Measurement of the Alcohols’ Permeability of the Cell Membrane by Calculating Betacyanin Leakage from Spectrophotometric Readings
The polarity of molecules is based on dipole, which is the separation of electric charges in a molecule. Non polar molecules have an equal sharing of electrons or a symmetrical arrangement of polar molecules, for example, an hydrogen atom (H2(g)). There is no net charge. Polar molecules however have an unequal sharing of an electrons or an asymmetrical arrangement of bonds, for example, a molecule of water (H2O(l)). There is a net charge as the more electronegative atom, O2, pulls the electrons towards itself, allowing it to have a slightly negative charge and the hydrogen to have a slightly positive charge. These differences affect the solubility of such substances. Non-polar substances will more readily dissolve in other non-polar substances, and polar will more readily dissolve in other polar substances; like dissolves like. Cells are the most basic unit of life. They are smallest entity of matter than can survive on their own. All cells have certain characteristics in common. These are ribosomes, DNA or genetic material, cell membrane and cytosol. The experiment was based on the features of the cell membrane. The cell membrane surrounds and encloses the cytosol, which is a gel-like substance in which all other cell organelles are suspended. The cell membrane is semipermeable; selecting what goes in and out of the cell. This selectivity is due to the type of molecules that make up the cell membrane – phospholipids. These are structures that contain a hydrophobic (non-polar) lipid tail and a hydrophilic (polar) phosphate head. The most dominant part is the lipid tails as they stretch across to make the membrane, forming a lipid bilayer. Substances that have to go across the membrane must be able to pass these. Since like dissolves like, nonpolar molecules can dissolve in the lipid bilayer and pass through the membrane rapidly. Polar molecules do not cross easily as they only have the small polar phosphate heads to dissolve across, which are far apart from each other as well.
Betacyanin is a red-purple pigment in the vacuole of cells. In the lab it was used to determine the rate of penetration of the cell membrane for each of the alcohols, based on the absorption readings of the betacyanin solution in a spectrophotometer. Light is sent from one end of the equipment through a sample and onto the other side of the equipment which measures the amount of light reaching it. Depending on the sample, it will absorb light of specific wavelengths (due to excitement of electrons in the atoms of its molecules) and hence decreasing the amount of light that will leave the sample. The intensity of the light before it reaches the sample and after it leaves the sample tells how much light was absorbed. So for this experiment, the higher the absorbance reading the more betacyanin was released from the beet root. This means that the specific alcohol dissolved most easily across the cell membrane. The objective of the lab was to test the permeability of the cell membrane to different alcohols by calculating the rate of betacyanin leakage. From least to greatest, it was hypothesized that the methanol would be the least permeable to the cell membrane, hence producing the least amount of betacyanin leakage; and also that it would have the lowest absorbance reading as well. Next would be ethanol, followed by propanol. Finally, butanol was hypothesized to be the most permeable to the cell membrane, hence producing the greatest amount of betacyanin leakage; and also that it would have the highest absorbance reading as well. This is based on the structure of the molecules. Methanol is the most polar of the group of alcohols tested and so will have a hard time getting across the cell membrane’s non-polar lipid bilayer. Ethanol is less polar than methanol due to an additional methyl group (which makes a...
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