Cell signaling
Cells do not work in isolation but continually ‘talk’ to each
other by sending and receiving chemical signals to each other.
This process is known as cell signaling
Cell signaling has a number of important steps
A signaling cell produces a signal molecule
The signal molecule is recognised by a target cell by
means of a receptor protein
The receptor protein performs the first step in a series
of transduction processes by converting the incoming
extra-cellular signal to an intracellular signal that
directs the cells behaviour
Principals Of Cellular Signalling
Receptor protein
‘recognises’ signal
molecule *
Change of molecule from
A-B is an example of
SIGNAL
TRANSDUCTION
Intracellular signal
molecule B - OUT
Extracellular signal
molecule A - IN
* - this receptor protein may be on cell surface or inside cell
Signal molecules may be hydrophilic in nature ( e.g
peptide hormones and neurotransmitters) or
hydrophobic in nature (e.g. steroid hormones)
Hydrophilic and hydrophobic signal molecules cross
cell membranes by two different routes.
Hydrophobic signal molecules
Hydrophobic signal molecules include steroid hormones
such as cortisol, oestrogen (estradiol), progesterone and
testosterone and thyroid hormones such as thyroxine.
Steroid hormones diffuse directly through the cell
membrane and bind to an inactive intracellular receptor
protein known as a gene regulatory protein located in
the cytosol or in the nucleus. On binding the
intracellular receptor becomes active allowing it to bind
to the equivalent regulatory sequence in the DNA
The Effect Of Cortisol On Target Cells
Cortisol
Intracellular
Gene
Regulatory
Protein
Intracellular gne
regulatory protein is
activated by slight
change in shape (not
shown)
Plasma
membrane
Activated Gene
regulatory proteinsteroid complex
moves into nucleus
Nuclear
membrane
Activated
target gene
Activated receptorsteroid complex binds
to the regulatory
region of the target
gene and activates
transcription
DNA
TRANSCRIPTION
RNA