BioPHYSICAL chemistry

Properties of lipids described using the chemical potential

The lipids forming cell membranes represent an example of a biological

system that can be divided into different phases. The membranes serve

to establish distinct compartments in the cells. Membranes are complex

assemblies of lipids, proteins, and other components organized in asym-

metric bilayers with the inner and outer components having different

compositions of lipids. The formation of the lipids into bilayers reflects

fundamental thermodynamic properties of the lipids. Lipids are molecules

that contain both a hydrophilic head group and a hydrophobic fatty

acid tail (Figure 4.3). In general, the types of lipids found in membranes

are glycerophospholipids, in which the hydrophobic portion contains

two fatty acids joined to glycerol, and sphingolipids, which have a single

fatty acid joined to sphingosine. In addition, membranes possess sterols

that have a rigid set of hydrocarbon rings. In glycerophospholipids and

some sphingolipids, a polar head group is joined to the hydrophobic por-

tion by a phosphodiester linkage. The polar head group of phospholipids

is usually a common alcohol such as serine, ethanolamine, choline, or

glycerol. The fatty acid chains present in cells commonly are composed

of 16 –18 carbons and may be either saturated or unsaturated.

At low concentrations, the lipids will exist as monomers, but as the con-

centration increases the formation of more complex structures is favored

as a result of hydrophobic interactions. This behavior can be modeled

by considering the chemical potentials of solutions. The amount of each

component in a solution can be described by themole fraction,Xi, which

is defined as the amount of theith component,ni, divided by the total

number of molecules,n:

(4.3)

For an ideal solution, thechemical potential,μi, of theith component is

related to the mole fraction Xiaccording to:

μi=μ^0 i+RTln Xi (4.4)

where the quantity μ^0 iis the standard-state chemical potential which equals

the molar energy of a pure compound:

μi=μ^0 i for Xi= 1 (4.5)

As an example, consider a two-component system, A and B. The chemical

potentials for each component can be expressed in terms of their standard-

state chemical potentials and mole fractions:

X

i

i=

Amount of molecule

Total number of moleculess

=

n

n

i

74 PARTI THERMODYNAMICS AND KINETICS

Polar head group

Hydrocarbon tails

Figure 4.3A
schematic diagram of
a phospholipid.