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Cell Communication
AP Biology
Communication Methods
 Cell-to-cell contact
 Local signaling
 Long distance signaling
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Cell-to-Cell Communications
 Cell junctions directly connect the

cytoplasm of adjacent cells
 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
Local Signaling

Adjacent cells are signaled.

Chemical messengers released
 Ex: Neurotransmitters via neurons
Local signaling
Target cell
Electrical signal
along nerve cell
triggers release of
neurotransmitter
Neurotransmitter
diffuses across
synapse
Secretory
vesicle
Local regulator
diffuses through
extracellular fluid
(a) Paracrine signaling. A secreting cell acts
on nearby target cells by discharging
molecules of a local regulator (a growth
factor, for example) into the extracellular
fluid.
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Target cell
is stimulated
(b) Synaptic signaling. A nerve cell
releases neurotransmitter molecules
into a synapse, stimulating the
target cell.
Long Distance Signaling
 Use of hormones
Both plants and animals
use hormones (e.g. Insulin)
Can affect many cells in
Other parts of the body


Long-distance signaling
Endocrine cell
 Protein or Steroid types
Blood
vessel
Hormone travels
in bloodstream
to target cells
Target
cell
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Figure 11.4
(c) Hormonal signaling. Specialized
endocrine cells secrete hormones
into body fluids, often the blood.
Hormones may reach virtually all
C body cells.
How Do Cells Communicate?
 Signal Transduction Pathways
 Convert signals on a cell’s surface
into cellular responses
 Are similar in microbes and
mammals, suggesting an early origin
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3 Phases of Signal Transduction
EXTRACELLULAR
FLUID
1 Reception
Plasma membrane
CYTOPLASM
2 Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Signal
molecule
Figure 11.5
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Step One - Reception
 Reception occurs when a signal molecule
(ligand) binds to a receptor protein.
 Receptor protein is on the cell surface
 Ligand and receptor have a unique bonding
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Step Two - Transduction
 Signal initiated by conformational change
of receptor protein
 Signal is turned into a cellular response.
 Signaling cascades relay signals to target
 Multistep pathways can amplify a signal

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Second messengers involved
 A 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
Cyclic AMP example…
First messenger
(signal molecule
such as epinephrine)
Adenylyl
cyclase
G protein
G-protein-linked
receptor
GTP
ATP
cAMP
Protein
kinase A
Cellular responses
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1 A signal molecule binds
2 Phospholipase C cleaves a
to a receptor, leading to
activation of phospholipase C.
plasma membrane phospholipid
called PIP2 into DAG and IP3.
EXTRACELLULAR
FLUID
3 DAG functions as
a second messenger
in other pathways.
Signal molecule
(first messenger)
G protein
DAG
Ex:
Inositol P3
and
calcium
GTP
PIP2
G-protein-linked
receptor
Phospholipase C
IP3
(second messenger)
IP3-gated
calcium channel
Endoplasmic
reticulum (ER)
Various
proteins
activated
Ca2+
Cellular
response
Ca2+
(second
messenger)
4 IP3 quickly diffuses through
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the cytosol and binds to an IP3–
gated calcium channel in the ER
membrane, causing it to open.
5 Calcium ions flow out of
6 The calcium ions
the ER (down their concentration gradient), raising
the Ca2+ level in the cytosol.
activate the next
protein in one or more
signaling pathways.
Step Three - Response
 Cell signaling leads to regulation of
cytoplasmic activities or
transcription
 Signaling pathways regulate a
variety of cellular activities
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Pathways can also regulate genes by
activating transcription factors that turn
genes on or off
Growth factor
Receptor
Phosphorylation
cascade
Reception
Transduction
CYTOPLASM
Inactive
transcription Active
transcription
factor
factor
P
Response
Figure 11.14
DNA
Gene
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NUCLEUS
mRNA
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, diverse functions
 Only results in 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
 Regulates/coordinates many cell
functions
Signal
Signal-binding site
molecule
Helix in the
Membrane
Tyr
Tyr
Tyr
Tyrosines
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine
kinase proteins
(inactive monomers)
CYTOPLASM
Signal
molecule
Tyr
Tyr
Tyr
Tyr Tyr
Tyr Tyr
Tyr Tyr
Dimer
Figure 11.7
Activated
relay proteins
Tyr
Tyr
Tyr
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Tyr
Tyr
Tyr
6
ATP
P Tyr
P Tyr
P Tyr
6 ADP
Activated tyrosinekinase regions
(unphosphorylated
dimer)
Tyr P
Tyr P
TyrP
Fully activated receptor
tyrosine-kinase
(phosphorylated
dimer)
P Tyr Tyr P
P Tyr TyrP
P Tyr Tyr P
Inactive
relay proteins
Cellular
response 1
Cellular
response 2
Ion channel receptors
When ligand binds, channel
can open or close.
Ex: neurotransmitters bind as
ligands for ion channels
Signal
molecule
(ligand)
Gate closed
Ligand-gated
ion channel receptor
Ions
Plasma
Membrane
Gate open
Cellular
response
Gate close
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Figure 11.7
*Intracellular Receptors
 Target protein is INSIDE the cell
 Must be hydrophobic molecule
Hormone
EXTRACELLULAR
(testosterone)
FLUID
Receptor
protein
Plasma
membrane
Hormonereceptor
complex
1 The steroid
hormone testosterone
passes through the
plasma membrane.
2 Testosterone binds
to a receptor protein
in the cytoplasm,
activating it.
3 The hormone-
DNA
4
mRNA
NUCLEUS
Figure 11.6
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CYTOPLASM
receptor complex
enters the nucleus
and binds to specific
genes.
New protein
The bound protein
stimulates the
transcription of
the gene into mRNA.
5 The mRNA is
translated into a
specific protein.
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.

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Quorum Sensing (survival purpose)
Yeast Sexual Reproduction
1
Yeast cells identify
their mates by cell
signaling.
Suggests early
evidence of 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/