DIRECTING TRAFFIC ACROSS THE SURFACE MEMBRANE 
46 
he oily lipids of a 
cell’s surface 
membrane serve 
admirably to 
prevent the cell’s 
water-soluble contents from 
leaking out. However, in pre- 
venting such leaks, the cell is 
confronted with another 
problem — 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 sub- 
stances across the membrane. 
Transport may be either “pas- 
sive,” which requires no 
energy, or “active,” which 
uses ATP. Also, a molecule 
may either pass directly 
through the membrane (usually 
through a pore created by a 
specific transmembrane pro- 
tein) or it may be carried in 
when a bit of the surface 
membrane folds inward 
around the entering particle, 
then pinches off and carries 
the particle into the cell. The 
method used to import a sub- 
stance depends on a 
combination of its size, chem- 
ical composition, electrical 
charge, abundance, and abil- 
ity to dissolve in lipids. 
Oxygen, nitrogen, and other 
small molecules that can dis- 
solve easily in lipids move 
readily back and forth across 
the membrane. Importantly, 
because of its small size and 
the distribution of its electri- 
cal charge, a water molecule 
can also pass relatively easily 
through the membrane even 
though water is quite insolu- 
ble in od. 
In contrast, large molecules, 
such as proteins and sugars, 
cannot pass through the 
membrane unassisted. A vari- 
ety of transport systems, 
many of which involve sur- 
face proteins, are used to 
ferry these substances into 
and out of the cell. Surface 
