Control of Cells by Chemical Messengers
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.
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.
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.