Membranes in biological cells are composed of two-tailed phospholipid molecules with hydrophilic heads and hydrophobic tails. These molecules can move parallel to the surface of the membrane, but not perpendicular to it. So it is like a 2D liquid. A diagram of how the lipids are arranged in a membrane is shown in the figure at the right.

One of the interesting questions is: Why are membranes made up of two-tailed lipids instead of single-tailed ones? They're more difficult to make and one might expect evolution to select against them.

Source: Wikimedia Commons

One way to see why is to consider the fact that the tails want to keep out of water. In principle, a lipid could just slip out of the membrane -- "evaporating" from it, as it were. But that would cost some energy. Putting one of the hydrophobic tails in water costs about 50 kJ/mol. Putting a double-tailed membrane in water costs twice as much.

1. A phospholipid molecule could get that energy from the thermal energy of the surrounding liquid by thermal fluctuations (check out the Boltzmann factor). What is the ratio of the probability that a 2-tailed lipid will evaporate into the water surrounding the membrane compared to the probability that a 1-tailed lipid would as a result of thermal fluctuations? Explain your reasoning.
why

2. Some lipid molecules could be expected to evaporate from a membrane over the life of a membrane. Explain why having membranes made up of two-tailed lipids is evolutionarily favored compared to having them made up of one-tailed lipids.