Biochemistry Experiment on Osmosis

EXPERIMENT 1D – OSMOSIS I.

Introduction

In humans, there is a need to maintain an “internal balance” despite internal or external environmental changes in order to survive and ensure health. The aforementioned “internal balance,” otherwise known as homeostasis, includes numerous processes whose ultimate goal is to normalize imbalances in the body. These processes include temperature control, pH balance, blood pressure, respiration and water and electrolyte balance among others. One of the fundamental processes in the body that maintains homeostasis is osmosis. Osmosis is defined as the net diffusion of water across a selectively permeable membrane from a region of low solute concentration to a region of high solute concentration to reach a state of equilibrium. The cells of the human body have semipermeable cell membranes through which water can diffuse through to maintain its balance with the external environment. To practically demonstrate the principle of osmosis, the use of a semipermeable cellophane film will be utilized in this experiment. II.

Objectives

This experiment aims to demonstrate the principle of osmosis through a semipermeable membrane; to be able to identify different factors that influence osmotic pressure and to determine how these factors affect the rate of osmosis. III.

Materials and Methods

The experiment was conducted using the following materials: thistle tubes, beakers, iron stands, rubber bands, 10% solution of sugar and 10% aqueous gelatin solution and was then performed as follows: 1. The thistle tube was filled with 10% sugar solution until its expanded part. 2. The mouth of the tube was covered with a cellophane membrane and secured by tying it in its constricted portion using a rubber band. 3. It was ensured that the solution reached the base of the vertical part of the tube when it was allowed to stand with the cup-shaped-end down. 4. The thistle tube was suspended in a beaker of water. The surface of the sugar solution and that of water were on the same level. 5. The tube was secured to an iron ring using a string. 6. A similar set-up was prepared using 10% aqueous solution of gelatin. 7. The rise of the solution was observed in both tubes and their speed and height were compared at the end of the period.

IV.

Results

Figure 1. Results of Set-Up with 10% Sugar Solution (L: Before; R: End of the Period). For the first set-up which contained 10% sugar solution, the water in the stem of the thistle tube rose up to the level near the rim of the beaker (approximately 1 inch relative to the set-up at the beginning of the period). Figure 1 shows the elevation of fluid in the stem of the thistle tube for the sugar set-up.

Figure 2. Results of Set-Up with 10% Aqueous Gelatin Solution (L: Before; R: End of the Period) The second set-up’s speed of water elevation is creeping. Level of elevation was also minimal (only around 0.2 inches relative to the set-up at the beginning of the period). Figure 2 shows the elevation of fluid in the stem of the thistle tube for the gelatin set-up. Comparing the former and the latter set-ups, the sugar set-up had a rate and height of elevation much higher than the gelatin set-up’s. V.

Interpretation and Discussion

Water, being a small molecule, is able to readily pass through a semi-permeable membrane enabling it to diffuse along its own concentration gradient. Most solutes, however, are not able to permeate through the membrane and will therefore not diffuse despite an existing concentration gradient. As aforementioned, osmosis is defined as the net diffusion of water across a selectively permeable membrane from a region of low solute concentration to a region of high solute concentration to reach a state of equilibrium. This can be seen in Figure 3.

Figure 3. Visual Depiction of Osmosis Across a Semi-Permeable Membrane. Water molecules, being small molecules and are able to readily cross the semi-permeable membrane, move to the region with high solute concentration (in this case, sugar) to reach equilibrium. [Source: Britannica Encyclopedia Kids] In application to the experiment, water should spontaneously diffuse from the outside to the inside across the semi-permeable cellophane film since the solute concentration inside the membrane is higher than that of the outside. When the internal and external environments reach equilibrium, water diffusion ceases due to the generation of a pressure that prevents the inward flow of water because the minimum pressure needed to stop osmosis was attained. This pressure is referred to as osmotic pressure. The osmotic pressure generated is influenced by the following factors: number of solute particles in a given amount of water, molar concentration of the substance and the molecular weight of the molecules. The van’t Hoff Equation summarizes how these factors affect osmotic pressure and this equation is given by (Equation 1):

Where: π is the osmotic pressure; i is the number of particles into which the substance dissociates; also known as the van’t Hoff factor; M is the molarity of the solution (moles of solute per liter of solution); R is the universal gas constant; and T is the temperature in Kelvin In the experiment, the rate of osmosis is to be compared with different types of heterogeneous solutions: 10% sugar solution and 10% gelatin solution where the former is a crystalline solution and the latter a colloidal solution. In determining which factor inherent to the solution caused the results, the characteristics of a crystalloid and a colloid solution must be understood.

A crystalloid is a substance, solution or mixture that resembles the properties of a crystal. These solutions, compared to colloids, generally have smaller molecular sizes. Their small molecular sizes will allow these substances to readily cross a semi-permeable membrane as long as permeability will allow it. An example of a crystalloid solution is sugar solution. On the other hand, colloids have larger molecules compared to crystalloids. Aqueous gelatin solution is an example of a colloidal solution. Based on Equation 1, there are only two factors than can influence osmotic pressure: molarity of the solution and temperature. R and I are both constant for the two set-ups. Both systems were also performed under constant room temperature, therefore molarity being the factor that plays a dominant role in this experiment. Both heterogeneous solutions used in the experiment were of the same concentration (10% w/v). The mass concentration of a solution is given by the following equation in terms of molarity (Equation 2):

where molarity is in mol/L and molecular weight is in grams/mole. Given a constant mass concentration (rho) for both solutions, it follows that the larger the molecule, the higher is its molecular weight and therefore the lower is its molarity. Therefore, sugar whose molecules are smaller than that of gelatin’s have a higher molarity—and ultimately a greater osmotic pressure which accounts for the results seen in the experiment. VI.

Conclusion

Osmosis is the net diffusion of water from an area of high solute concentration to a low solute concentration until it reaches equilibrium. Diffusion of water ceases when osmotic pressure—the pressure needed to stop movement of water—is generated at equilibrium. According to the van’t Hoff equation, the factors that affect osmotic pressure are temperature, number of particles into which the substance dissociates to and the concentration of the substance. In this particular experiment, the concentration of the substance played a great role in influencing osmotic pressure. The crystalloid sugar solution, which has a lower molecular weight, has more moles than the larger colloidal gelatin solution given a constant volume. Therefore, the difference in molarity accounts for the differences in rate and extent of osmosis in the two set-ups where the sugar solution had a faster and greater rate of osmosis than the gelatin solution. VII.

References

Hall, John E. Guyton and Hall Textbook of Medical Physiology. 12th Edition. 2010. Koeppen Bruce M., Stanton Bruce A. Berne & Levy Physiology. 6th Updated Edition. 2009. “Lab #5: Osmosis, Tonicity and Concentration.” Indiana University. PDF File. Accessed 14 June 2013. < http://www.indiana.edu/~nimsmsf/P215/p215notes/LabManual/Lab5.pdf>

Nelson, David L., Cox Michael M. Lehninger Principles of Biochemistry. 5th Edition. 2008. “Osmosis.” Art. Britannica Online for Kids. Web. Accessed 14 June 2013. . Reasoner, Amanda. “Teaching Osmosis and Diffusion through Kidney Dialysis.” Yale-New Haven Teachers Institute. Web. Accessed 12 June 2013. < http://www.yale.edu/ynhti/nationalcurriculum/units/2011/7/11.07.07.x.html>