Cells are continually receiving information from their surroundings, and
they must be able to respond appropriately. Most extracellular chemical
signals fall into one of three categories: (1) proteins and peptides; (2) peptide
neurotransmitters; (3) and steroids and other membrane soluble molecules.
Physical signals such as electromagnetic radiation (light) and heat
are also important. Growth, proliferation, differentiation, movement, and
programmed cellular death all depend upon signals altering a cell's physiology,
often through the activation and repression of genes. Signals may
induce transitory or permanent changes in cells.
Chemical signals specifically bind protein receptors found either on
the plasma membrane or in the cell's cytoplasm. The signal pathways
consist of a few nonprotein second messengers such as calcium ions
(Ca2+), cyclic adenosine monophosphate (cAMP), cyclic guanosine
monophosphate (cGMP), diacylglycerol (DG) and inositol triphosphate (IP3) which transduce or send the signal to the cellular components involved
in the response.
Common second messengers are:
cAMP, cGMP, Ca2+, DG, IP3