When a receptor protein acts as an enzyme, the receptor protein activates a second messenger that acts as a signal molecule within the cell. Resources: HOLT Biology Textbook - 9th grade edition Chapter 4, Cells and Their Environment.
A person that acts as a messenger between two people is not helping anyone. This type of situation can cause problems and should be dealt with without a messenger.
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Ribosomes
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Possible activation of several different second messenger systems.
The use of a second messenger
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Arginine
Second messenger
A neurotransmitter whose function depends on a second messenger is known as a norepinephrine. It is a hormone that is released by the sympathetic nerves and the adrenal medulla.