AP Biologyeasymcq1 pt
Which type of molecule typically serves as the FIRST messenger?
A.An extracellular ligand such as a hormone
B.cAMP
C.A transcription factor
D.Calcium ions
Explanation
Core Concept
The first messenger is the extracellular signal — hormone, neurotransmitter, or growth factor — that binds the receptor and starts the pathway.
Correct Answer
AAn extracellular ligand such as a hormone
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