Ch. 11 Cell
Communication
AP Biology
Communication Methods
 Cell-to-cell contact
 Local signaling
 Long distance signaling
AP Biology
Evolutionary Significance
 Unicellular and multicellular cell
communication have similarities
 Yeast cells signal for sexual
reproduction through signal
transduction process.

A signal is received (for reproduction)
and is then converted into a specific
cellular response in a series of steps
 Bacteria secrete molecules to sense
density of own population.
AP Biology
Quorum Sensing (senses population densities-
Yeast Sexual Reproduction
1
Exchange of
mating factors.
Each cell type
secretes a
mating factor
that binds to
receptors on
the other cell
type.
Yeast cells identify
their mates by cell
signaling.
Receptor

a
Yeast cell,
mating type a
2
3
AP Biology
 factor
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
Yeast cell,
mating type 

a
a/
AP Biology
AP Biology
Cell-to-Cell Communications
 Cell junctions directly connect the
cytoplasm of adjacent cells (juxtacrine=
cells are touching or are next to)
membranes
 Ex: cardiac Plasma
cells
for rhythmicity
AP Biology
Gap junctions
between animal cells
Plasmodesmata
between plant cells
Cell-to-Cell Communications
 Cell-cell recognition

Surface receptors can give/send
information
 Ex: specific immune response
AP Biology
Local Signaling

Adjacent cells are signaled.

Chemical messengers released
 Ex: Neurotransmitters via neurons (synaptic
signaling), animal growth factors (paracrine
Local signaling
signaling= cells nearby)
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.
AP Biology
Target cell
is stimulated
(b) Synaptic signaling. A nerve cell
releases neurotransmitter molecules
into a synapse, stimulating the
target cell.
Long Distance Signaling
Long-distance signaling
Blood
vessel
Endocrine cell
 Use of hormones
Endocrine (= within) signaling
 Both plants and animals
use hormones (e.g. insulin,
pheromones)
Can affect many cells in
other parts of the body


 Can be protein or steroid
 Animals can also pass
Hormone travels
in bloodstream
to target cells
Target
cell
signals electrically
AP Biology
Figure 11.4 C
(c) Hormonal signaling. Specialized
endocrine cells secrete hormones
into body fluids, often the blood.
Hormones may reach virtually all
body cells.
AP Biology
Pathways of
Communication?
 Signal Transduction Pathways
 Convert signals on a cell’s surface
into cellular responses
 Are similar in microbes and
mammals, suggesting an early origin
AP Biology
3 Phases of Signal Transduction
EXTRACELLULAR
FLUID
1 Reception
CYTOPLASM
Plasma membrane
2
Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Signal
molecule
Figure 11.5
AP Biology
Step 1. 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
AP Biology
Step 2. 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

Second messengers involved
AP Biology
 Ex. A phosphorylation cascade
Signal molecule
Receptor
A protein kinase is an
enzyme that transfers
phosphate groups
1 A relay molecule
ATP to a protein
activates protein kinasefrom
1.
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
ADP
Pi
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.
3 Active protein kinase 2
then catalyzes the phosphorylation (and activation) of
protein kinase 3.
P
Active
protein
kinase
2
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
P
ADP
Figure 11.8
AP Biology
P
PP
i
Active
protein
Cellular
response
Transduction “second messenger”
cyclic AMP (cAMP) helps to
broadcast signals
First messenger
(signal molecule
such as epinephrine)
Adenylyl
cyclase
G protein
G-protein-linked
receptor
GTP
ATP
cAMP
Protein
kinase A
AP Biology
Cellular responses
Transduction
“second
messenger”
Ca2+helps to
broadcast
signals for
muscle cell
contraction,
secretion,
cell division,
AP Biology
etc.
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
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
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 3: Response
 Cell signaling leads to regulation of
cytoplasmic activities or genetic
activities (transcription)
 Signaling pathways regulate a
variety of cellular activities
AP Biology
Pathways can also regulate genes by
activating transcription factors that turn
genes on or off
Growth factor
Reception
Receptor
Phosphorylation
cascade
Transduction
CYTOPLASM
Inactive
transcription
factor
Active
transcription
factor
P
Response
Figure 11.14
DNA
Gene
AP Biology
NUCLEUS
mRNA
Types of Receptors
 There are three main types of
plasma membrane receptors:
 G-protein-linked
 Tyrosine kinases
 Ion channel
AP Biology
G-protein coupled receptors
(GPCRs)
 most common, many diverse functions
 Binds to energy-rich molecule GTP

(Guanosine-5'-triphosphate is a substrate for
RNA synthesis during transcription or for
DNA during replication)
 GPCRs also functions in embryonic
development & sensory reception
 Most medicines todays impact the G
protein pathway. Also, many diseases
(cholera, botulism, whooping cough)
produce toxins which interfere w/
AP Biology
pathway.
G-protein coupled receptors
(GPCRs)
G-protein-linked
Receptor
Plasma Membrane
Activated
Receptor
Signal molecule
GDP
CYTOPLASM
G-protein
(inactive)
Enzyme
GDP
GTP
Activated
enzyme
GTP
GDP
Pi
Cellular response
AP Biology
Inactivate
enzyme
Receptor tyrosine kinases
 Multiple pathway response
 Enzymatic activity- a “kinase” is an
enzyme that catalyzes the transfer of
phosphate groups
 Regulates/coordinates many cell
functions like cell growth and cell
reproduction
 Abnormal receptor tyrosine kinases
are linked to many types of cancer
(ex. breast cancer)
AP Biology
Receptor tyrosine kinases
Signal-binding site
Signal
molecule
Signal
molecule
Helix in the
Membrane
Tyrosines
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine
kinase proteins
(inactive monomers)
CYTOPLASM
Dimer
Figure 11.7
Activated
relay proteins
Tyr P
Tyr
Tyr
P Tyr
Tyr P
P Tyr
Tyr
Tyr
P Tyr
Tyr P
P Tyr
Tyr P
Tyr P
P Tyr
Tyr P
Tyr
Tyr
6
Activated tyrosinekinase regions
(unphosphorylated
dimer)
AP Biology
ATP
6 ADP
P Tyr
Fully activated receptor
tyrosine-kinase
(phosphorylated
dimer)
Inactive
relay proteins
Cellular
response 1
Cellular
response 2
Ion channel receptors
• Has an area that acts as a
“gate” when the receptor
changes shape
• When ligand binds,
channel can open or
close.
• Ex: neurotransmitters
bind as ligands for ion
channels
• Some controlled by
Figure 11.7
AP
Biology
electrical
signals instead
Signal
molecule
(ligand)
Gate closed
Ligand-gated
ion channel receptor
Ions
Plasma
Membrane
Gate open
Cellular
response
Gate close
*Intracellular Receptors
 Target protein is INSIDE the cell
 Must be hydrophobic molecule
Hormone
EXTRACELLULAR
(testosterone) FLUID
Plasma
membrane
Receptor
protein
Hormonereceptor
complex
DNA
AP Biology
Figure 11.6
CYTOPLASM
hormone testosterone
passes through the
plasma membrane.
2 Testosterone binds
to a receptor protein
in the cytoplasm,
activating it.
3 The hormonereceptor complex
enters the nucleus
and binds to specific
genes.
4 The bound protein
stimulates the
transcription of
the gene into mRNA.
mRNA
NUCLEUS
1 The steroid
New protein
5 The mRNA is
translated into a
specific protein.
Apoptosis
 Programmed cell death
 Cell shrinks and forms lobes (blebbing)
 Common during embryonic

development
Ex. Mitochondrial membrane leaks
proteins that promote apoptosis.
AP Biology
Why cell suicide?
 Development and maintenance in
animals (also in fungi and yeast)
 Studied in worms (nematodes)
 Development of nervous system,
morphogenesis of hands, feet, paws
(failure results in webbing)
 Failure of apoptosis linked to
degenerative diseases like Parkinsons
and Alzheimers or cancers like
melanoma
AP Biology
That’s about it!
AP Biology