What is a second messenger?

A. A substance that reacts with a first messenger.

B. A small substance generated in response to receptor binding that activate or inactivate specific proteins.

C. A substance that reacts with a hormone or other ligand

D. A substance released into the intracellular space.

The correct answer is B. A small substance generated in response to receptor binding that activate or inactivate specific proteins.

Explanation:

An example of second messengers are AMPcyclic, Ca2 + ion. In the case of cAMP, the enzyme adenylate cyclase is synthesized from ATP (this is found within the cell membrane). The AMPC is involved in cell signal transduction pathways before external and / or internal stimuli, (example: adrenaline hormone), being able to activate the enzyme called protein kinase A (PKA), which phosphorylates its target and thus the signal is transmitted.