Basic terminology – In classic cell signaling, a hormone or extracellular ligand is best described as which “messenger” in the communication chain?
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AFirst messenger
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BSecond messenger
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CBoth (a) and (b)
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DNone of these
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EThird messenger that acts after Ca2+
Answer
Correct Answer: First messenger
Explanation
Introduction:Signal transduction uses a hierarchical terminology: the extracellular signaling molecule, the receptor and associated enzymes, and intracellular second messengers. This question focuses on the correct label for the ligand itself in this sequence.
Given Data / Assumptions:
- Ligands (hormones, neurotransmitters, growth factors) originate outside the cell or at its surface.
- Second messengers are generated inside the cell after receptor activation.
- Downstream effectors (e.g., kinases) interpret second messengers.
Concept / Approach:The extracellular ligand that binds a receptor is termed the first messenger. Receptor engagement triggers intracellular enzymes to produce second messengers such as cAMP, cGMP, IP3, DAG, and Ca2+. These propagate the message to effectors that alter cell behavior.
Step-by-Step Solution:
Identify the position of the ligand: outside the cell or at the membrane.Define first messenger: the original extracellular signaling molecule.Contrast with second messengers: intracellular molecules generated upon receptor activation.Select “First messenger” as the correct classification.Verification / Alternative check:Canonical GPCR signaling: epinephrine (first messenger) → β-adrenergic receptor → adenylyl cyclase → cAMP (second messenger) → PKA activation, confirms the terminology.
Why Other Options Are Wrong:
- Second messenger: reserved for intracellular mediators.
- Both/none/third messenger: do not match standard nomenclature.
Common Pitfalls:Calling Ca2+ or cAMP “hormones”; they are intracellular messengers, not extracellular ligands.
Final Answer:First messenger