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Computer Interfacing

The Effect of Alcohol on Biological Membranes


The primary objective of this experiment is to determine the stress that various alcohols have on biological membranes. Membranes within cells are composed mainly of lipids and proteins and often serve to help maintain order within a cell by containing cellular materials. Different membranes have a variety of specific functions.

One type of membrane-bound vacuole found in plant cells, the tonoplast, is quite large and usually contains water. In beet plants, this membrane-bound vacuole also contains a water-soluble red pigment, betacyanin, that gives the beet it's characteristic color. Since the pigment is water soluble and not lipid soluble, it remains in the vacuole when the cells are healthy. If the integrity of a membrane is disrupted, however, the contents of the vacuole will spill out into the surrounding environment. This usually means the cell that made the vacuole is dead.

In this experiment, you will test the effect of three different alcohols, methanol, ethanol, and 1-propanol, on membranes. Ethanol is found in alcoholic beverages. Methanol, sometimes referred to as wood alcohol, can cause blindness and death. 1-propanol is fatal if consumed. One possible reason why they are so dangerous to living organisms is that they might damage cellular membranes. Methanol, ethanol, and 1-propanol are very similar alcohols, differing by the number of carbon and hydrogen atoms within the molecule. Methanol, CH3OH, is the smallest, ethanol, CH3CH2OH, is intermediate in size, and 1-propanol, CH3CH2CH2OH, is the largest of the three molecules.

If beet membranes are damaged, the red pigment will leak out into the surrounding environment. The intensity of color in the environment should be proportional to the amount of cellular damage sustained by the beet.

To measure the color intensity, you will be using the colorimeter shown in Figure 1. In this device, blue light from the LED light source will pass through the solution and strike a photocell. The alcohol solutions used in this experiment are clear. If the beet pigment leaks into the solution, it will color the solution red. A higher concentration of colored solution absorbs more light and transmits less light than a solution of lower concentration. The computer-interfaced colorimeter monitors the light received by the photocell as either an absorbance or a percentage transmittance value.

Figure 1

You are to prepare six solutions of differing alcohol concentrations (0%, 20%, 40%, 60%, 80%, and 100%) for each of the three alcohols. A small piece of beet is placed in each solution. After ten minutes, each alcohol solution is transferred to a small, rectangular cuvette that is placed into the colorimeter. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. The absorbance is directly related to the amount of red pigment in the solution. By plotting the percent alcohol vs. the amount of pigment (that is, the absorbance), you can assess the amount of damage various alcohols cause to cell membranes.

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