Essential Cell Biology 5th edition

General Principles of Cell Signaling
539
extracellular signal molecule
intracellular signaling
pathway
cell-surface
receptor protein
nucleus
FAST
(<sec to mins)
ALTERED
PROTEIN
FUNCTION
DNA
RNA
SLOW
(mins to hrs)
ALTERED PROTEIN SYNTHESIS
ALTERED CYTOPLASMIC MACHINERY
plasma
membrane
ALTERED CELL BEHAVIOR
Figure 16–7 Extracellular signals can act slowly or rapidly. Certain types of cell
responses—such as cell differentiation or increased cell growth and division (see
Figure 16–6)—involve changes in gene expression and the synthesis of new proteins;
they therefore occur relatively slowly. Other responses—such as changes in cell
movement, secretion, or metabolism—need not involve changes in gene expression
and therefore occur more quickly (see Figure 16–5).
been received. Some extracellular signals act swiftly: acetylcholine can
stimulate a skeletal muscle cell to contract within milliseconds and a salivary
gland cell to secrete within a minute or so. Such rapid responses are
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possible because, in each case, the signal affects the activity of proteins
that are already present inside the target cell, awaiting their marching
orders.
Other responses take more time. Cell growth and cell division, when
triggered by the appropriate signal molecules, can take many hours
to execute. This is because the response to these extracellular signals
requires changes in gene expression and the production of new proteins
(Figure 16–7). We will encounter additional examples of both fast and
slow responses—and the signal molecules that stimulate them—later in
the chapter.
Cell-Surface Receptors Relay Extracellular Signals via
Intracellular Signaling Pathways
The majority of extracellular signal molecules are proteins, peptides, or
small, hydrophilic molecules that bind to cell-surface receptors that span
the plasma membrane (see Figure 16–4A). Transmembrane receptors
detect a signal on the outside and relay the message, in a new form,
across the membrane into the interior of the cell.
The receptor protein performs the primary step in signal transduction: it
recognizes the extracellular signal and generates new intracellular signals
in response (see Figure 16–2B). The resulting intracellular signaling
process usually works like a molecular relay race, in which the message
is passed “downstream” from one intracellular signaling molecule to
another, each activating or generating the next signaling molecule in the
pathway, until a metabolic enzyme is kicked into action, the cytoskeleton
is tweaked into a new configuration, or a gene is switched on or off. This
final outcome is called the response of the cell (Figure 16–8).