C h a p t e r 1
Cell Structure and Function
13
proteins with their ligand-binding site on the outer surface
of the cell membrane. Instead of having a cytosolic domain
that associates with a G protein, their cytosolic domain
either has intrinsic enzyme activity or associates directly
with an enzyme. There are several classes of enzyme-
linked receptors, including one widely used in hormonal
control of cell function. The binding of the hormone to
a special transmembrane receptor results in activation of
the enzyme adenylyl cyclase at the intracellular portion of
the receptor. This enzyme then catalyzes the formation of
the second messenger cAMP, which has multiple effects
on cell function. Insulin, for example, acts by binding to
an enzyme-linked receptor (see Chapter 33).
Ion Channel–Linked Receptors.
Ion channel–linked
receptors are involved in the rapid synaptic signaling
between electrically excitable cells. Many neurotransmit-
ters mediate this type of signaling by transiently opening
or closing ion channels formed by integral proteins in the
cell membrane (to be discussed). This type of signaling is
involved in the transmission of impulses in nerve and mus-
cle cells.
Intracellular Receptors
Some messengers, such as thyroid hormone and steroid
hormones, do not bind to membrane receptors but move
directly across the lipid bilayer of the cell membrane and
are transported to the cell nucleus, where they influence
DNA activity (see Chapter 31). Many of these hormones
bind to a receptor within the cytoplasm, and the receptor–
hormone complex enters the nucleus. There it binds to
DNA, initiating processes that increase the production
of proteins that alter cell function.
MembraneTransport Mechanisms
The lipid layer of the cell membrane serves as a bar-
rier against the movement of water and water-soluble
substances between the intracellular and extracellular
fluids, while allowing a few lipid-soluble (e.g., alco-
hols with lower numbers of hydrocarbons, oxygen,
nitrogen) and uncharged molecules (glycerol, water) to
cross the cell membrane by simple diffusion. The cell
membrane also contains large numbers of protein mol-
ecules, many of which insert completely through the
membrane (Fig. 1-10). Most ions and small molecules
rely on these proteins for transport.
Different membrane proteins function in different
ways. For example,
channel proteins
form water-lined
passageways through the membrane and allow free
movement of water as well as selected ions or molecules.
Membrane transport proteins
bind molecules or ions and
undergo a series of conformational changes to transfer
the bound solute across the membrane. Some transport
proteins, called
uniporters
, simply mediate the move-
ment of a single solute from one side of the membrane
to the other, whereas others function as coupled trans-
porters in which the transfer of one solute depends on
the transfer of a second solute (Fig. 1-11). This coupled
transport involves either the simultaneous transport in
the same direction, performed by transporters called
symporters
, or the transport of a second solute in the
opposite direction, by transporters called
antiporters
.
All channels and many transporters allow solutes to
cross the membrane only passively by
passive transport
or
facilitated diffusion
. Cells also require transport pro-
teins that actively pump certain solutes across the mem-
brane against an electrochemical gradient, in a process
called
active transport
. Active transport is directional
and requires an energy source such as ATP. The cell mem-
brane can also engulf substances, forming a membrane-
bound vesicle; this vesicle is brought into the cell by
endocytosis
. The process by which cellular vesicles fuse
to the cell membrane releasing contents outside of the
cell is called
exocytosis
.
Many integral transmembrane proteins form the ion
channels found on the cell surface. These channel pro-
teins have a complex morphology and are selective with
respect to the substances that can transverse the channel.
Diffusion
through
lipid bilayer
Diffusion
through a
channel
Passive transport
Facilitated
diffusion
Active transport
Carrier
protein
Channel
protein
Vesicular transport
ATP ADP
Lipid
bilayer
Concentration
gradient
FIGURE 1-10.
Mechanisms of membrane transport. Passive transport represents the net movement
from a region of higher to lower concentration and active transport (energy requiring) from a region
of lower to higher concentration. Vesicular transport involves the formation of membrane-enclosed
vesicles or sacs that serve as transport vehicles from extracellular materials that are being moved
into the cell (endocytosis) or intracellular materials that are being moved out of the cell (exocytosis).
ADP, adenosine diphosphate; ATP, adenosine triphosphate.
