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 ions
such as Na+ or Ca2+.
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 or
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 - cAMP, Ca2+, 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
tyrosine-kinase receptor
pathways.
Calcium Ions
• Works because of differences in
concentration between
extracellular and intracellular
environments. (10,000X)
• Used in plants, muscles and other
places.
Homework
•Read Chapter 11
•Chapter 11 – Mon. Nov. 5
•Ps Lab – Wed. Nov. 7
Inositol Trisphosphate (IP3)
• 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
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
Apoptosis video
• http://www.youtube.com/watch?v=
DR80Huxp4y8
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
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