Biology, Eighth Edition (Raven)

Chapter 46: The Endocrine System

Membrane-Bound Receptors, G Proteins, and Ca2+ Channels

How can one type of signal produce different responses in different cells? Cell signaling pathways have many elements in common. G proteins and calcium channels are used in a number of different cell types to produce different responses. How is this possible? Different types of cells have different receptor proteins on their membranes. A liver cell will have different receptors from a muscle cell, for example. A liver cell will not respond to a ligand (external signal molecule) for a muscle cell because it lacks the appropriate receptor molecule. Even though other aspects of cell signaling may be similar in the two cells they can produce completely different reactions because the initial receptor molecule is different.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	The alpha subunit of the G-protein is activated by...

		A)	separating from the gamma and beta subunits.
		B)	the G-protein changing conformation.
		C)	binding to the calcium ions.
		D)	replacing the GDP with GTP.
		E)	replacing the GTP with GDP.

2.	The activated alpha subunit then binds to...

		A)	the calcium ion channel.
		B)	the calcium ions.
		C)	the gamma and beta subunits.
		D)	GDP
		E)	GTP

3.	As a result...

		A)	the calcium channel opens and calcium ions leave the cell.
		B)	the calcium channel opens and calcium ions enter the cell.
		C)	the calcium channel closes.
		D)	the calcium ions bind to calmodulin.
		E)	the calcium ions bind to the ligand receptor site.

4.	The G-protein changes conformation when the GTP replaces the GDP on the alpha subunit.

		A)	True
		B)	False

5.	The combination of the calcium and the calmodulin produces the response of the cell to the ligand.

		A)	True
		B)	False