Video Transcript
Which of the following conditions
is most likely to damage a cell membrane and increase its permeability? (A) Low temperatures, four to 10
degrees Celsius, (B) hypertonic solutions, (C) organic solvents, or (D) polar
solvents.
To answer this question, let’s
first discuss the composition of a cell membrane, as the composition will give us
information about physical and chemical properties of the cell membrane.
Cell membranes are formed from two
layers of phospholipids. A phospholipid is made up of two
distinct parts: the hydrophobic tail and the hydrophilic head. “Hydrophobic” refers to the fear of
water, and “hydrophilic” refers to the affinity to water. As the cells and the cells’
surroundings contain water, hydrophilic phospholipid heads face outward and the
hydrophobic tails are contained within the membrane. Embedded within the two layers of
phospholipids are various molecules, such as different proteins and cholesterol. Let’s now have a look through the
answer choices to see how the listed conditions act on a cell membrane.
Temperature has a significant
impact on the fluidity of cellular membranes. At low temperatures, phospholipids
tend to move less and become more tightly organized in a crystal-like structure. Therefore, low temperatures make
membranes more rigid, which can interfere with key functions, like the passage of
certain substances.
More rigidity also makes membranes
more prone to break. Cholesterol limits this phenomenon
by disrupting the phospholipids’ tight and regular organization with its bulky tail,
thereby increasing membrane fluidity. So, at low temperatures, the
membrane is prone to get damaged by breaking, but only if cholesterol levels within
the membrane are low. In addition, we mentioned that at
low temperatures, the permeability of the membrane is reduced, not increased as the
question states. Answer option (A) is therefore most
probably not the answer option we are looking for.
The hyper- from “hypertonic”
indicates that the solution has a high solute concentration. If a cell is placed in a hypertonic
solution, the rate of movement of molecules from outside the cell to its inside may
be increased. Also, the water movement out of the
cell might be increased. These movements will, however, most
likely not drastically affect membrane permeability. And answer option (B) can be
excluded.
Cell membranes already live in a
polar environment, as water is a polar substance. We already know that the polar
heads of the phospholipids, which make up a cell membrane, face this polar
solvent. So answer option (D) is
incorrect.
This would indicate that it is
organic solvents which likely damage a cell membrane and increase its
permeability. Organic solvents that are less
polar than water, such as alcohols or nonpolar solvents, are indeed capable of
dissolving plasma membranes. An example of this is an
antiseptic. The alcohol disrupts the membranes
of bacterial cells by dissolving the fatty acids of the phospholipids, killing the
bacteria.
Therefore, the correct answer
describing the likely effects of different conditions on increasing the permeability
of the cell surface membrane is (C), organic solvents.
