Characteristics Of Life

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• Chapter 11 Cell Communication
• Why do cells communicate?
• A. Regulation cells need to control cellular
  processes
• Ex
• B. Environmental Stimuli cells need to be able
  to respond to
• signals from their environment
• Ex

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II. Stages of cell signaling A. Reception
signaling molecule binds to a
receptor protein in the membrane
B. Transduction passing on
the signal (can occur in one step, but is
usually a sequence of changes in a series of
relay molecules) transduction pathway
C. Response - cellular changes because
of the signal (catalysis by an enzyme,
rearrangement of the cytoskeleton, activation
of a specific gene.)
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III. More Detail A. Reception 1. Signal
will only be heard by a specific cell. 2.
Types of signaling a. Direct Contact
cell junctions signaling substances
dissolved in cytosol are shared (ex
plasmodesmata, gap junctions) membrane
bound cell-surface molecules that touch
another cell (cell recognition, embryonic
development, immuneresponse) b.
Local signaling influence cells in the
vicinity Paracrine Signaling
Synaptic Signaling c.
Long-distance Signaling -- Use Hormones(Endrocrine
signaling)
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Local and Long Distance Signaling Examples
Growth Factor
Nervous System
Hormones
Cell secretes molecule (local regulator) into
extracellular fluid to influence neighboring cells
More specialized than paracrine. Nerve cell
releases a neurotransmitter that stimulates the
target cell.
Specialized cells release hormones into
circulatory system that carries them to target
cells in other parts of the body.
AKA endocrine signaling
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3. Signal molecules a. often water
soluble b. usually too large to travel
through membranes c. behave as ligands
smaller molecule binding to a larger
one. 4. Receptor molecules a. Usually a
proteinmost are located within the cell
membrane (G-protein coupled, tyrosine-kinase, ion
channels) b. some are located inside the
cellin the cytoplasm or nucleus of
target cell (usually acts as a transcription
factor to turn on specific genes). (see slide
15 for more detail) c. Changes shape when
bound to signal molecule d. Transmits
information from the exterior to the interior
of the cell.
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e. Types of receptor molecules page 211, 212,
213 G-protein coupled receptor Very
widespread and diverse in functions. Ex -
vision, smell, blood vessel development. Many
diseases work by affecting g-protein linked
receptors. Ex - whooping cough, botulism,
cholera, some cancers (toxins interfere
with G-protein function) Up to 60 of all
medicines exert their effects through G-protein
linked receptors. Tyrosine-kinase
Ion channels Intracellular
Receptors
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G-Protein Coupled Receptor (page 211) Works
with the help of a G-protein (protein that binds
to GTP) The receptor has a variety of binding
sites for different signal molecules and for
different G-proteins. There are different
G- proteins out there. All G-protein coupled
receptors have a similar structure. (7 alpha
helices spanning the membrane) The loops are
binding sites for signals and G-proteins
Examples yeast mating factors
epinephrine neurotransmitters
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How the G-Protein Coupled Receptor works (page
211) Works with G-protein, an intracellular
protein that binds with GDP (G-protein is
inactive) or GTP (G-protein is active). When
signal binds to receptor, the receptor changes
shapethis allows it to bind to an inactive
G-protein that is on the cytoplasmic side of the
membrane. GTP displaces GDP to activate the
G-protein. Now active (GTP is bound to it), the
G-protein binds to an enzyme and alters the
enzymes shape and activity. It triggers the next
step in a pathway leading to a cellular
response. At this point, the G-protein acts as a
GTPase enzyme to hydrolyze GTP to make GDP making
itself inactive. It is now available for reuse.

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Tyrosine-Kinase Receptors (page 212) What
is Tyrosine? ______________ What is a Kinase?
___________________ Receptor extends through the
cell membrane. Intracellular part functions as a
kinase, which transfers P from ATP to amino
acid tyrosine on a substrate protein Often
activate several different pathways at once,
usually 10 or more (a single binding of signal
triggers multiple pathways to occur, helping
regulate complicated functions such as cell
division and cell growth). Abnormal receptors
(meaning they function in the absence of a signal
molecule) contribute to some kinds of cancer.

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How the Tyrosine-kinase receptor works (page
212) 1. Ligand binding causes two receptor
polypeptides to aggregate forming a dimer
which activates the tyrosine kinase region
(inside cell). 2. Each tyrosine kinase adds a P
(from ATP) to a tyrosine on the other
polypeptide. 3. Now activated, the receptor is
recognized by specific relay proteins inside
the cell. The relay proteins bind to a specific
phosphorylated tryosine causing it to change
shapethis triggers a transduction pathway
leading to a cellular response.
Before signal binds, receptors exist as
individual polypeptides
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Ion-Channel Receptors (page 213) Receptors
that act as gates (open or close in response to
chemical signal) that either allows or blocks
the flow of ions such as Na or Ca2 through a
channel in the receptor. Ex - nervous system
signals. neurotransmitter released by one nerve
cell to send signal to a neighboring one.
It binds to ion channel receptor
causing gate to openletting ions into
cell. This triggers an electrical signal
(ions are charged atoms) down the
receiving cell. There are some gated ion
channels that are controlled by electrical
signals instead of ligands. These are
voltage-gated ion channels.
Chapter 48
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Intracellular Receptors (page 210, then
continues on 213) Proteins located in the
cytoplasm or nucleus that receive a signal that
CAN pass through the cell membrane. Activated
protein turns on genes in nucleus. Ex -
steroids (hormones) they are
lipids, so they can diffuse
through if not too big. NO - nitric
oxide
Enters nucleus with hormone attached
Chapter 45
When in the nucleus, the activated receptor
protein acts as a transcription factor to turn on
a specific gene
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B. Signal Transduction 1. The further
amplification and movement of a signal in the
cytoplasm to target molecules. 2. Often has
multiple steps using relay proteins such as
Protein Kinases. a. Remember, a protein
kinase is any enzyme that transfers P
from ATP to a protein. b. About 2 of our
genes are for Protein Kinases. c. When a
protein is phosphorylated, it changes shape
and activates it. d. Usually adds P to
amino acids Serine or Threonine.
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3. Protein phosphatases are enzymes that remove
phophate groups from proteins. a.
this is called dephosphylation b. this
inactivates the protein kinases. c. this
provides a way to turn off the signal
transduction pathway when the initial
signal is no longer present. d. this also
makes the protein kinases available for
reuse.
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4. Secondary Messengers a. Small water
soluble non-protein molecules or ions that
pass on a signal. b. Spread throughout
cell by diffusion. c. Activates relay
proteins. d. cyclic AMP (cAMP) and Ca2 ions
are widely used A form of AMP made directly
from ATP by Adenylyl cyclase. Short lived -
converted back to AMP. Activates a number of
Protein Kinases.
Enzyme found in the cell membrane
Works in response to signal
Doesnt stay in cytoplasm long
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Figure 11.11 in your book. Page 216
Epinephrine
Adenylyl cyclase
G Protein-coupled receptor
ATP
cAMP
Protein kinase A
Hydrolysis of glycogen
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e. Calcium Ions More widely used than
cAMP. Used as a secondary messenger in both
G-protein pathways and tyrosine-kinase receptor
pathways. Pathway causes an increase in
cytosolic concentration Normally Ca2 are
actively transported out of the cell and
imported from the cytosol into the ER,
mitochondria, chloroplasts Ca2 release
involves IP3 (inositol triphophate) or DAG
(diacylglycerol) Used in plants, animal
muscle contraction, secretion of certain
substances, and cell division
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f. Inositol Triphosphate (IP3) and
diacylglycerol (DAG) These messengers are made
by cleavage of a certain kind of phospholipid
in the plasma membrane. Sent to Ca2 channel on
the ER. Allows flood of Ca2 into the cytoplasm
from the ER.
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C. Cellular Responses 1. Cytoplasmic
Regulation a. Rearrangement of the
cytoskeleton. b. Opening or closing of an ion
channel. c. Alteration of cell
metabolism. 2. Transcription
Regulation in the nucleus (DNA --gt RNA). a.
Activating protein synthesis for new
enzymes. b. Transcription control factors are
often activated by a Protein
Kinase. c. can turn a gene on or off.
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3. Amplification a. Protein Kinases often
work in a cascade with each being able
to activate several molecules. b. Result -
from one signal, many molecules can be
activated because each protein stays in the
active
form long enough to process many molecules before
they become inactive again.
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4. Question

• a. If liver and heart cells both are
  exposed to ligands, why does one respond
  and the other not?
• b. Different cells have different
  collections of receptors, relay
  proteins, and proteins needed to carry out the
  response.

5. Alternate Explanation