Essential Cell Biology 5th edition

CHAPTER TWELVE
12
Transport Across Cell
Membranes
To survive and grow, cells must be able to exchange molecules with
their environment. They must import nutrients such as sugars and amino
acids and eliminate metabolic waste products. They must also regulate
the concentrations of a variety of inorganic ions in their cytosol and organelles.
A few molecules, such as CO 2 and O 2 , can simply diffuse across
the lipid bilayer of the plasma membrane. But the vast majority cannot.
Instead, their movement depends on specialized membrane transport
proteins that span the lipid bilayer, providing private passageways across
the membrane for select substances (Figure 12–1).
In this chapter, we consider how cell membranes control the traffic of
inorganic ions and small, water-soluble molecules into and out of the
cell and its membrane-enclosed organelles. Cells can also selectively
transfer large macromolecules such as proteins across their membranes,
but this transport requires more elaborate machinery and is discussed in
Chapter 15.
We begin by outlining some of the general principles that guide the passage
of ions and small molecules through cell membranes. We then
examine, in turn, the two main classes of membrane proteins that mediate
this transfer: transporters and channels. Transporters shift small
organic molecules or inorganic ions from one side of the membrane
to the other by changing shape. Channels, in contrast, form tiny hydrophilic
pores across the membrane through which substances can pass
by diffusion. Most channels only permit passage of ions and are therefore
called ion channels. Because these ions are electrically charged, their
movements can create a powerful electric force—or voltage—across the
membrane. In the final part of the chapter, we discuss how these voltage
differences enable nerve cells to communicate—and, ultimately, to shape
how we behave.
PRINCIPLES OF
TRANSMEMBRANE TRANSPORT
TRANSPORTERS AND THEIR
FUNCTIONS
ION CHANNELS AND THE
MEMBRANE POTENTIAL
ION CHANNELS AND NERVE
CELL SIGNALING