biological entity and the exterior. For cells, the membrane plays a role in maintaining
interior salt concentrations and pH at appropriate values. The key components of a
cellular membrane are the lipid molecules. Lipids are characterized by a hydrophilic
(polar) head group and a hydrophobic (non-polar) tail group. This amphipathic nature
results in self-assembly into the lipid bilayer structures seen in cells.
A given biological membrane will be composed of a number of different types of these
lipids. Cell membranes in particular are composed of phospholipids.
CH2-hΓfCH2)3
CH2
о
I
O=P-O
I
O
I
CH2-CH-CH2
-O O
I I
choline
phosphate
glycerol
r C=O
I
CH2
CH2
-CH3
I
CH2
CH2
CH
W ----
CH
∖
CH2
∖
CH2
CH3
double
bond
fatty
acids
Figure 1.1: Structure of a Phospholipid molecule. The head group is hydrophilic. The tail
group is comprised of an oily fatty acid group and is thus hydrophobic. The average lipid
molecule is on the order of 2 nm in length.