Diffusion and Osmosis Laboratory - Lab Report Example

Diffusion and Osmosis Laboratory Report For a cell to survive, the exchange of water from outside to cell to inside and vice versa is extremely important since water serves as the medium for transport of metabolic ingredients and byproducts. It can occur mainly through two biophysical processes, osmosis and diffusion. Depending on the biological process, the cell exerts its option of choosing either of the two in a given exchange process. Physical movement of water occurs in both of these processes depending on the physical characteristics of the solutions that are being exchanged. The cell and its extracellular environments are fluid in that concentrations of substances dissolved in it vary. Depending on varying concentration of the substances dissolved in water, be it extracellular or intracellular, dissolved substances or water move from one compartment to another directed by physical principles of gradients. To accomplish this, the cell utilizes the processes of osmosis and diffusion as appropriate. These phenomena can be visualized outside the cell by simulating these in vitro. Since cell membrane is a semipermeable membrane and exchanges occur through this, these experiments must utilize a construct of the semipermeable membrane that can be made with a commercially available dialysis tubing and bag. These membranes have microscopic pores through which small molecules like water can pass, but larger molecules, such as sugar cannot pass through them. Thus through this molecular net, solutions of different solute concentrations can be observed to pass in a measurable fashion. This experiment has been designed to prove the hypothesis of the mechanisms of osmosis and diffusion through such a semipermeable membrane. Introduction Diffusion is defined as the movement of molecules from a site of higher concentration to that of a lower concentration. Technically speaking, this difference in concentration creates a concentration gradient, and the steepness of the gradient determines the rate of diffusion. In an attempt to find out the reason as to why it occurs, it can be attributed to a great deal of space between the molecules of all substances that are candidates for diffusion. Liquids have moderate intermolecular space. Moreover, all molecules are in a state of constant random movement so that they collide and intermingle. It is expected that in solids diffusion would occur slowly for tight packing of the molecules. Liquids and gases would diffuse freely because their molecules are spaced widely. Thus, it can be stated that any solute will tend to uniformly occupy the entire space available to it. This movement, known as diffusion, is due to the spontaneous Brownian random movement that all molecules experience. The net result of diffusion is the movement of substances according to their difference in concentrations, from regions of high concentration to regions of low concentration. Diffusion is an effective way for substances to move short distances. Diffusion across a membrane has no preferential direction; it can occur from the outside of the cell toward the inside or from the inside of the cell toward the outside. It is however determined the extend of permeability through a membrane, which in the case of a living cell is the protein-lipid-protein plasma membrane, and therefore, diffusion across the membrane usually implies that the diffusing solute enters the lipid bilayer to cross it, the solute's solubility in a lipid solvent compared with its solubility in water is important in determining its permeability through it. Hydrophilic substances, such as ions and sugars, do not interact well with the lipid component of the membrane, hence would diffuse across the membrane more slowly. This is, however, not applicable in physical membrane such as a dialysis membrane (Campbell NA, Reece JB, Mitchell LG., 1999). If equal volumes of pure water and a strong sugar solution are taken, the pure water will have more water molecules and would have higher water concentration that a sugar solution since in the sugar solution, sugar molecules will take up some space. Similarly, a weak sugar solution will have a higher concentration of water molecules than a strong sugar solution. If a strong sugar solution is separated from a weak sugar solution by a membrane through which both sugar and water molecules can pass, as expected sugar molecules will diffuse from the stronger side to the weaker side in an attempt to equalize the concentrations of sugar on both sides of the membrane. In an attempt to uniformly distribute on both sides of the membrane, likewise, water molecules would also diffuse from the weak to strong solution until a uniformity of distribution is achieved (Campbell NA, Reece JB, Mitchell LG., 1999). In the experimental situation when a semipermeable membrane is chosen to be used, the idea is to mimic the cellular plasma membrane as nearly as possible. A semipermeable membrane such as a dialysis membrane would only allow water, but not sugar to pass through. When a stronger sugar solution is separated from pure water, under these circumstances, sugar molecules cannot diffuse from a high to a low concentration, that is, across a gradient. Only the water molecules can move and pass through such a membrane. Since the pure water has larger concentration of water molecule than that in the sugar solution, according to the concentration gradient, water will diffuse from the weak to the string solution until a uniform distribution is achieved. This process is known as osmosis and is applicable to any solvent. The spontaneous movement of water across a membrane driven by a gradient of water concentration is the process known as osmosis. The water moves from an area of high concentration of water to an area of low concentration. Since concentration is defined by the number of particles per unit of volume, a solution with a high concentration of solutes has a low concentration of water, and vice versa. Osmosis can, therefore, be viewed as the movement of water from a solution of high water concentration, that is, low concentration of solute toward a solution with a lower concentration of water, that is, high solute concentration. Osmosis is a passive transport mechanism that tends to equalize the total solute concentrations of the solutions on both sides of every membrane. If a cell that is normally in osmotic equilibrium is transferred to a more dilute solution, water will enter the cell, the cell volume will increase, and the solute concentration of the cytoplasm will be reduced. If the cell is transferred to a more concentrated solution, water will leave the cell, the cell volume will decrease, and the solute concentration of the cytoplasm will increase. The driving force for the movement of water across the plasma membrane is the difference in water concentration between the two sides of the membrane. Taking the condition of this proposed experiment, when a membrane separates two solutions of different osmotic pressure, water will move from the solution with low osmotic pressure with high water concentration to the solution of high osmotic pressure, that is, low water concentration. In this context, the term selectively permeable means that the membrane is permeable to water but not solutes (Campbell NA, Reece JB, Mitchell LG. 1999). Objective The objective of this experiment is to observe how selectively a semi-permeable membrane such as dialysis tubing acts to control diffusion and osmosis of various substances into and out of a compartment depending on varying concentrations of solutes or solvents in a solution. Hypothesis In osmosis, water molecules diffuse through a semi-permeable membrane from a weak to strong solution. Methods and materials Materials: 3 beakers, distilled water, 1.5 M sucrose solution, 3.0 M sucrose solution, 25 cm dialysis strips, clamps for dialysis tubing, disposable syringe, plastic tubing. Method: 1. Before beginning the experiments hands are washed thoroughly. 2. Three pieces of precut dialysis tubing were soaked in distilled water. Each tubing was wiped off and gently rubbed back and forth between thumb and index finger to open up the tubing. For each tube, one end of the tubing was gentle folded on itself to close that end and a clamp was applied. Figure 1: Dialysis Tubing and clamp 3. The end of the dialysis tubing was rubbed with hand to find out the opening. Through this opening, a 20 cm segment of plastic tubing was inserted. With the disposable syringe prefilled with a 1M sucrose solution, a plastic piece of tubing was attached to it. The contents of the syringe were gently and carefully pushed into it. Figure 2: 3M Sucrose solution pushed into the dialysis tubing with the syringe. 4. While filling it was important to avoid any air bubbles in the dialysis tubing. Moreover, the tubing was filled up to its volume to allow for space in case some solution or material enters into the dialysis tubing during the experiment. 5. There should be enough tubing remaining to allow for closure of this end of the tubing following filling. Following this, the end of the tubing is again twisted and folded on itself, and a clamp was applied to close it securely. It was important to leave space for expansion and to avoid any air bubbles, which would be taken care of if there was any. 6. Each of these three bags were rinsed off and blotted with paper. 7. Each of these dialysis bags were weighed and recorded in the data table. 8. Next, three 250 mL beakers were taken and filled up with approximately 200 mL of distilled water, 200 mL of 1.5M sucrose, and 200 mL of 3M sucrose respectively. Each beaker was labeled according to the contents of the dialysis tubing solution it holds and placed on a folded tissue paper. Figure 3: Dialysis bag set up in a beaker of distilled water 9. Each of the bags was then immersed into the respective labeled beakers, and time was recorded using a chronometer. 10. This was allowed to stand there for 75 min. However, every 15 min, respective dialysis tubing from each beaker was taken out, mopped off, and weighed. The respective weights were recorded and tabulated. Results Time, mi Beaker 1 Beaker 2 Beaker 3 distilled water 1.5M sucrose 3.0M sucrose 0 6.92 g 7.02 g 7.12 g 15 8.00 g 7.11 g 6.88 g 30 8.82 g 7.02 g 6.56 g 45 9.48 g 6.97 g 6.27 g 60 10.02 g 6.92 g 6.06 g 75 10.33 g 6.82 g 5.75 g Table 1: Weight of dialysis tubing in three beakers 11. The mass difference and changes in weight were calculated and the percent differences in mass were computed for each entry. Time, mi Beaker 1 Beaker 2 Beaker 3 distilled water 1.5M sucrose 3.0M sucrose 0 0 0 0 15 1.08 g 0.09g -0.24 g 30 1.90 g 0.00 g -0.56 g 45 2.56 g -0.05 g -0.85 g 60 3.10 g -0.1 g -1.06 g 75 3.41 g -0.2 g -1.06 g Table 2: Weight change of dialysis tubing from time 0 (Calculations for Weight Change (bag weight at 15 min - bag weight at 0 min) of each from 0 min in Table 1) 12. This data are graphically represented with the independent variable in the X-axis and the dependent variable in the Y-axis. Figure 4: Graph of Dialysis bag weight change in three different solutions Dicussion As seen in the graph and evident from the table, the sac placed in distilled water gained most of weight, whereas the sac placed in 1.5M, the change was weight is negligible on the negative site, meaning weight was lost, but not to a great extent. In the contrary, the sac placed in the 3M sucrose solution containing beaker had the maximum weight loss, in fact with advance of time, it kept on losing weight. When two solutions have the same concentration of solutes, they are said to be isotonic to each other. Therefore, if these two solutions are separated by a semi-permeable membrane, water will move between the two solutions, but there will be no appreciable change in the amount of water in either solution. If two solutions differ in the concentration of solutes that each has, the one with more solute is hypertonic to the one with less solute. The solution that has less solute is hypotonic to the one with more solute. If two solutions are separated by a semi-permeable membrane, the solution that is hypertonic to the other must have more solute and therefore less water. At standard atmospheric pressure, the water potential of the hypertonic solution is less than the water potential of the hypotonic solution, so the net movement of water will be from the hypotonic solution into the hypertonic solution. In case of beaker 1 containing distilled water, distilled water being hypotonic in comparison to the bag containing 1M sucrose, there will be osmosis or diffusion of water into the dialysis bag, and it will keep on gaining weight until the concentrations of water in both the compartments become equal. This is evident by the blue curve in the graph. In the second beaker that contains 1.5M sucrose, the solution outside the dialysis bag is just a little more hypertonic than that in the interior. Since in this case, the solution within the bag contains more water molecules, water will flow into the beaker, and the dialysis bag will lose weight although at a very slow rate and in insignificant amount, until the concentrations become equal. Therefore, the dialysis bag will lose weight albeit in a smaller and negligible rate. In the third beaker, the solution outside the dialysis bag is comparatively more concentrated than the solution inside the dialysis bag in terms of sucrose, so bag will contain more water molecules per unit weight than the beaker. Water would therefore flow out through this semi-permeable membrane from inside the bag, and the bag will lose weight (more than the previous dialysis bag). The curve demonstrates this. Thus, this experiment demonstrates that osmosis occurs when different concentrations of water are separated by a differentially permeable membrane, and the same principle applies to the plasma membrane of a living cell. This experiment demonstrates osmosis by using dialysis tubing to simulate the plasma membrane, which is differentially permeable membrane and allows the passage of water preventing the passage of larger molecules in a solution. The movement of water in this experiment is demonstrated by weighing the dialysis bags every 15 minutes for 75 minutes from the beginning of the experiment. If the bag gains weight, then water has moved into it by osmosis, and vice versa. As discussed earlier, this experiment proves the hypothesis that in osmosis, water molecules diffuse through a semi-permeable membrane from a weak to strong solution. Applied in case of a cell, if a cell is hypertonic to its surroundings, water rushes into the cell, causing it to expand, causing the cell to burst, called cytolysis. Error It is to be mentioned that actually osmosis is a factor related to concentration of the solute or the solvent and can be calculated by the osmotic pressure. Osmotic pressure is again related to the percentage change in weight that is calculated by using the following formula: % Change = [(Final Mass -Initial Mass)/Initial Mass] x 100%. Since a simplistic approach has been taken, these data cannot be used to predict the osmosis with accuracy. Moreover, this is a simulation experiment, where only the physical property of a biological membrane has been considered to predict osmosis and diffusion. Similar phenomena occur in a living cell; however, the semi-permeability of a living cell membrane is biologic where many other biological factors intervene to dictate diffusion or osmosis through the membrane both outward and inward. Moreover, unlike this dialysis bag, other solute molecules pass through the cell membrane since there is presence of multiple pores or channels of varying sizes. Thus this experiment would not exactly mimic a living cell, and the results will be also not identical. However, despite lack of accurate predictability, these experiments may allow the examiner to understand the process. Conclusion 1. Osmosis and diffusion are processes of movement of solute or solvent through a semi-permeable membrane. 2. Movement of water happens from hypotonic to hypertonic compartment. References Campbell NA, Reece JB, Mitchell LG. (1999). Biology: Fifth Edition. California: Benjamin Cummings. 1175 p. Read More