Cell Communication Chapter 11:. Why do cells communicate? Regulation - cells need to control...
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Transcript of Cell Communication Chapter 11:. Why do cells communicate? Regulation - cells need to control...
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Cell Communication
Chapter 11:
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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.
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Cell Communication
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Stages of C.S.
1. Reception - receiving the signal.
2. Transduction - passing on the signal.
3. Response - cellular changes because of the signal.
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Reception
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Transduction
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Response
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Reception
The target cell’s detection of a signal coming from outside the cell.
May occur by: Direct Contact Through signal molecules
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Direct Contact
When molecules can flow directly from cell to cell without crossing membranes.
Plants - plasmodesmata Animals - gap junctions
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Direct Contact
May also occur by cell surface molecules that project from the surface and “touch” another cell.
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Signal Molecules
The actual chemical signal that travels from cell to cell.
Often water soluble. Usually too large to travel through
membranes.
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Signal Molecules
Behave as “ligands”: a smaller molecule that binds to a larger one.
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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.
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Receptor Molecules
1. G-Protein linked
2. Tyrosine-Kinase
3. Ion channels
4. Intracellular
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G-protein linked
Plasma membrane receptor. Works with “G-protein”, an intracellular
protein with GDP or GTP.
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G-protein
GDP and GTP acts as a switch. If GDP - inactive If GTP - active
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G-protein
When active (GTP), the protein binds to another protein (enzyme) and alters its activation.
Active state is only temporary.
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G-protein linked receptors
Very widespread and diverse in functions.
Ex - vision, smell, blood vessel development.
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G-protein linked receptors
Many diseases work by affecting g-protein linked receptors.
Ex - whooping cough, botulism, cholera, some cancers
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G-protein linked receptors
Up to 60% of all medicines exert their effects through G-protein linked receptors.
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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.
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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.
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Intracellular Proteins
Become activated, cause the cellular response.
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Tyrosine-Kinase Receptors
Often activate several different pathways at once, helping regulate complicated functions such as cell division.
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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+.
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Ion-channel Receptors
Activated by a ligand on the extracellular side.
Causes a change in ion concentration inside the cell.
Ex - nervous system signals.
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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
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Intracellular Signals
Activated protein turns on genes in nucleus.
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Comment
Most signals never enter a cell. The signal is received at the membrane and passed on.
Exception - intracellular receptors
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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.
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Protein Phosphorylation
The addition of Pi to a protein, which activates the protein.
Usually adds Pi to Serine or Threonine.
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Protein Kinase
General name for any enzyme that transfers Pi from ATP to a protein.
About 1% of our genes are for Protein Kinases.
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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.
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Secondary Messengers
Small water soluble non-protein molecules or ions that pass on a signal.
Spread rapidly by diffusion. Activates relay proteins.
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Secondary Messengers
Examples - cAMP, Ca2+, inositol trisphosphate
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cAMP
A form of AMP made directly from ATP by Adenylyl cyclase.
Short lived - converted back to AMP. Activates a number of Protein Kinases.
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Calcium Ions
More widely used than cAMP. Used as a secondary messenger in
both G-protein pathways and tyrosine-kinase receptor pathways.
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Calcium Ions
Works because of differences in concentration between extracellular and intracellular environments. (10,000X)
Used in plants, muscles and other places.
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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.
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Start here Or Start here
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Cellular Responses
Cytoplasmic Regulation Transcription Regulation in the nucleus
(DNA --> RNA).
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Cytoplasmic Regulation
Rearrangement of the cytoskeleton. Opening or closing of an ion channel. Alteration of cell metabolism.
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Transcription Regulation
Activating protein synthesis for new enzymes.
Transcription control factors are often activated by a Protein Kinase.
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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.
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Alternate explanation
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Comment
Chapter focused only on activating signals. There are also inactivation mechanisms to stop signals.
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Summary
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).