Essential Cell Biology 5th edition

534 CHAPTER 16 Cell Signaling
considering how these intricate signaling networks ultimately interact to
control complex behaviors.
(A)
(B)
10 µm
Figure 16–1 Yeast cells respond
to mating factor. Budding yeast
(Saccharomyces cerevisiae) cells are
(A) normally spherical, but (B) when they
are exposed to an appropriate mating
factor produced by neighboring yeast
cells, they extend a protrusion toward
the source of the factor. (Courtesy of
Michael Snyder.)
ECB5 16.01/16.01
GENERAL PRINCIPLES OF CELL SIGNALING
Information can come in a variety of forms, and communication frequently
involves converting the signals that carry that information from
one form to another. When you receive a call from a friend on your
mobile phone, for instance, the phone converts radio signals, which
travel through the air, into sound waves, which you hear. This process of
conversion is called signal transduction (Figure 16–2).
The signals that pass between cells are simpler than the sorts of messages
that humans ordinarily exchange. In a typical communication
between cells, the signaling cell produces a particular type of extracellular
signal molecule that is detected by the target cell. As in human conversation,
most animal cells both send and receive signals, and they can
therefore act as both signaling cells and target cells.
Target cells possess proteins called receptors that recognize and respond
specifically to the signal molecule. Signal transduction begins when the
receptor on a target cell receives an incoming extracellular signal and
then produces intracellular signaling molecules that alter cell behavior.
Most of this chapter is concerned with signal reception and transduction—the
events that cell biologists have in mind when they refer to
cell signaling. First, however, we look briefly at a few of the different
types of extracellular signals that cells send to one another—and what
happens when target cells receive those signals.
Signals Can Act over a Long or Short Range
Cells in multicellular organisms use hundreds of kinds of extracellular
signal molecules to communicate with one another. The signal molecules
can be proteins, peptides, amino acids, nucleotides, steroids, fatty
acid derivatives, or even dissolved gases—but they all rely on just a handful
of basic styles of communication for getting the message across.
In multicellular organisms, the most “public” style of cell–cell communication
involves broadcasting the signal throughout the whole body by
secreting it into an animal’s bloodstream or a plant’s sap. Extracellular
signal molecules used in this way are called hormones, and, in animals,
the cells that produce hormones are called endocrine cells (Figure 16–3A).
Part of the pancreas, for example, is an endocrine gland that produces
several hormones—including insulin, which regulates glucose uptake in
cells all over the body.
Somewhat less public is the process known as paracrine signaling. In this
case, rather than entering the bloodstream, the signal molecules diffuse
locally through the extracellular fluid, remaining in the neighborhood of
sound
OUT
extracellular signal
molecule
IN
target cell
Figure 16–2 Signal transduction is the
process whereby one type of signal is
converted into another. (A) When a mobile
telephone receives a radio signal, it converts
it into a sound signal; when transmitting
a signal, it does the reverse. (B) A target
cell converts an extracellular signal into an
intracellular signal.
(A)
radio
signal
IN
(B)
intracellular
signaling
molecule
OUT