overall rate of osmosis was -5.05g/minute. This rate is significantly higher than the rate of osmosis in Bag C and is an indication that solute concentration affects the rate of osmosis and, therefore, the higher the concentration, the greater the rate of osmosis (Marvel & Kepler, 2009).
Bag E had an unknown sucrose concentration. From the graph, the dialysis tubing contained a hypertonic solution to that in the beaker indicates a total weight gain. The overall rate of osmosis in this system was -2.275g/minute. The weight gain for this bag was somewhere between that of Bag B and bag C. This means that the concentration of the sucrose in Bag E was between 1% and 25%. However, the concentration seems to be around 10% since the value are far greater than those of 1% and less than half those of 25%.
Conclusion
In conclusion, a simulation of osmosis across the cell membrane using dialysis bags is a good model of cells under different conditions in order that can be used in measuring the direction and the rate of osmosis. The experiment answers the question that solute concentration affects the rates of osmosis with the solution with high solute concentration having the greatest rate of osmosis while the solution with low solute concentration giving the least rate of osmosis.References
Beckett, B., 1986. Biology: a modern introduction. Oxford: Oxford University Press.
Kent, M., 2000. Advanced Biology. Oxford: Oxford University Press.
Marvel, S. C. & Kepler, M. V., 2009. A Simple Membrane Osmometer System & Experiments that Quantitatively Measure Osmotic Pressure. The American Biology Teacher, 71(6), pp. 355-362.
Rowland, M., 1992. Biology. Cheltenham: Nelson Thornes.
