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Signal Transduction by Extracellular Receptors
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How can molecules that cannot penetrate the cellular membrane cause changes within a cell? Many signaling molecules are proteins or other molecules that cannot diffuse across the lipid bilayer of biological membranes. Signaling molecules called first messengers, are able to affect events inside of cells by reacting with receptor proteins embedded in the lipid bilayer. The animation shows details of how this is accomplished in several specific examples. In every case, the first messenger changes the conformation of the receptor protein. Sometimes this opens a channel through the membrane. In many other cases the conformational change catalyses a chemical reaction inside the cell. The messenger is essentially activating an enzyme, one that is embedded in the membrane. The initial reaction is the first step in a pathway that leads to the cell’s response.

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





1Ion channels...
A)open when a first messenger binds to a membrane receptor protein / tyrosine kinase complex, which opens the ion channel and prompts the cell response.
B)open when a first messenger binds to a membrane receptor protein / G protein complex, which opens the ion channel and prompts the cell response.
C)open when a first messenger binds to a membrane receptor protein, which opens the ion channel and prompts the cell response.
D)open when a first messenger binds to a membrane receptor protein / JAK kinase, which opens the ion channel and prompts the cell response.
E)open when a first messenger binds to a membrane receptor protein / docking protein complex, which opens the ion channel and prompts the cell response.


2The function of a docking protein is...
A)to bind to the first messenger / receptor protein complex which activates an effector protein which initiates multiple steps leading to a cell response.
B)to bind with the JAK kinase / receptor protein / first messenger complex, which then binds to the phosphate from an ATP molecule, and initiates multiple steps leading to a cell response.
C)to bind to the first messenger / receptor protein complex which opens an ion channel and promotes the cell response.
D)to bind to a phosphate from ATP and the tyrosine kinase / receptor protein / first messenger complex to initiate multiple steps leading to a cell response.
E)to bind with a G protein which activates an effector protein which initiates multiple steps leading to a cell response.


3The function of a G protein is...
A)to bind to the first messenger / receptor protein complex which opens an ion channel and promotes the cell response.
B)to relay a signal from the first messenger / receptor protein complex to activate an effector protein in the cell membrane which leads to multiple steps which produce a cell response.
C)to bind to a phosphate from ATP and the tyrosine kinase / receptor protein / first messenger complex to initiate multiple steps leading to a cell response.
D)to generate a change in membrane potential or a second messenger system which promotes the cell response.
E)to bind with the JAK kinase, which then initiates multiple steps leading to a cell response.


4First messengers are by definition hydrophobic.
A)True
B)False


5The common thread for signal transduction by extra cellular receptors is that a first messenger is unable to pass through the cell membrane, and therefore must activate a membrane receptor protein to initiate an intracellular response.
A)True
B)False





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