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Unformatted text preview: Released from carrier protein to diffuse across cell membrane into
target cells. Act by binding to intracellular protein receptors . Signal transduction pathways via
Act as cofactors/coenzymes to modulate
intracellular enzyme activity
Signal molecule ( first messenger ) – Do not enter target cells. Act as ligand by binding to protein
receptor on cell surface.
1. Since the signal molecule (first messenger) does not enter the cell,
the receptor/ ligand complex causes a second messenger to be
produced or released within the cell.
2. This second messenger acts as a coenzyme/cofactor to regulate
cellular enzymes ﬁ change the activity of the cell. Heyer Receptor Activated relay
molecule ( second messenger ) Inactive
enzyme 2 Coordination of Body Functions Intracellular Receptors for Lipophilic
1. Steroid diffuses across
membrane into cell
binds to DNA
3. Transcription factor —
turns genes on/off
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- Fall '10