cyclic amp
Cyclic amp is produced by a membrane bound enzyme which is activated by the binding of a hormone and ligand on the other side of that membrane. When a door bell is rung, an electric pulse is sent to a bell which is located within the house. The bell ringing alerts the occupant that someone is at the door and that they must do something. The similarity is that both systems send a signal across a barrier to stimulate an action within a closed space.
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