Question

A cell membrane consists of a lipid bilayer, which is a double layer of phospholipids. Each phospholipid has a hydrophilic phosphate head and a hydrophobic lipid tail. These phospholipids are not chemically bound to one another, but rather maintain their structure due to the fact that the lipid tails do not like water. They're hydrophobic^2. If they all buddy up, then they can all stay dry. (See figure.) One way to deal with this quantitatively is to realize that there is an energy cost associated with a lipid leaving the bilayer and entering the surrounding water. For a single-tailed phospholipid, this energy cost is about 50 kJ/mol. For a double-tailed phospholipid, it's nearly double that. We can model this as a two-state system, wherein lipids can either be in the membrane or out of it. Estimate (to within an order of magnitude) the number of phospholipids in the membrane of an average eukaryotic cell. What is the probability that any individual single-tailed phospholipid would find itself removed from the lipid bilayer due to thermal fluctuations? What is the probability that any individual double-tailed phospholipid would find itself removed from the lipid bilayer due to thermal fluctuations? Why is it that cell membranes consist of double-tailed rather than single-tailed phospholipids?

Answer 1

a) phospholipid is one the important component of the lipid bilayer. There are approximately 5x106 phospholipid molecules in a 1um x 1um area of lipid bilayer.

b) The probability is very high that any single tailed phospholipid would be removed from the lipid bilayer due to thermal fluctuation. Because the thermal forces the bonds in the lipid bilayer is disintegrated.

c) The probability of double tailed phospholipid being removed from the lipid bilayer is comparatively lower than single tailed. The cohesive force between the two tails helps them retain the lipid bilayer structure.

d) The cell membranes contain double-tailed phospholipids because the bond between the double tailed is stronger and the membrane does not get disrupted easily which prevents the cell from rupturing.