Chapter 11
Cell
Communication
Question?
How
do cells communicate?
By “cellular” phones.
But seriously, cells do need
to communicate for many
reasons.
Why do cells
communicate?
Regulation
- cells need to
control cellular processes.
Environmental Stimuli cells need to be able to
respond to signals from
their environment.
Cell Communication
Cell Signaling (C.S.)
Is
a relatively “new” topic in
Biology and AP Biology.
Appears to answer many
questions in medicine.
Is a topic you’ll be hearing
more about in your future.
Stages of C.S.
1. Reception - receiving the
signal.
2. Transduction - passing on
the signal.
3. Response - cellular
changes because of the
signal.
Reception
Transduction
Response
Reception
The
target cell’s detection of
a signal coming from
outside the cell.
May occur by:
Direct
Contact
Through signal molecules
Direct Contact
When
molecules can flow
directly from cell to cell
without crossing
membranes.
Plants - plasmodesmata
Animals - gap junctions
Direct Contact
May
also occur by cell
surface molecules that
project from the surface and
“touch” another cell.
Signal Molecules
The
actual chemical signal
that travels from cell to cell.
Often water soluble.
Usually too large to travel
through membranes.
Double reason why they
can’t cross cell membranes.
Signal Molecules
Behave
as “ligands”:
a smaller molecule that
binds to a larger one.
Receptor Molecules
Usually
made of protein.
Change shape when bind to
a signal molecule.
Transmits information from
the exterior to the interior of
a cell.
Receptor Mechanisms
1. G-Protein linked
2. Tyrosine-Kinase
3. Ion channels
4. Intracellular
G-protein linked
Plasma
membrane receptor.
Works with “G-protein”, an
intracellular protein with
GDP or GTP.
G-protein
GDP
and GTP acts as a
switch.
If GDP - inactive
If GTP - active
G-protein
When
active (GTP), the
protein binds to another
protein (enzyme) and alters
its activation.
Active state is only
temporary.
G-protein linked
receptors
Very
widespread and
diverse in functions.
Ex - vision, smell, blood
vessel development.
G-protein linked
receptors
Many
diseases work by
affecting g-protein linked
receptors.
Ex - whooping cough,
botulism, cholera, some
cancers
G-protein linked
receptors
Up
to 60% of all medicines
exert their effects through
G-protein linked receptors.
Tyrosine-Kinase
Receptors
Extends
through the cell
membrane.
Intracellular part functions
as a “kinase”, which
transfers Pi from ATP to
tyrosine on a substrate
protein.
Mechanism
1. Ligand binding - causes two
receptor molecules to
aggregate.
Ex growth hormone
2. Activation of Tyrosine-kinase
parts in cytoplasm.
3. Phosphorylation of tyrosines
by ATP.
Intracellular Proteins
Become
activated & cause
the cellular response.
Tyrosine-Kinase
Receptors
Often
activate several
different pathways at once,
helping regulate
complicated functions such
as cell division.
Ion-channel Receptors
Protein
pores in the
membrane that open or
close in response to
chemical signals.
Allow or block the flow of
+
2+
ions such as Na or Ca .
Ion-channel Receptors
Activated
by a ligand on the
extracellular side.
Causes a change in ion
concentration inside the cell.
Ex - nervous system signals.
Intracellular Signals
Proteins
located in the
cytoplasm or nucleus that
receive a signal that CAN
pass through the cell
membrane.
Ex - steroids (hormones),
NO - nitric oxide
Intracellular Signals
Activated
protein turns on
genes in nucleus.
Comment
Most
signals never enter a
cell. The signal is received
at the membrane and passed
on.
Exception - intracellular
receptors
Signal-Transduction
Pathways
The
further amplification
and movement of a signal in
the cytoplasm.
Often has multiple steps
using relay proteins such as
Protein Kinases.
Protein Kinase
General
name for any
enzyme that transfers Pi
from ATP to a protein.
About 1% of our genes are
for Protein Kinases.
Protein
Phosphorylation
The
addition of Pi to a
protein, which activates the
protein.
Usually adds Pi to Serine or
Threonine.
Amplification
Protein
Kinases often work
in a cascade with each being
able to activate several
molecules.
Result - from one signal,
many molecules can be
activated.
Secondary
Messengers
Small
water soluble
non-protein molecules or
ions that pass on a signal.
Spread rapidly by diffusion.
Activates relay proteins.
Secondary
Messengers
Examples
2+
Ca ,
- cAMP,
inositol trisphosphate (IP3)
cAMP
A
form of AMP made directly
from ATP by Adenylyl cyclase.
Short lived - converted back to
AMP.
Activates a number of Protein
Kinases.
Calcium Ions
More
widely used than
cAMP.
Used as a secondary
messenger in both G-protein
pathways and tyrosinekinase receptor pathways.
Calcium Ions
Works
because of
differences in concentration
between extracellular and
intracellular environments.
(10,000X)
Used in plants, muscles and
other places.
Inositol Trisphosphate
(IP3)
Secondary
messenger attached
to phospholipids of cell
membrane.
Sent to Ca channel on the ER.
2+
Allows flood of Ca into the
cytoplasm from the ER.
Start here
Or Start
here
Cellular Responses
Cytoplasmic
Regulation
Transcription Regulation in
the nucleus (DNA --> RNA).
Cytoplasmic
Regulation
Rearrangement
of the
cytoskeleton.
Opening or closing of an ion
channel.
Alteration of cell
metabolism.
Transcription
Regulation
Activating
protein synthesis
for new enzymes.
Transcription control factors
are often activated by a
Protein Kinase.
Question
If
liver and heart cells both
are exposed to ligands, why
does one respond and the
other not?
Different cells have different
collections of receptors.
Alternate explanation
Comment
Chapter
focused only on
activating signals. There are
also inactivation
mechanisms to stop signals.
Signaling Efficiency
Often
increased by the use
of scaffolding proteins.
Scaffolding proteins – a
protein that holds or groups
signal pathway proteins
together.
Apoptosis
Programmed
cell death
Uses cell signaling pathways
DNA is chopped up
Cell shrinks and becomes
lobed (blebbing)
Pieces are digested by
specialized scavenger cells
WBC before and after
Apoptosis
Balance
between signals for
“live” or “die”
Triggered by mitochondria
damage, neighbor cells,
internal signals
Involved with Parkinson’s
Alzheimer’s, Cancer
Summary
Don’t
get bogged down in
details in this chapter.
Use the KISS principle.
Know - 3 stages of cell
signaling.
Know - At least one example
of a receptor and how it
works (in detail).
Summary
Know
- protein kinases and
cascades (amplification)
Know – example of a
secondary signal
Apoptosis