Lab Report

Observations involving the movement of particles across a membrane Joshua Huddleston, Erika Kight, Leeann Wilt BIOL 152-28 Jessica Sprague 10/20/2010

Introduction

Movement is key to life. From entire clusters of galaxies to the microscopic electron, the unending dance across space creates the known (and unknown) universe. Through movement, particles attempt to achieve balance. At the molecular level, particles in a solution move from a level of high concentration to a level of low concentration to reach equilibrium in a process known as diffusion (White & Campo 2008). The area along which the concentration conce ntration of particles decreases is known as the con centration gradient (Campbell et al. 2008). A specific form of diffusion, called osmosis, involves the movement of relatively small water molecules across a membrane whe n the larger solute  particles cannot pass through (Mader 2007). In an isotonic environment, the the number of   particles inside and outside of the membrane are at equilibrium, and while particles continue to move, there is no net shift in either direction. In a hypotonic h ypotonic solution, there are more particles inside the membrane than outside. If the membrane allows the solute  particles to pass, they will move outside of the membrane to achieve balance. If it is impermeable to the membrane, water will move into the cell instead. In a hypertonic environment, more particles exist outside the membrane than inside it. Particles will move into the membrane if the barrier allows it, if not, water will travel outside the membrane in attempt to reach equilibrium (White & Campo 2008). An experiment w as conducted involving dialysis bags used as a membrane placed in various solutions. The  purpose to the experiment was to determine what type of environment the dialysis bags were in, as well as how this affects the movement of particles into our out of the dialysis  bags.. The null hypothesis is the difference in concentration gradient will not affect the

Observations involving the movement of particles across a membrane Joshua Huddleston, Erika Kight, Leeann Wilt BIOL 152-28 Jessica Sprague 10/20/2010 weight of the dialysis bag. The alternate hypothesis is that the greater the concentration gradient, the greater the change in weight of the dialysis bag. Materials and Methods

Four dialysis bags were each tied off at one end. Bag 1 was filled with 10mL of  water, bag 2 with 10mL of solution A, bag 3 with 10mL of solution B, and bag 4 was filled with 10mL of solution C. One solution was 50% sucrose, another 25% sucrose, and one contained no sucrose, but it was unknown which solution was which. The bags were then tied off and each weighed to the nearest 0.1g. Three beakers were filled with 300mL of solution C, and a fourth beaker was filled with 300 mL of solution B. Bags 1,2, and 3 were placed in beakers containing solution C, and bag 4 was placed in the beaker  containing solution B. After 7 minutes, each bag was removed and weighed. This continued at intervals of 7 minutes for 35 minutes. Changes in weight were recorded at each interval for each bag. The control was bag 1 in solution C, as it contained only water. The independent variable was the concentration gradient, and the resulting dependent variable was the movement of particles into or out of the bag, directly observed by measuring the weight of the bag. Results

The data, illustrated by the graph created below utilizing the measurements collected, indicates that there was little to no net movement of particles in bag 1 (the control group), with a starting weight of 12.5 g and an ending weight of 13.1 g for a net gain of 0.6 g. Bags 2 and 3 both gained weight over the course of the time trials (each an interval of 7 minutes), with bag 3 at a slightly increased overall rate, with a total gain of  5.4 g while bag 2 gained a total of 5.1 g. Bag 4 lost weight after trial 1, the weight steadily decreasing from 11.5 g to 8.6 g for an overall loss of 2.3 g.

Observations involving the movement of particles across a membrane Joshua Huddleston, Erika Kight, Leeann Wilt BIOL 152-28 Jessica Sprague 10/20/2010

Discussion

The lack of change in weight of the control containing water (bag 1) indicates that it was subjected to an isotonic environment. This leads to the assumption that solution C is water or another solution not containing sucrose. Because bags 2 and 3 gained weight over the course of the experiment and were in solution C, already confirmed to have the same tonicity as water, bags 2 and 3 were in a hypotonic environment. The bags gained weight as a result of the movement of water into the bags, meaning that the bags were selectively permeable to the sucrose molecules. If the bags did allow sucrose molecules to pass, the bags would have lost weight as a result of sucrose moving into the outside solution (White & Camp 2008). This means that osmosis, not simple diffusion, was responsible for the reaction to the concentration gradient. Because bag 3 had a larger net gain of weight and a higher overall rate, it can be confirmed that solution A was 50% sucrose and solution B was 25% sucrose. Bag 4 lost weight as water left the cell because it was in a hypertonic environment. Water left the cell through osmosis in order to equalize the concentrations of particles inside and outside of the dialysis bag (Campbell et al. 2008). The experiment confirmed previous findings on the behavior of solutions in environments of various relative tonicities. Water moved into and out of the cell when the sucrose molecules could not, illustrating the concepts of osmosis (Mader 2007). The higher the sucrose concentration in relation to the solution inside or outside of the bag , the faster the bag gained or lost weight. For instance, bag 3 (containing 50% sucrose) was  placed into solution C (containing no sucrose), causing it to gain 5.4 g in the form of  water. This causes the null hypothesis to be rejected and the conclusion to be reached that

Observations involving the movement of particles across a membrane Joshua Huddleston, Erika Kight, Leeann Wilt BIOL 152-28 Jessica Sprague 10/20/2010 the greater the concentration gradient, the greater the rate of osmosis and the overall net change of weight of the bag. Sources of error include the possibility of a leaking bag or one that was not tied off adequately, which would destroy the selectively permeable membrane and render  results inaccurate. Residual solution may not have been completely wiped off each time the bags were to be measured, resulting in inexact measurements. Other groups may have also failed to dry off their bags adequately, resulting in residual solution on the scales used to weigh the bags. Finally, bags may have not been completely submerged in solution, resulting in incomplete exposure to the environment and inadequate surface area for which osmosis to work to its fullest potential. The biological significance of this experiment is that the cell, with a selectively  permeable membrane, must be maintained in a fairly isotonic environment or will risk  gaining too much water and bursting in a hypotonic environment, or losing too much water in a hypertonic environment and shrink to the point of lysing. A real world application would be the treatment of a patient with generalized dehydration. A hypotonic of hypertonic solution should not be used, as it would cause a shift in the  balance of fluid between the tissues and the vascular system. A solution such as 0.9% normal saline should be infused, as it is isotonic and would allow for hydration of the  body without causing a significant change in the balance of fluid. An extension of the experiment would be to conduct the trials with the solutions at different temperatures and see if a hotter or colder environment would affect the rate of osmosis. Literature Cited

White, M. E. and Campo, F. M. 2008. Investigations in Biology, 4th ed. The McGraw-Hill Co. Inc., New York, NY, USA.

Observations involving the movement of particles across a membrane Joshua Huddleston, Erika Kight, Leeann Wilt BIOL 152-28 Jessica Sprague 10/20/2010 Mader, S. S. 2007. Selections from Biology, 9th ed. The McGraw-Hill Co. Inc., New York, NY, USA. Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. 2008. Biology, 8th ed. Pearson Benjamin Cummings, San Francisco, CA, USA.