Shannon Hastings. Movement of Molecules Across Cell Membranes Lab Report. Lab Section 2

Transcription

1 Shannon Hastings Movement of Molecules Across Cell Membranes Lab Report Lab Section 2

2 Introduction The main objective of this lab was to give us an understanding and knowledge about osmosis and diffusion in living cells while being exposed to different extracellular solutions. The lab goals also included to test for the movement of molecules and determine the direction of diffusion in or out of the dialysis bag by using indicator chemicals. Some of the new topics introduced in this lab were diffusion, osmosis, high/low concentration areas, and selectively permeable membranes. Diffusion is the movement of molecules through a selectively permeable membrane and osmosis is the movement of water through a semi permeable membrane. High/low concentration areas can be explained as the vicinities where more or less molecules are located. Semi permeable membranes are the wall that (dis)allows molecules to pass through. (Definitions were taken from the glossary in the Essential Biology with physiology textbook.) My hypothesis for this lab was that the salt and sugar would diffuse through the dialysis bag and starch would stay in the bag. I also thought that the water would osmose into the bag from the surrounding cup and cause the bag to gain weight. My hypothesis was correct except for the sugar; it did not diffuse out of the bag. Materials and Methods This lab was broken into four sections, Diffusion and osmosis, Controls for the diffusion and osmosis experiment, Completing the diffusion and osmosis experiment, and Osmosis in Elodea cells In the first part, we collected a plastic cup, one piece of dialysis tubing and two twist ties. After getting the wet dialysis tubing, I folded the tube vertically and horizontally and then tied it. I opened the other end of the tube, my partner inserted eight full droppers of stock solution into the tube, and then I tied the other end the same way I tied the other. My partner and I then weighed the bag at 24.3g on a digital balance. Next, we inserted the dialysis bag into a cup

3 of water and waited 30 minutes to finish the lab so that the solutions and water would have a chance to mix. During this time, we completed comprehension check and the next activity. The next part of the lab allows us to determine which molecules will be able to cross the dialysis bag membrane. We used Iodine to test for the presence of starch (the solution will turn black if there is Iodine); Silver Nitrate to test for the presence of chloride ions (the solution will turn cloudy white if there are salt ions present); and we used Benedict to test for sugar presence (the solution will turn either green, yellow, orange or red if there is sugar present). We used a large beaker, filled it one-third with water, and heated it over a burner until almost boiling. We then gathered six test tubes, a test tube rack and a test tube holder for the experiment. There were two tubes (a control and experimental tube) set up to test for salt, sugar or starch. We filled three tubes with the stock solution and added one dropper full of each test solution to the appropriate tube. Next, we lowered each tube into the hot water and waited to see if there were any changes in the solutions color. The results can be found in the next section. The last part of the lab was to determine osmosis with the dialysis tubing bag. First, I removed the bag from the water and weighed it the same way I did initially. The bag gained 0.7g, with the final weight being 25g. My hypothesis was correct about weight change and the percent weight change was 2.88%. I believe that the water moved through the membrane to cause the weight change. We then filled the other three with water from the cup that held the dialysis tubing and tested the tubes the same as in the previous section. The results can also be found in the next section. The last part of the lab consisted of us comparing two elodea cells. The cells were prepared as wet mount slides, one was made with culture water, and the other was made with 20% saline solution. The results can be found in the next section.

4 Results The results from the first experiment can be found in the following table: Indicator Solution Shows Presence of Initial Color Final Color Results (+ or -) Silver Nit. Test Cl+ ions Clear Cloudy White + Control Tube (Salt) Iodine Test Starch Brown Black + Control Tube Benedict s Test Sugar Cloudy Blue Orange + Control Tube Silver Nit. Test Cl+ ions White White + Exper. Tube (Salt) Iodine Test Starch Light Brown Light Brown - Exper. Tube Benedict s Test Sugar Clear Blue Orange + Exper. Tube Discussion For the first activity (before the experiment), I determined the following in the comprehension check: (1) The glucose solution consists of 10% glucose and 90% water; (2) The highest concentration of salt, sugar and starch is inside the bag while the highest concentration of water is outside the bag; and (3) If the dialysis bag gains weight during the experiment, I ll know that water entered the bag through osmosis and that if the bag loses weight during the experiment, the water exited the bag through osmosis. Before the second activity, I answered the following questions: (1) if starch leaves the dialysis tubing bag, I expect that iodine test results from the water in the jar to be positive, (2) if sugar doesn t leave the dialysis bag, the Benedict s test results on the water will be negative, and (3) if salt leaves the dialysis bag, Silver nitrate test results on the water will be positive. According to the test results in the table above, Cl+ ions and sugar were the only molecules that were able to diffuse out of the dialysis bag. All of the molecules (salt, sugar and starch) did not leave the dialysis bag because the starch molecules were too big to diffuse through the semi permeable membrane. The experiment illustrated the concept of dialysis because some molecules

5 (sugar and salt) diffused through the selectively permeable membrane while starch did not which is similar to the dialysis process of cleaning the blood and extracting certain blood minerals. While examining the elodea cells, I determined that the cell membrane was only visible in 20% saline solution. The culture water proved to be the control because it was used to test against the 20% saline solution. Osmosis caused the difference in appearance of the cells. Also, the cell wall prevented the elodea cells from shriveling or exploding during the experiment. References My two resources for this lab report were the lab manual and class textbook stated below. Thinking about Biology, An Introductory Laboratory Manual: Fourth Edition Essential Biology with Physiology: Fourth Edition