MCB 21l - Lab 1 Report
MCB 21l - Lab 1 Report
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This 3 page Reader was uploaded by Sami Smith on Friday February 14, 2014. The Reader belongs to a course at a university taught by a professor in Fall. Since its upload, it has received 227 views.
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Date Created: 02/14/14
Movement of Electrolytes Across an Artificial Membrane I Introduction Background When there are solutions of different concentrations of ions on opposite sides of a membrane the concentrations of each attempt to reach equilibrium If the membrane is permeable to only one ion the concentration for that ion still reaches equilibrium across the membrane For every permeable ion that passes through the membrane one of the opposite charge is left behind The opposite charges of the two ions still attract each other through the membrane preventing further diffusion of the permeable ion Equilibrium is reached and there will be a slight positive charge on side and a slight negative charge on the other The voltage difference is a condition called the Nernst potential which can be described through the equation EX RTA EX Nernst potential or equilibrium voltage for ion X across the membrane R gas constant T Temperature Z charge of the ion X A concentration of permeable ion on side A XB concentration of permeable ion on side B Hypothesis The voltage difference across the membrane will change when two solutions of different concentrations are placed on either side of a membrane permeable to only one of the ions The difference will change according to the equation for the Nernst potential II Materials and Methods One cation selective membrane and one anion selective membrane were used Each one was placed between two hollowed Lucite blocks clamped together by a vise Solutions with different concentrations were placed on each side of the membrane and the voltage difference across the membrane was measured through calomel electrodes The voltage difference was recorded using a hand held volt meter III Results Semipermeable Membrane Experiments The tables attached at the end of the lab report show the potential differences found with both membranes for sodium chloride and with the cationpermeable membrane for magnesium sulfate The attached graph plots the data from the tables and ts a linear regression line for each curve Analysis The table below compares the values for the slope and Yintercept obtained from the linear regression in the graph attached to those calculated based on the temperature in which the experiments were conducted That temperature was 25 C From Regression Calculated Cation membrane NaCl Slope 38088 5877 YIntercept 88611 127889 Anion membrane NaCl Slope 31661 5877 YIntercept 77506 127889 CationAnion membrane MgSO4 Slope 27596 2939 YIntercept 62821 63 95 5 Simulation Experiments A computer model of a cell membrane was created in Excel to see how it would react under different conditions Either the permeabilities or the concentrations were changed to see how it would affect the change in the Nernst potential and the change in the membrane potential The two tables below show the results of the simulation under the different changes Changing the Permeabilities PNa PK EK ENa Em 1 10000 98 67 98 10 10000 98 67 97 1000 10000 98 67 57 10000 10000 98 67 3 100000 10000 98 67 44 1000000 10000 98 67 63 Changing the ionic concentrations Na inside Naouts1de Kms1de Koutside ENa EK Em 12 145 155 4 67 95 98 12 145 155 20 67 55 55 60 145 155 4 24 98 98 Three main points can be gathered from the simulation Changing the permeabilities only affects the membrane potential Changing the ionic concentrations only affects the Nernst potential for the speci c ion that is being altered Changing the ionic concentrations only affects the membrane potential if the membrane is permeable to the ion with an altered concentration IV Discussion The results from the experiments con rmed that the potential difference across the membrane will change when the concentrations are altered and the membrane is selectively permeable to only one ion The measured potential differences are close to the theoretical potential differences obtained from the Nemst equation and the graph of the potential differences versus the logarithm of permeant ion shows this as the points can be fitted with a straight line This suggests that there can be a potential difference across a membrane if the membrane is permeable to only one charge Cell membranes come across more many different kinds of ions and have ion channels to control the in ux and ef ux Having selectively permeable membranes allows them to control this while creating potential differences at the surfaces This energy can be used by the cell for other functions The slopes and the yintercepts from the regression line did not match the theoretical calculations The differences could be attributed to the membranes used One of the membranes yielded data sporadically and another was replaced because it yielded no data The calomel electrodes also yielded data sporadically and were also replaced The inconsistency in lab materials could have affected the measured slopes and yintercepts A solution to this problem might have been to replace the equipment immediately so that all measurements were taken using the same materials This might also lead to more accurate measurements as there would be more time to do each one The testing of the equipment took up time that could have been used toward taking the measurements more carefully Further causes may have resulted from the setup of the vise and residual substances The blocks were difficult to position so that they were touching the bottom of the vise and aligned with each other The equipment might have had traces of other substances outside of the experiment Indeed the measuring process yielded more data after rinsing the materials after every measurement There could be another experiment using real membranes from different areas of the body For example membranes lining different organs could be used The membranes could be tested to see if the potential differences for certain ions are the same throughout the body Variations in the potential differences might be explained by the conditions in the body facing the membrane
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