DIRECTING TRAFFIC 
ACROSS THE SURFACE 
MEMBRANE 
The oily lipids of a cell's surface 
membrane serve admirably to pre- 
vent the cell's water-based contents 
from leaking out. However, in 
"solving" this problem, the cell is 
confronted with another — how to 
transport wastes and cell products 
out of the cell and allow nutrients 
and other substances in, without 
either shrinking or swelling too much. 
Over eons, cells have evolved a 
wide variety of transport mechanisms 
to ferry substances across the hydro- 
phobic barrier. Transport may be 
either "passive," which requires no 
energy, or "active," which uses ATP. 
Also, a molecule may either pass 
directly through the lipid layer or it 
may be carried in by a surface 
protein in a process called receptor- 
mediated endocytosis. (Endocytosis 
can also occur without the involve- 
ment of a surface protein. In both 
styles of endocytosis, a bit of the 
surface membrane folds inward 
around the entering particle, then 
pinches off and carries the particle 
into the cell. The opposite process, 
exocytosis, occurs when vesicles 
moving from the cell's interior fuse 
with the surface membrane and spill 
their contents outside of the cell.) 
The method used to import or export 
substances depends on a combina- 
tion of the transported item's size, 
chemical composition, electrical 
charge, and abundance (concentra- 
tion), as well as on its ability to 
dissolve in lipids. 
Oxygen, nitrogen, and other small 
molecules that can dissolve easily in 
lipids move readily back and forth 
across the bilayer. Importantly, 
because of its small size and the 
distribution of its electrical charge, 
a water molecule can also pass rela- 
tively easily through the membrane 
even though water is quite insoluble 
in oil. 
In contrast, large molecules, such 
as proteins and sugars, cannot pass 
through the lipid bilayer. A variety 
of transport systems, many of which 
involve surface proteins, are used to 
ferry these substances into and out of 
the cell. Surface membrane lipids 
are also highly impermeable to all 
ions, no matter how small they are. 
ATP-requiring protein "pumps" are 
employed to transport ions. 
One well-studied pump system is 
the sodium-potassium pump. This 
membrane protein consumes more 
than a third of the cell's total ATP 
production in an endless cycle of 
pumping sodium ions out of the cell 
while drawing potassium ions in. 
This frantic work results in an "ion 
gradient" in which there is a high 
concentration of sodium ions outside 
the cell and a high concentration of 
potassium ions inside. This creates a 
source of potential energy analogous 
to the potential energy created when 
water is held behind a dam. If the 
dam is lowered, water will flow over 
it and can be used to turn turbines or 
do other work. In the same way, if 
the membrane pumps momentarily 
permit an inward flux of sodium ions 
(accompanied by an outward rush of 
potassium ions), a variety of tasks, 
49 
