THE SURFACE MEMBRANE, VERSATI 
44 
T n the beginning,'’’ 
wrote biologist Gerald 
Weissmann, “there 
must have been a 
membrane! Whatever 
flash of lightning there was 
that organized purines, 
pyrimidines, and amino acids 
into macromolecules capable 
of reproducing themselves, it 
would not have yielded cells 
but for the organizational 
trick afforded by the design 
of a membrane wrapping.” 
Weissmann imagines these 
primitive membranes forming 
bubbles in which the first 
macromolecules were enclosed 
and protected from dissipation 
in the salty primordial seas. 
A cell’s outer membrane is 
often thought of as a bound- 
ary that distinguishes the 
living cell from its surround- 
ings. And, indeed, surface 
membranes are crucial in 
keeping cells intact. Moreover, 
the internal membranes that 
wrap around many organelles 
in eukaryotic cells separate 
the cytoplasm into discrete 
regions, somewhat like the 
walls that form rooms in a 
house. These inner membranes 
enable the cell to perform 
many otherwise incompatible 
biochemical activities simulta- 
neously, thereby greatly 
increasing the cell’s efficiency. 
Yet despite its barrier func- 
tions, the cell membrane — 
which is often less than 0.01 
micrometer thick — is not 
impassive. Rather, it is 
exquisitely sensitive to its 
surroundings and selectively 
allows certain substances to 
enter and leave the cell while 
barring others. It takes in 
nutrients and excretes wastes. 
It sends and receives chemical 
and electrical messages, 
including signals for the cell 
to manufacture proteins or to 
divide. In multicellular 
organisms, it joins with other 
cells to form tissues. 
LE GATEKEEPER 
These myriad abilities are 
due to the membrane’s com- 
position. Although surface 
membranes differ in their 
precise composition depend- 
ing on the cell’s type, and 
although a membrane’s con- 
figuration changes from 
moment to moment, all mem- 
branes are composed of two 
basic kinds of molecules — 
proteins and lipids (fats). 
In 1972, S. Jonathan Singer 
and Garth Nicolson of the 
University of California, San 
Diego proposed a model to 
describe the relationship of 
proteins and lipids in an ide- 
alized membrane. They 
compared the proteins to 
“icebergs floating in a sea of 
lipids,” and suggested that 
some of the proteins are 
folded so that the “tips” poke 
above and below the plane of 
the membrane, while the mid- 
dle of the protein is 
embedded in the membrane 
itself. 
