Question

Which one of the following is NOT a second messenger in the signal transduction pathway?

• A. cAMP
• B. Inositol 1,4,5 triphosphate
• C. Acetyl choline
• D. Ca2+

Hint:

Signaling Messengers:
First Messengers (Extracellular) = Acetylcholine, Epinephrine, Insulin.
Second Messengers (Intracellular) = cAMP, cGMP, $\text{IP}_3$, DAG, $\text{Ca}^{2+}$, NO.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Signal transduction pathways distinguish extracellular primary messengers (hormones/neurotransmitters) from intracellular second messengers.

Step 2: Detailed Explanation:
Evaluating signaling messengers:
1. Second Messengers (Intracellular): Small, diffusible intracellular signaling molecules generated in response to receptor activation that amplify and relay signals inside the cytosol. Universal examples include cAMP, cGMP, Inositol 1,4,5-trisphosphate ($\text{IP}_3$), Diacylglycerol (DAG), and Calcium ions ($\text{Ca}^{2+}$).
2. Acetylcholine (ACh): An extracellular neurotransmitter hormone acting as a First Messenger that binds cell-surface nicotinic or muscarinic receptors.

Step 3: Final Answer:
Hence, Acetyl choline is NOT a second messenger, matching option (C).