Osmosis and Diffusion in the Cell Membrane
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Currently, the accepted model of the structure of the cell membrane is the fluid mosaic model. The model explains how the membrane controls what enters and leaves the cell. The main component of the membrane is the phospholipid bilayer. This bilayer acts like a gate, allowing nonpolar molecules such as oxygen and carbon dioxide to cross over with ease but limits the passage of polar molecules like sugars.
In order for cells to survive, they need to take in nutrients and to eliminate waste. Therefore, there has to be methods to allow substances to travel across the cell membrane. The two main types of movements that cells utilize are passive transport, which does not require energy, and active transport, which does involve the use of energy. This investigation will focus on passive transport, specifically simple diffusion and osmosis. In simple diffusion, molecules tend to spread out evenly, moving from an area of high concentration to an area of low concentration. It will continue to move down its concentration gradient until the concentration is uniform throughout. Osmosis is just the diffusion of water across a selectively permeable membrane from a more dilute region to a more concentrated region. Osmosis is crucial to the survival of an organism because it controls the balance of water between the cell and its surroundings. In order for simple diffusion and osmosis to happen, there must be a moist and permeable membrane and a concentration gradient to move down.
Depending on how much solute there is, a solution can be either isotonic, hypotonic, or hypertonic. In an isotonic solution, the cell has the same solute concentration as the solution and thus no net movement of water occurs. Water would flow in and out of the cell at the same rate. However if a cell's surrounding has more solute, than that solution would be considered hypertonic. In an attempt to correct the offset of concentration, water from the cell would leave to make the cell's surrounding less concentrated. There is a net movement of water away from the cell and in most cases the cell would shrivel up and die. On the other hand, a hypotonic solution is when a solution has less solute than the cell. To make the cell less concentrated, water would enter the cell at a faster rate than it leaves. The cell swells up with water and sometimes even burst if too much water enters. In all three types of solution, water is trying to reach a state of equilibrium.
In this experiment we will a) determine the size of several small molecules based on whether or not they diffuse across the semi-permeable membrane, and b) study the relationship between solute concentration and the movement of water, and how it affects osmosis. The hypotheses for these objectives are as follows:
Since glucose is a simple sugar, or monosaccharide, it will be able to diffuse across the membrane with much more ease than starch, a polysaccharide. Polysaccharides are bigger because they are composed of monosaccharides bonded together.
As molarity and solute concentration increases, so will the net movement of water from the beaker into the bag. Water is trying to reach equilibrium by moving into the more concentrated region so that it can dilute the solution.
This experiment was divided into 2 sections. Part 1 tested diffusion while part 2 investigated osmosis. In both section, moist 30-cm pieces of dialysis tubing were used to represent the semi-permeable membrane of cells. The tubing has pores that allow for some substances, such as water, to pass through while it blocking others. For the first part of the experiment we formed a bag out of the tubing by tying one end of it with string and poured 15 mL of the clear 15% glucose/ 1% starch solution into it. We will be testing this to see whether or not the solution is able to diffuse out of the tubing. We used two substances to represent a few of the many things that try to diffuse through the cell membrane.
Next, we dipped one strip of glucose tes-tape into the solution in the bag and another strip into 185 mL of distilled water in the beaker. The purpose of this is to check if the glucose is present in the water and to see that glucose really is in the solution. The strip dipped into water exhibited no change in color but the one soaked in the solution changed to a green color indicating that glucose was there. Afterward, we added about 4 mL of Lugol's solution (KI) in the beaker of distilled water. Normally, the KI solution is a light brown/golden yellow color but when starch is present, it produces a navy blue black color. When KI was mixed with the water it turned the water a clear yellow. Again we used a tes-tape strip to test for glucose and we got a negative reading. Finally we tied the other end of the tubing and submerged the bag into the solution. We have to let the bag be immersed for 30 minutes to allow the solution enough time to diffuse and reach equilibrium.
Soon we were able to see the solution inside the bag turn into a dark blue black color. The color of the water of the beaker remained a transparent yellow. When time elapsed, we used the test stripes for both the beaker and the bag and both strips turned green.
For the second part of the experiment, we used dialysis tubings to make six bags. This time around we poured 25 mL of varying concentrations of sucrose into the each bag. The six bags held distilled water, a 0.2 M, 0.4 M, 0.6 M, 0.8 M, and 1.0 M solution of sucrose respectively. Sucrose is a disaccharide and commonly known as table sugar. It was used because sucrose is found commonly in human bodies. Many different concentrations of sucrose were used to measure the relationship between molarity and osmosis while the bag full of distilled water was the control.
After securing the contents of the bags, we individually weighed each bag using an electronic balance. We filled six beakers with 185 mL of distilled water. Since the bags have a higher concentration of solute, osmosis will occur to try to dilute the solution in the bag so that equilibrium between the contents of the bag and beaker is reached. We simultaneously submerged the six bags into each beaker, which gives each bag equal amount of time to go through osmosis. After about 25-30 minutes of waiting we removed the bags from the water, blotted the excess water, and massed them again using the balance. Finally we checked to see if there was a difference between the initial mass (before submerging) and final mass (after the 30 minutes) of the bag.
Through osmosis, water is both leaving and entering the bag. Glucose also is leaving the bag through the pores. This is evident when we used the test strips to check for glucose. In the initial solution, before the bag was added, the test strip showed no change in color, but letting the bag sit for 30 minutes, the strip turned green, indicating that glucose was present. Lugol's solution also was entering the bag. When there is no starch, the KI does not react and remain a yellow tint. However, if starch is introduced, the KI mixes with it and turns the solution dark.
The tubing represented the semi-permeable cell membrane. One way substances enter and leave the membrane is through simple diffusion in which substances go from an area of high concentration to low concentration. Since the glucose and starch in the bag were of higher concentration than that of the beaker, they naturally wanted to diffuse through the tubing and into the beaker. The same thing occurs with Lugol's solution, except that it wants to enter the bag instead. KI and glucose were able to pass through the tubing but starch was too big to fit through the pores.
This experimented could be modified to allow quantitative data that shows that water diffused into the dialysis bag. One would use an electronic scale to mass the contents of the bag before and after submerging it into the beaker. The difference between the final and initial mass would show how much water diffused.
Water molecules are probably the smallest because it is only made up 3 small molecules and able to easily diffuse through the tubing. KI molecules are next because it consists of two larger molecules, followed by glucose molecules because they are made up of many carbons, hydrogens, and oxygens. These three molecules were able to diffuse through the membrane pores. This leaves the starch molecules as the largest since they were unable to diffuse through and because they are polysaccharides.
The glucose and KI solution would diffuse out of the bag while water would diffuse into the bag. Starch is unable to diffuse so it would remain in the beaker only. When the KI diffuses through it will mix with the starch outside and thus change the color of the water in the beaker to a blue black color.
According to our results, it seems that as the molarity of the sucrose in the dialysis bag increases so does the change in mass. This is due to osmosis. Water from outside would enter the bag in an attempt to dilute the sucrose solution and the higher the concentration the more water would come in to dilute it.
If all bags were placed into a 0.4 M sucrose solution, then all of the bags would try to reach equilibrium relative to the 0.4 M sucrose solution. The 0.6, 0.8, and 1.0 M bags would gain water because the concentrations inside these bags are higher and water would enter to lessen the molarity. In the distilled water and 0.2 M bags, water would actually flow into the beaker because the beaker has the higher concentration. The 0.4 M bag is already in equilibrium with the beaker. Since the beaker solution is isotonic, there would be no net movement of water.
We calculated the percent change in mass and not the actual change in mass because the initial mass of each bag was not the same as the others. We use the percent change because the mass difference has to be relative to that particularly initial mass. If all of the initial masses were the same, we would be able to use the actual change in mass instead.
The percent change in mass is equal to the final mass minus the initial mass and the result of divided over the initial mass and then multiplied by 100 percent. In an equation form it would be [(final mass - initial mass)/initial mass] X 100% = percent change. Thus percent change of mass for this particular problem would be [(18 g - 20 g)/18 g] X 100% = (-2 g/18 g) X 100% = -0.1111 X 100% = 11.11%
The sucrose solution in the bag would have been hypotonic to the distilled water in the beaker since water entered the bag and left entered the beaker of water.
For the first experiment, the glucose test strip that was initially dipped into the beaker containing distilled water and Lugol's solution remained a yellow color. However, the strip that was initially dipped into the bag of 15% glucose/1% starch turned green (Table 1). If the strip turns a green color, it means that glucose is present in that solution. Although we used a solution labeled 15% glucose/1% starch, we tested it just to make sure glucose really was there. After letting the bag sit in the beaker, two test strips were used to see if there was glucose after the experiment was completed. The strips were dipped into the beaker and bag and both turned green. Also the bag turned a dark color which indicated that there was starch still there.
For the second experiment, except for the 0.6 M bag, the initial mass of each bag starting with the most dilute concentration, consecutively got higher. This same exact trend occurred with the final masses of the bags and all bags displayed increased mass. Also starting with the least concentration solution, the percent change in mass generally increased as the molarity increased (Table 2). Compared to the class averages, most of our values were around the class values. Some of our data were a little higher, especially the percent change for the 1.0 M solution (Table 3).
Our data provided support for our hypothesis that glucose would be able to diffuse through. Since glucose was not present in the beaker initially but was there after the bag was submerged, this means that glucose must have been able to diffuse through the pores of the tubing. Since the bag is the only source of glucose, diffusion is the only method in which glucose could have entered into the beaker. Starch on the other hand was unable to diffuse across the membrane. It remained in the bag only and hence the bag was the only section that turned that distinct dark blue color.
The results from part 2 also matched the general accepted knowledge of osmosis. Osmosis will usually occur when there are unbalanced regions of concentrations. In trying to establish equilibrium, water from the less concentrated solution will flow through the cell membrane into the more concentrated solution to try to lower the concentration. This is what basically happened with our 5 bags of sucrose. There was a net movement of water from the beaker and into the bag.
A few errors occurred while the experiment took place. In the first part, the inside of the bag did not turn to the blue black color when we added the 4 mL of Lugol's solution. In the end we had to add a few more drops in before the inside of the bag would change color. In the second part, the reason why some of our numbers are higher than the class average was probably because our group was one of the first groups to have the set-up of the six bags in the six beakers. Our experiment would have had more time to allow osmosis to occur than other groups did.
Our data and results can be applied to the further studies of osmosis and diffusion, especially to passive transport in human cells.
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