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.
– 2 reasons 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 - 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 Gprotein 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.
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
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