Download 11 Cell Communication

Chapter 11:
Cell Communication
Why do cells communicate?
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Regulation - cells need to control
cellular processes.
Environmental Stimuli - cells need to
be able to respond to signals from their
environment.
Cell Communication
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
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The target cell’s detection of a signal
coming from outside the cell.
May occur by:
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Direct Contact
Through signal molecules
Direct Contact
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When molecules can flow directly from
cell to cell without crossing
membranes.
Plants - plasmodesmata
Animals - gap junctions
Direct Contact
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May also occur by cell surface
molecules that project from the surface
and “touch” another cell.
Signal Molecules
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The actual chemical signal that travels
from cell to cell.
Often water soluble.
Usually too large to travel through
membranes.
Signal Molecules
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Behave as “ligands”: a smaller
molecule that binds to a larger one.
Receptor Molecules
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Usually made of protein.
Change shape when bind to a signal
molecule.
Transmits information from the exterior
to the interior of a cell.
Receptor Molecules
1. G-Protein linked
2. Tyrosine-Kinase
3. Ion channels
4. Intracellular
G-protein linked
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Plasma membrane receptor.
Works with “G-protein”, an intracellular
protein with GDP or GTP.
G-protein
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GDP and GTP acts as a switch.
If GDP - inactive
If GTP - active
G-protein
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When active (GTP), the protein binds
to another protein (enzyme) and alters
its activation.
Active state is only temporary.
G-protein linked receptors
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Very widespread and diverse in
functions.
Ex - vision, smell, blood vessel
development.
G-protein linked receptors
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Many diseases work by affecting gprotein linked receptors.
Ex - whooping cough, botulism,
cholera, some cancers
G-protein linked receptors
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Up to 60% of all medicines exert their
effects through G-protein linked
receptors.
Tyrosine-Kinase Receptors
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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
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Become activated, cause the cellular
response.
Tyrosine-Kinase Receptors
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Often activate several different
pathways at once, helping regulate
complicated functions such as cell
division.
Ion-channel Receptors
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Protein pores in the membrane that
open or close in response to chemical
signals.
Allow or block the flow of ions such as
Na+ or Ca2+.
Ion-channel Receptors
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Activated by a ligand on the
extracellular side.
Causes a change in ion concentration
inside the cell.
Ex - nervous system signals.
Intracellular Signals
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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
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Activated protein turns on genes in
nucleus.
Comment
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Most signals never enter a cell. The
signal is received at the membrane
and passed on.
Exception - intracellular receptors
Signal-Transduction Pathways
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The further amplification and
movement of a signal in the cytoplasm.
Often has multiple steps using relay
proteins such as Protein Kinases.
Protein Phosphorylation
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The addition of Pi to a protein, which
activates the protein.
Usually adds Pi to Serine or
Threonine.
Protein Kinase
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General name for any enzyme that
transfers Pi from ATP to a protein.
About 1% of our genes are for Protein
Kinases.
Amplification
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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
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Small water soluble non-protein
molecules or ions that pass on a
signal.
Spread rapidly by diffusion.
Activates relay proteins.
Secondary Messengers
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Examples - cAMP, Ca2+, inositol
trisphosphate
cAMP
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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
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More widely used than cAMP.
Used as a secondary messenger in
both G-protein pathways and tyrosinekinase receptor pathways.
Calcium Ions
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Works because of differences in
concentration between extracellular
and intracellular environments.
(10,000X)
Used in plants, muscles and other
places.
Inositol Trisphosphate (IP3)
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Secondary messenger attached to
phospholipids of cell membrane.
Sent to Ca channel on the ER.
Allows flood of Ca2+ into the cytoplasm from
the ER.
Start here
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Cellular Responses
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Cytoplasmic Regulation
Transcription Regulation in the nucleus
(DNA --> RNA).
Cytoplasmic Regulation
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Rearrangement of the cytoskeleton.
Opening or closing of an ion channel.
Alteration of cell metabolism.
Transcription Regulation
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Activating protein synthesis for new
enzymes.
Transcription control factors are often
activated by a Protein Kinase.
Question
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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
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Chapter focused only on activating
signals. There are also inactivation
mechanisms to stop signals.
Summary
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Don’t get bogged down in details in this
chapter.
Know - 3 stages of cell signaling.
Know - At least one example of a
receptor and how it works (in detail).