fibers somewhat like hot-air balloons 
are tethered by guy wires before 
liftoff. 
The lipids that make up the bulk of 
a cell's surface membrane fall into 
three classes: phospholipids, steroids 
(primarily cholesterol), and glycol i p- 
ids. About half of the molecules in 
an average membrane are phospho- 
lipids. Each phospholipid molecule 
has a water-seeking (or hydrophilic) 
phosphate "head" and two flexible, 
water-avoiding (or hydrophobic) lipid 
"tails." In a surface membrane, 
phospholipids spontaneously arrange 
themselves into a bilayer (double 
layer) with phosphate heads touching 
the watery interior and exterior of the 
cell, and lipid tails buried in the 
middle of the layer. 
Cholesterol is abundant in many 
animal cell membranes; sometimes 
there is as much as one cholesterol 
molecule for every phospholipid. 
Cholesterol is a rigid molecule that 
gives the surface membrane strength. 
It is manufactured within the cell (on 
the endoplasmic reticulum) and is 
also brought into the cell from the 
blood. Cholesterol is present only in 
animal cells; plant cells are stiffened 
by a very rigid cell wall composed 
mainly of cellulose. 
Glycolipids are composed of a 
sugar ("glyco" is derived from the 
Greek word for sweet) and a lipid 
portion, and make up about 5 
percent of the lipid population. A 
person's blood group (O, A, B, or 
AB) is determined by the particular 
kind of glycolipids present on the 
surface of his or her red blood cells. 
The surface membrane of a cell play' many roles, including keeping 
47 
