Although such tripartite pro- 
teins were unknown at the 
time Singer and Nicolson 
proposed their model, they 
have since been shown to play 
important roles in a number 
of biological processes, 
including those that involve 
the transport of molecules 
into the cell. Many other 
membrane proteins that are 
attached either to the inner 
or outer face of the surface 
membrane have also been 
studied in detail in the years 
since Singer and Nicolson 
proposed their so-called 
fluid-mosaic model. 
The lipids that make up the 
bulk of a cell’s surface mem- 
brane fall into three classes: 
phospholipids, steroids 
(primarily cholesterol), and 
glycolipids. About half of the 
molecules in an average 
membrane are phospholipids. 
Each phospholipid molecule 
has a water-seeking phosphate 
Lipid “ tail ” 
“head” and two flexible, 
water-avoiding lipid “tails.” 
In a surface membrane, 
phospholipids spontaneously 
arrange themselves into a 
double layer with the phosphate 
heads touching the watery 
interior and exterior of the 
cell, and the lipid tails buried 
in the middle of the layer. 
Cholesterol is a rigid molecule 
that helps stabilize the mem- 
branes of animal cells. It is 
manufactured within the cell 
(in 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. 
Proteins 
is. Surface 
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 (0, A, 
B, or AB) is determined by 
the particular kind of 
glycolipids present on the 
surface of his or her red 
blood cells. 
45 
ig keeping the 
suostances to 
enter and leave the cell. 
