Question

The effects of temperature and lipid composition on membrane fluidity are often studied by using artificial membranes containing only one or a few kinds of lipids and no proteins. Assume that you and your lab partner have made the following artificial membranes: Membrane 1: Made entirely from phosphatidylcholine with saturated 16-carbon fatty acids. Membrane 2: Same as membrane 1, except that each of the 16-carbon fatty acids has a single cis double bond. Membrane 3: Same as membrane 1, except that each of the saturated fatty acids has only 14 carbon atoms. After determining the transition temperatures of samples representing each of the membranes, you discover that your lab partner failed to record which membranes the samples correspond to. The three values you determined are -36∘C, 23∘C, and 41∘C. Assign each of these transition temperatures to the correct artificial membrane, and explain your reasoning.

Answer 1

The fluidity of the membrane is primarily based upon the tendency of intermolecular forces between the phospholipids' fatty acids. These forces are high between the straight-chain alkanes as they coordinate well with each other that is, enhancing the association between the chains of fatty acids.

These forces are disturbed by cis double bonds as they result in kinks in the fatty acid chains that is, inhibiting them from getting lined up well. To some extent, the length of the chain also predicts fluidity. The longer fatty acids can produce more associations with each other, thus, they are less fluid in comparison to the shorter fatty acids that produce lesser associations.

Of the three membranes, the least fluid will be membrane 1 comprising the saturated 16-carbon fatty acids. It will be solid at the highest temperature, therefore, membrane 1 will exhibit a transition temperature of 41 degrees Celsius.

The most fluid membrane will be membrane 2 as it comprises unsaturated 16-carbon fatty acids. This is due to the fact that double bonds result in the enhancement of fluidity. Thus, this membrane will be solid at the lowest temperature, hence, this membrane will exhibit a transition temperature of -36 degrees Celsius.

The membrane 3 will exhibit a transition temperature of 23 degrees Celsius due to the presence of shorter chain length in comparison to membrane 1. However, its transition temperature is more than membrane 2 as membrane 2 exhibit number of double bonds.