How do cell-surface receptors relay extracellular signals via intracellular signaling pathways?
A.
The majority of extracellular signal molecules are proteins, peptides, or small, hydrophilic molecules that bind to cell-surface receptors that span the plasma membrane. Transmembrane receptors detect a signal on the outside and relay the message, in a new form, across the membrane into the interior of the cell.
B.
The receptor protein performs the primary step in signal transduction: it recognizes the extracellular signal and generates new intracellular signals in response.
C.
The resulting intracellular signaling process usually works like a molecular relay race, in which the message is passed “downstream” from one intracellular signaling molecule to another, each activating or generating the next signaling molecule in the pathway, until a metabolic enzyme is kicked into action, the cytoskeleton is tweaked into a new configuration, or a gene is switched on or off.
D.
This final outcome is called the response of the cell.