Chapter 11
Cell Communication
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The Cellular “Internet”
 Within multicellular organisms, cells must


communicate with one another to
coordinate their activities
A signal transduction pathway is a series
of steps by which a signal on a cell’s
surface is converted into a specific
cellular response
Signal transduction pathways are very
similar in all organisms, even organisms
as different as unicellular yeasts and
multicellular mammals
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Communication Methods
 Cell-to-cell contact
 Local signaling
 Long distance signaling
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Cell-to-Cell Communications
 Cell (gap) junctions must directly connect the
cytoplasm of adjacent cells; Protein channels
connecting two adjoining cells
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 Animal cells use gap junctions to send signals



Ex: cardiac cells for rhythmicity;
Surface receptors can give/send information
Ex: specific immune response
Plasma membranes
Gap junctions
between animal cells
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Plasmodesmata
between plant cells
Cell-Cell Communication
 Plant cells use plasmodesmata to send
signals
Cells must be in direct contact
 Gaps in the cell wall connecting the two
adjoining cells together

Plasmodesmata
between plant cells
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Local Signaling
 Other types of signaling over a short
distance

Cell-cell recognition
 Membrane bound cell surface molecules
 Glycoproteins
 Glyolipids

Local regulators
 Growth factors
 Only work over a short distance
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Long-Distance Signaling
 Nervous System in Animals

Electrical signals through neurons
 Endocrine System in Animals

Uses hormones to transmit messages
over long distances
 Plants also use hormones
Some transported through vascular
system
 Others are released into the air

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Local/Long-Distance Signaling
 Messenger molecules can also be secreted by the signaling cell
 Paracrine signaling:


One cell secretes (releases) molecules that act on nearby “target”
cells
Example: growth factors
 Synaptic Signaling:

Nerve cells release chemical messengers (neurotransmitters) that
stimulate the target cell
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Long-Distance Signaling
 Endocrine
(hormone) signaling

Specialized cells
release hormone
molecules, which
travel (usually by
diffusion through
cells or through the
circulatory system) to
target cells elsewhere
in the organism
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The Three Stages of Cell Signaling
 There are 3 stages at the “receiving end” of
a cellular conversation:
1.
2.
3.
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Reception
Transduction
Response
Step One - Reception
EXTRACELLULAR
FLUID
CYTOPLASM
Plasma membrane
1 Reception
Receptor
The receptor and signaling molecules
fit together (lock and key model,
induced fit model, just like enzymes!)
Signaling
molecule
 Signaling molecule (ligand)
binds to the receptor protein
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Step 1: Reception
 The target cell “detects” that there is a signal
molecule coming from outside the cell


The signal is detected when it binds to a protein on the
cell’s surface or inside the cell
The signal molecule “searches out” specific receptor
proteins
 The signal molecule is a ligand
 It is a molecule that specifically binds to another one (think
enzymes!)
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Step Two - Transduction
CYTOPLASM
EXTRACELLULAR
FLUID
Plasma membrane
1 Reception
2 Transduction
Receptor
2nd
Messenger!
Relay molecules in a signal transduction pathway
Signaling
molecule
 The signal is converted into a form that
can produce a cellular response
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Step 2: Transduction
 This stage converts the signal into a
form that can bring about a specific
cellular response
One signal-activated receptor activates
another protein, which activates
another molecule, etc., etc.
 These act as relay molecules
 Often the message is transferred using
protein kinases, which transfer
phosphate groups from ATP molecules
to proteins

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 Step 2 – Transduction (phosphorylation cascade)
Signal molecule
Receptor
Activated relay
molecule
Inactive
protein kinase
1
2 Active protein kinase 1
transfers a phosphate from ATP
to an inactive molecule of
protein kinase 2, thus activating
this second kinase.
Active
protein
kinase
1
Inactive
protein kinase
2
ATP
PP
Inactive
protein kinase
3
5 Enzymes called protein
phosphatases (PP)
catalyze the removal of
the phosphate groups
from the proteins,
making them inactive
and available for reuse.
Figure 11.8
P
Active
protein
kinase
2
ADP
Pi
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1 A relay molecule
activates protein kinase 1.
3 Active protein kinase 2
then catalyzes the phosphorylation (and activation) of
protein kinase 3.
ATP
ADP
Pi
Active
protein
kinase
3
PP
Inactive
protein
P
4 Finally, active protein
kinase 3 phosphorylates a
protein (pink) that brings
about the cell’s response to
the signal.
ATP
ADP
Pi
PP
P
Active
protein
Cellular
response
Step Three - Response
CYTOPLASM
EXTRACELLULAR
FLUID
Plasma membrane
1 Reception
2 Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Signaling
molecule
Can be catalysis, activation of a gene,
triggering apoptosis, almost anything!
 The transduced signal triggers a
cellular response
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Step 3: Response
 The signal that was
passed through the
signal transduction
pathway triggers a
specific cellular
response


Examples: enzyme
action, cytoskeleton
rearrangement,
activation of genes, etc.,
etc.
Diagram example:
transcription of mRNA
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The Specificity of Cell Signaling
 The particular proteins
that a cell possesses
determine which signal
molecules it will
respond to and how it
will respond to them
 Liver cells and heart
cells, for example, do
not respond in the
same way to
epinephrine because
they have different
collections of proteins
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Types of Receptors
 There are three main types of
plasma membrane receptors:
 G-protein-linked
 Tyrosine kinases
 Ion channel
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G-protein-linked receptors
 Very common
 Results in a single pathway response
G-protein-linked
Receptor
Plasma Membrane
GDP
CYTOPLASM
G-protein
(inactive)
Enzyme
Activated
Receptor
GDP
Signal molecule
GTP
Activated
enzyme
GTP
GDP
Pi
Cellular response
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Inactivate
enzyme
Receptor tyrosine kinases
 Multiple pathway response
Signal-binding site
Signal
molecule
Helix in the
Membrane
Signal
molecule
Tyrosines
Tyr
Tyr
Tyr
CYTOPLASM
Receptor tyrosine
kinase proteins
(inactive monomers)
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Dimer
Figure 11.7
Activated
relay proteins
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
6
ATP
Activated tyrosinekinase regions
(unphosphorylated
dimer)
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6 ADP
P Tyr
P Tyr
P Tyr
Tyr P
Tyr P
Tyr P
Fully activated receptor
tyrosine-kinase
(phosphorylated
dimer)
P Tyr
P Tyr
P Tyr
Tyr P
Tyr P
Tyr P
Inactive
relay proteins
Cellular
response 1
Cellular
response 2
Ion Channel Receptors
 Very important in
1
Gate
closed
Ions
Signaling
molecule
(ligand)
the nervous system
 When ligand binds,
channel can open
or close.
depolarization
 Triggered by
neurotransmitters
Ligand-gated
ion channel receptor
2
Gate open
Cellular
response

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Plasma
membrane
3
Gate closed
*Intracellular Receptors
 Target protein is INSIDE the cell
 Must be hydrophobic molecule
Hormone
EXTRACELLULAR
(testosterone)
FLUID
Why can the
signal molecule
meet its target
INSIDE the cell?
Receptor
protein
Plasma
membrane
Hormonereceptor
complex
2 Testosterone binds
to a receptor protein
in the cytoplasm,
activating it.
NUCLEUS
CYTOPLASM
receptor complex
enters the nucleus
and binds to specific
genes.
4
mRNA
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hormone testosterone
passes through the
plasma membrane.
3 The hormone-
DNA
Figure 11.6
1 The steroid
New protein
The bound protein
stimulates the
transcription of
the gene into mRNA.
5 The mRNA is
translated into a
specific protein.
Fig. 11-11
First messenger
Adenylyl
cyclase
G protein
G protein-coupled
receptor
GTP
ATP
cAMP
Transduction in a
G-protein pathway
Second
messenger
Protein
kinase A
Cellular responses
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Growth factor
Receptor
Response
Reception
 Many possible
outcomes
 This example
shows a
transcription
response
Phosphorylation
cascade
CYTOPLASM
Inactive
transcription
factor
Active
transcription
factor
P
DNA
Gene
NUCLEUS
mRNA
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Transduction
Response
 The signal
can also
trigger an
activator or
inhibitor
 The signal
can also
trigger
multiple
receptors and
different
responses
Activation
or inhibition
Response 4
Response 5
Cell C. Cross-talk occurs Cell D. Different receptor
between two pathways.
leads to a different response.
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Yeast Sexual Reproduction
1
Yeast cells identify
their mates by cell
signaling.
2
3
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Exchange of
mating factors.
Each cell type
secretes a
mating factor
that binds to
receptors on
the other cell
type.
Mating. Binding
of the factors to
receptors
induces changes
in the cells that
lead to their
fusion.
New a/ cell.
The nucleus of
the fused cell
includes all the
genes from the
a and a cells.
 factor
Receptor
a

Yeast cell,
mating type a
 factor
Yeast cell,
mating type 

a
a/
Evolutionary Significance
 Unicellular and multicellular cell
communication have similarities
 Yeast cells signal for sexual
reproduction through signal
transduction process.
 Bacteria secrete molecules to sense
density of own population.

Quorum Sensing (survival purpose)
TEDED on Quorum Sensing
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