Chapter 11 Cell Communication

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Chapter 11Cell Communication
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Cell Signaling Evolved early in the History of
Life
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Communication between mating yeast cells.
Yeast cells use chemical signaling
Induced changes lead to fusion.
The resulting a/a cell is diploid.
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• Local Chemical Signaling
• Paracrine
• Secreting cell acts on nearby target cell by
  discharging molecules into the extracellular
  fluid
• Synaptic
• Nerve cell releases neurotransmitter into
  synapses

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Distant Cell Communication

• Hormones signal target cells at much greater
  distances.
• Secreted into the body fluids, they reach all
  parts of the body.
• Only specific target cells are recognized and
  respond

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Major Glands and their Hormones
Growth Hormone
Thyroxine
Epinephrine (adrenaline)
Insulin and Glucagon
Testosterone
Estrogen
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Local and Distant Cell Communication In Animals.
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• Cells also communicate by direct contact.
• Cell Junctions
• Signaling substances dissolved in the cell can
  pass freely from one cell to another
• Direct Contact
• Through molecules on the surface of the cell
• (embryonic development and the immune
  system)

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Communication by Direct Contact with Cells.
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The Three Stages of Cell Signaling

• For a chemical signal to elicit a specific
  response, the target cell must possess a
  signaling system for the signal.
• 1. Reception
• 2. Transduction
• 3. Response

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Overview of Cell Signaling.
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Reception signal (ligand) binds to a specific
cell surface protein. Transduction binding
results in conformation change of receptor. This
initiates transduction (one or many
steps) Response almost any cellular activity
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• Sutherland, 1971 Nobel Prize
• Epinephrine stimulates glycogen breakdown by
  activating glycogen phosphorylase
• Only when intact cells are used
• Plasma membrane required
• Glycogen phosphorylase required

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• Many signal molecules cannot pass freely through
  the plasma membrane
• Receptors are located on the plasma membrane
• These families are called
• - G-protein linked receptors
• - tyrosine kinase receptors
• - ion channel receptors

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The Structure of a G-protein linked Receptor.
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The Functioning of a G-Protein Linked-
Receptor(e.g. epinephrine receptor).
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• G-proteins bind guanine nucleotides
• GDP inactive state, GTP active state
• Signal binding changes receptor conformation
• Interacts with G-protein

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• G-protein binds GTP
• Activated G-protein binds to another protein
  (enzyme)
• Activation of subsequent target.
• Activation state is temporary

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• Critical metabolic and developmental processes
• Defects in G-protein signaling form the bases of
  many human disease states.
• Cholera, whooping cough and botulism, due to
  toxins that interfere with G-protein function.

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The Structure and Function of a Tyrosine Kinase
Receptor.
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Tyrosine Kinase Receptors

• Extracellular ligand-binding domain.
• Cytosolic domain with tyrosine kinase (tk) enzyme
  activity.

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Ligand binding causes dimerization. Activates
endogenous tk activity. Transfer of PO4 from ATP
to tyrosine autophosphorylation. Interaction
with other cellular proteins, resulting in
activation of relay proteins many
responses Protein phosphatase terminates the
signal process
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A ligand-gated ion-channel receptor.
(important in nervous system)
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Overview Animation

• Reception 11.2

http//bcs.whfreeman.com/thelifewire/content/chp15
/15020.html
http//www.wiley.com/college/fob/anim/
http//www.youtube.com/watch?v3nODx3cT1RU
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Not All Signal Receptors Are Located on the
Plasma Membrane

• Some receptors are proteins located in the
  cytoplasm or nucleus
• The signal molecules for these receptors must be
  able to pass through the plasma membrane
• Examples of signals that bind to intracellular
  receptors are Nitric oxide, steroid and thyroid
  hormones

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Pathways Relay Signals From Receptors to Cellular
Responses

• The signal molecule is not physically passed
  along the transduction pathway.
• The information is passed along.
• It is converted or transduced at each step.

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Protein Phosphorylation

• Transfer of a phosphate group
• From ATP to a protein substrate
• (serine or threonine residues)
• Catalyzed by protein kinases
• Regulates functional activity of
• proteins
• 1 of our genes code for protein
• kinases
• Effects of protein kinases are
• reversed by protein phosphatases

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A Phosphorylation Cascade.
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Cyclic AMP (cAMP)

• Some signaling systems rely on small non-protein
  water
• soluble molecules or ions.
• These are called second messengers.
• Examples cAMP

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cAMP as a Second Messenger.
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Cytoplasmic Response to a Signal.
Amplification
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Cellular Responses

• Signaling can effect the function or activity of
  proteins that carry out various processes such
  as
• Rearrangement of the cytoskeleton
• Opening or closing of an ion channel
• Serve at key points in metabolic pathways
• Modulation of gene expression in the nucleus

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Nuclear Response to a Signal.
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Signal Specificity

• A particular signal can bind to different cell
  types and result in different responses in each
  cell
• Example, epinephrine action
• In liver cells glycogen breakdown
• In cardiac muscle cells contraction

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