PROCESS DIAGRAM
Nonsteroid Hormones Bind to
Receptors on the Target Cell
Membrane
Nonsteroid hormones are water-soluble and cannot pass
through the cell membrane. Instead, they bind to receptors
on the surface of the cell. The hormone-receptor complex
elicits the formation of a
second messenger
inside the cell,
which alters the target cell’s activity (Figure 9.3). Second
messengers include
cyclic AMP (cAMP)
and calcium. cAMP
is made from adenosine triphosphate (ATP) but does not
participate in energy transformations. Instead, it is a chemi-
cal second message inside target
cells. In some cells, calcium can act
as a second messenger. The bind-
ing of a nonsteroid hormone to its
receptor opens calcium channels in
the membrane that allow calcium to
flow into the cell, alter the activities
of enzymes, and elicit responses.
cyclic AMP
(cAMP)
A form of
adenosine monophos-
phate in which the
phosphate is in a ring
structure.
H ow n o n stero id h o rm o n es w ork • Figure 9.3
Nonsteroid hormones do not enter the cell but rather act through second messengers, such
as cAMP, inside the cell. They achieve their physiological effects by activating existing pro-
teins rather than making new ones. Their effects are relatively short-lived.
Blood capillary
I Water-soluble non-steroid hormone
is the first chemical message
It binds to a specific receptor
on the cell membrane
of the target cell (activation).
Water-soluble hormone
Receptor
I cAMP activates protein
kinase enzymes, which
activate many other
proteins by phosphorylation
(adding a phosphate
group to them from ATP).
The hormone-receptor complex
stimulates the formation of a
second messenger inside the cell,
such as cyclic AMP (cAMP).
cAMP is made from ATP by an
enzyme called adenylate cyclase.
cAMP only lasts for a short time
before it becomes degraded.
The many phosphorylated proteins alter the
cell’s activity to elicit physiological responses.
Target cell
256 CHAPTER 9
The Endocrine System
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