Chapter 11 Cell Communication

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Chapter 11Cell Communication
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Question?

• How do cells communicate?
• By cellular phones.
• But seriously, cells do need to communicate for
  many reasons.

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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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Movie
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Cell Communication
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Cell Signaling (C.S.)

• Is a relatively new topic in Biology and AP
  Biology.
• Appears to answer many questions in medicine.
• Is a topic youll be hearing more about in your
  future.

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Movie
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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 cells 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 --gt
  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

• Dont 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).