Chapter 3
Cells: The Living Units
81
3
Nitric oxide
(
NO
) is an important signaling molecule even
though it doesn’t act in any of the ways we have already de-
scribed. One of nature’s simplest molecules, NO is made of a
single atom of nitrogen and one of oxygen. It is also an environ-
mental pollutant and the first gas known to act as a biological
messenger. Because of its tiny size, it slips into and out of cells
easily. Its unpaired electron makes it highly reactive and it reacts
with head-spinning speed with other key molecules to spur cells
into a broad array of activities. You will hear more about NO in
the neural, cardiovascular, and immune system chapters.
Role of Voltage-Gated Membrane Channel
Proteins: Electrical Signaling
In
electrical signaling
, certain plasma membrane proteins are
channel proteins that respond to changes in membrane poten-
tial by opening or closing the channel. Such voltage-gated chan-
nels are common in excitable tissues like neural and muscle
tissues, and are crucial to their normal function.
Check Your Understanding
16.
What term is used to indicate signaling chemicals that
bind to membrane receptors? Which type of membrane
receptor is most important in directing intracellular events by
promoting formation of second messengers?
For answer, see Appendix H.
The Cytoplasm
Describe the composition of the cytosol.
Discuss the structure and function of mitochondria.
Discuss the structure and function of ribosomes, the
endoplasmic reticulum, and the Golgi apparatus, including
functional interrelationships among these organelles.
Compare the functions of lysosomes and peroxisomes.
Cytoplasm
(“cell-forming material”), the cellular material be-
tween the plasma membrane and the nucleus, is the site of most
cellular activities. Although early microscopists thought that
the cytoplasm was a structureless gel, the electron microscope
reveals that it consists of three major elements: the cytosol, or-
ganelles, and inclusions.
Te
cytosol
(si
9
to-sol) is the viscous, semitransparent fluid
in which the other cytoplasmic elements are suspended. It is a
complex mixture with properties of both a colloid and a true
solution. Dissolved in the cytosol, which is largely water, are
proteins, salts, sugars, and a variety of other solutes.
Te
organelles
are the metabolic machinery of the cell. Each
type of organelle carries out a specific function for the cell—some
synthesize proteins, others package those proteins, and so on.
Inclusions
are chemical substances that may or may not be
present, depending on cell type. Examples include stored nu-
trients, such as the glycogen granules in liver and muscle cells;
lipid droplets in fat cells; pigment (melanin) granules in certain
skin and hair cells; and crystals of various types.
Roles of Plasma Membrane Receptors
A huge and diverse group of integral proteins and glycoproteins
that serve as binding sites are collectively known as
membrane
receptors
. Some function in contact signaling, and others in
chemical signaling. Let’s take a look.
Contact Signaling
Contact signaling
, in which cells come together and touch, is the
means by which cells recognize one another. It is particularly
important for normal development and immunity. Some bacte-
ria and other infectious agents use contact signaling to identify
their “preferred” target tissues.
Chemical Signaling
Most plasma membrane receptors are involved in
chemical sig-
naling
.
Ligands
are chemicals that bind specifically to plasma
membrane receptors. Ligands include most
neurotransmitters
(nervous system signals),
hormones
(endocrine system sig-
nals), and
paracrines
(chemicals that act locally and are rapidly
destroyed).
Different cells respond in different ways to the same ligand.
Acetylcholine, for instance, stimulates skeletal muscle cells to
contract, but inhibits heart muscle. Why do different cells re-
spond so differently? Te reason is that a target cell’s response
depends on the internal machinery that the receptor is linked
to, not the specific ligand that binds to it.
Tough cell responses to receptor binding vary widely, there
is a fundamental similarity: When a ligand binds to a mem-
brane receptor, the receptor’s structure changes, and cell pro-
teins are altered in some way. For example:
Catalytic receptor proteins
are membrane proteins that re-
spond to ligands by becoming activated enzymes.
Chemically gated channel-linked receptors
, common in mus-
cle and nerve cells, respond by transiently opening or closing
ion gates, which in turn changes the excitability of the cell.
G protein–linked receptors
exert their effect indirectly
through a
G protein
, a regulatory molecule that acts as a
middleman or relay to activate (or inactivate) a membrane-
bound enzyme or ion channel. Tis in turn generates one
or more intracellular chemical signals, commonly called
second messengers
, which connect plasma membrane
events to the internal metabolic machinery of the cell.
±wo important second messengers are
cyclic AMP
and
ionic calcium, both of which typically activate
protein
kinase enzymes
, which transfer phosphate groups from
A±P to other proteins. In this way, the protein kinases can
activate a whole series of enzymes that bring about the
desired cellular activity. Because a single enzyme can cata-
lyze hundreds of reactions, the amplification effect of such
a chain of events is tremendous, much like that stirred
up by a chain letter.
Focus on G Proteins
(Figure 3.16)
on p. 82 describes a G protein signaling system. ±ake a
moment to study this carefully because this key signaling
pathway is involved in neurotransmission, smell, vision,
and hormone action (Chapters 11, 15, and 16).
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