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Unit 1
Cell and Molecular
Biology
Section 7
Signalling
Cell Signalling
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
Hydrophilic signal molecules
Hydrophilic signal molecules such as peptide
hormones and neurotransmitters.
These cannot pass through a cell membrane and must
activate the surface receptor proteins
Once activated the surface receptor protein generates
an intracellular response
This process is called signal transduction
The mechanism by which hydrophilic extracellular molecules such as peptide
hormones generate an intracellular
response
Endocrine cell
Target cell
Receptor
Bloodstream

Hormones, such as peptide hormones, are
produced in endocrine glands, secreted into the
bloodstream and carried throughout the body.

These signal molecules only produce a response in
target molecules with the appropriate surface
receptor.

This type of cell signalling is used by the body to coordinate the bodies metabolism and causes
relatively slow, long lasting changes. Important
peptide hormone include insulin and glucagon
A second group of hydrophilic signalling molecules
are neurotransmitters
An electrical signal is passed along a nerve and on
reaching the terminal point stimulates the release of
neurotransmitter signalling molecules.
These diffuse across the gap between nerves
known as a synapse and lock onto receptors found
on the surface of the nearest nerve generating an
electrical impulse
Neurotransmitters produce a fast acting , short lived
response between nerves
The mechanism by which hydrophilic extra-cellular molecules
such as neurotransmitters generate an intracellular response
Neuron consists of dendron + cell body + axon
Dendron
Cell
body
Axon
Direction of nerve impulse
Target cell
Receptor
Synapse
showing
movement of
neurotransmitt
er molecules
When activated by signals from the surroundings, or other nerve cells, the
neuron sends electrical impulses along its axon at speeds of up to 100
meters/second. On reaching the axon terminal, the intracellular electrical
signals are converted to an extra-cellular form: each electrical impulse
stimulates the terminal to secrete a pulse of chemical signal called a
neurotransmitter. Neurotransmitters diffuse across the narrow gap, known
as a synapse, and bind to receptors on the surface of the target cell
In both cases above the extracellular
signal molecule binds to cell surface
receptors as the hydrophilic molecules
cannot cross the lipid bilayer. These
receptors act as transducers which
convert the signal on the outside of the
cell to an intracellular signal
There are three main classes of cellsurface receptors
Three types of signal transduction
mechanism:
ion channels
(a)
(b)
S
+ +
++ +
+
+
+
enzyme-linked
(c)
++ +
+
++ +
+
+
+
+
+
+
+
+
+
+
+ + +
+
closed
G-protein-linked
open
T
R
G
inactive
G
active
response
Transport ions rapidly across
membranes. Very important in
Muscles and nerves
Extracellular signal binds to
inactive form and activates
the enzyme function at the
cytosol side
G-linked protein actives
G-protein which in turn
starts sequence of
intracellular events
Class1 - Ion-channel receptors. These are found
on the surface of muscles and nerves and
tranduce a signal in the form of a
neurotransmitter into an electrical voltage.
Class 2 - G-protein-linked receptors. This is the
largest group. G-protein-linked receptors activate
a G-protein which sets off a chain of events
within the cell. These are found in all cells
Class 3 - Enzyme linked receptors . An enzyme
linked receptor binds an extracellular signal
molecule switching on an enzyme activity,
usually a kinase. on the other side of the
membrane. This kinase activity causes the
phosphorylation of other intracellular proteins.
These are found in all cells
G-protein-linked receptors
(a)
(b)
signal
AC
AC
GP
GP
GTP
GDP
ATP
cAMP
other
effects
The peptide hormone glucagon sets off a chain of reactions
as follows:
Glucagon molecule binds to G-linked protein

Inactive G-Protein is switched on by addition of
phosphate to GDP

Activated G protein binds to enzyme adenylate cyclase
(AC)

Enzyme AC breaks down ATP to cyclic AMP

Cyclic AMP causes intracellular effect e.g. breakdown of
glycogen

or fats or activates gene regulatory proteins which
switch on genes
Activity





Read DART pg 70 – 72
Scholar 7.2 – 7.3 (7.1 to a lesser extent)
Draw a diagram / make a poster to explain the
steps in extracellular hydrophobic signalling
Draw flow charts to show the processes in
extracellular hydrophilic signalling
http://www.sp.uconn.edu/~bi107vc/images/ani
m/SigtranRA.gif