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
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