Control of Cells by Chemical Messengers
125
First messenger
First messenger
First messenger
First messenger
Ion channel
Ion
Change in
membrane potential
and/or
cytosolic [Ca
2+
]
(multiple steps)
(multiple steps)
(a)
Receptor
Tyrosine kinase
ATP
ADP
PO
4
(multiple steps)
Docking
protein
Docking
protein
(b)
Effector
protein
(ion channel
or enzyme)
Activates
Generates
Second
messengers
(multiple steps)
Change in
membrane potential
G Protein
α
γ
α
β
γ
β
Receptor
Receptor
Receptor
(c)
(d)
CELL’S RESPONSE
CELL’S RESPONSE
CELL’S RESPONSE
CELL’S RESPONSE
Extracellular fluid
Plasma
membrane
Intracellular fluid
JAK kinase
+ ATP
Protein-PO
4
+ ADP
Protein
Figure 5–5
Mechanisms of action of water-soluble messengers (noted as “fi rst messengers” in this and subsequent fi gures). (a) Signal transduction
mechanism in which the receptor complex includes an ion channel. (b) Signal transduction mechanism in which the receptor itself functions as
an enzyme, usually a tyrosine kinase. (c) Signal transduction mechanism in which the receptor activates a JAK kinase in the cytoplasm.
(d) Signal transduction mechanism involving G proteins.
Figure 5–5
physiological
inquiry
Many cells express more than one of the four types of receptors depicted in this fi
gure. Why might this be?
Answer can be found at end of chapter.
Table 5–2
Classifi cation of Receptors Based on Their Locations and the Signal Transduction Pathways They Use
1. INTRACELLULAR RECEPTORS (Figure 5–4) (for lipid-soluble messengers) Function in the nucleus as transcription factors or
suppressors to alter the rate of transcription of particular genes.
2. PLASMA MEMBRANE RECEPTORS (Figure 5–5) (for water-soluble messengers)
a. Receptors that are ligand-gated ion channels.
b. Receptors that themselves function as enzymes, such as receptor tyrosine kinases.
c. Receptors that are bound to and activate cytoplasmic JAK kinases.
d. G-protein-coupled receptors that activate G proteins, which in turn act upon effector proteins—either ion channels or enzymes—in
the plasma membrane.
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