Essential Cell Biology 5th edition

390 CHAPTER 12 Transport Across Cell Membranes
Figure 12–1 Cell membranes contain
specialized membrane transport proteins
that facilitate the passage of selected
small, water-soluble molecules. (A) Proteinfree,
artificial lipid bilayers such as liposomes
(see Figure 11–13) are impermeable to
most water-soluble molecules. (B) Cell
membranes, by contrast, contain membrane
transport proteins (light green), each of
which transfers a particular substance across
the membrane. This selective transport can
facilitate the passive diffusion of specific
molecules or ions across the membrane
(blue circles), as well as the active pumping
of specific substances either out of (purple
triangles) or into (green bars) the cell.
For other molecules, the membrane is
impermeable (red squares). The combined
action of different membrane transport
proteins allows a specific set of solutes
to build up inside a membrane-enclosed
compartment, such as the cytosol or an
organelle.
SMALL,
NONPOLAR
MOLECULES
SMALL,
UNCHARGED
POLAR
MOLECULES
LARGER
UNCHARGED
POLAR
MOLECULES
IONS
O 2
CO 2
N 2
steroid
hormones
H 2 O
ethanol
glycerol
amino acids
glucose
nucleosides
H + , Na +
K + , Ca 2+
CI – , Mg 2+
HCO – 3
artificial
lipid
bilayer
Figure 12–2 The rate at which a solute
crosses a protein-free, artificial lipid
bilayer by simple diffusion depends on
its size and solubility. Many of the organic
molecules that a cell uses as nutrients (red)
are too large and polar to pass efficiently
through ECB5 an e12.02/12.02
artificial lipid bilayer that does
not contain the appropriate membrane
transport proteins.
(A) protein-free, artificial
lipid bilayer (liposome)
PRINCIPLES OF TRANSMEMBRANE TRANSPORT
ECB5 e12.01/12.01
(B) cell membrane
As we saw in Chapter 11, the hydrophobic interior of the lipid bilayer
creates a barrier to the passage of most hydrophilic molecules, including
all ions. These molecules are as reluctant to enter a fatty environment
as hydrophobic molecules are reluctant to interact with water. But cells
and organelles must allow the passage of many hydrophilic, water-soluble
molecules, such as inorganic ions, sugars, amino acids, nucleotides,
and other cell metabolites. These molecules cross lipid bilayers far too
slowly by simple diffusion, so their passage across cell membranes must
be accelerated by specialized membrane transport proteins—a process
called facilitated transport. In this section, we review the basic principles
of such facilitated transmembrane transport and introduce the various
types of membrane transport proteins that mediate this movement. We
also discuss why the transport of inorganic ions, in particular, is of such
fundamental importance for all cells.
Lipid Bilayers Are Impermeable to Ions and Most
Uncharged Polar Molecules
Given enough time, virtually any molecule will diffuse across a lipid
bilayer. The rate at which it diffuses, however, varies enormously depending
on the size of the molecule and its solubility properties. In general,
the smaller the molecule and the more hydrophobic, or nonpolar, it is, the
more rapidly it will diffuse across the lipid bilayer.
Of course, many of the molecules that are of interest to cells are polar
and water-soluble. These solutes—substances that, in this case, are dissolved
in water—are unable to cross the lipid bilayer without the aid of
membrane transport proteins. The relative ease with which a variety of
solutes can cross a lipid bilayer that lacks membrane transport proteins
is shown in Figure 12–2.
1. Small, nonpolar molecules, such as molecular oxygen (O 2 , molecular
mass 32 daltons) and carbon dioxide (CO 2 , 44 daltons), dissolve
readily in lipid bilayers and therefore diffuse rapidly across them;
indeed, cells depend on this permeability to gases for the cell respiration
processes discussed in Chapter 14.
2. Uncharged polar molecules (those with an uneven distribution of
electric charge) also diffuse readily across a bilayer, but only if they
are small enough. Water (H 2 O, 18 daltons) and ethanol (46 daltons),
for example, cross at a measurable rate, whereas glycerol (92 daltons)
crosses less rapidly. Larger uncharged polar molecules, such
as glucose (180 daltons), cross hardly at all.
3. In contrast, lipid bilayers are highly impermeable to all charged
substances, including all inorganic ions, no matter how small. The