Transcript Lesson 7 Hydrophilic signalling

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 extra-cellular 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
Target cell
Direction of nerve impulse
Receptor
Synapse showing
movement of
neurotransmitter
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 cell-surface receptors
Three types of signal transduction
mechanism:
ion channels
(a)
(b)
S
+ +
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+
+
+
enzyme-linked
(c)
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+
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+
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+
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+
+
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+ + +
+
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
The peptide hormone glucagon sets off a chain of reactions as follows:other
• Glucagon molecule binds to G-linked protein
• Inactive G-Protein is switched on by addition of phosphate to GDP effects
• 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
ATP
cAMP