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Cell Communication
Overview: The Cellular
Internet
 Cell-to-cell
communication is
absolutely essential for multicellular
organisms
 Nerve cells must communicate pain
signals to muscle cells (stimulus) in
order for muscle cells to initiate a
response to pain
 Biologists
have discovered some
universal mechanisms of cellular
regulation
External Signals
Signal Transduction Pathway

Yeast cells identify their mates by cell
signaling (early evidence of signaling)
1
2
3
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/
Signal Transduction Pathways
 Convert signals on a cell’s
surface into cellular responses
 Are similar in microbes and
mammals, suggesting an early
origin
Local and Long-Distance
Signaling





Cells in a multicellular organism (tissues,
organs, systems) communicate via chemical
messengers
A hormone is a chemical released by a cell in
one part of the body, that sends out messages
that affect cells in other parts of the
organism
All multicellular organisms produce hormones
Plant hormones are also called phytohormones
Hormones in animals are often transported in
the blood

Animal and plant cells
 Have cell junctions that directly connect
the cytoplasm of adjacent cells
Plasma membranes
Gap junctions
between animal cells
Plasmodesmata
between plant cells
Figure 11.3 (a) Cell junctions. Both animals and plants have cell junctions that allow molecules
to pass readily between adjacent cells without crossing plasma membranes.
 In
local signaling, animal cells
 May communicate via direct contact
Figure 11.3(b) Cell-cell recognition. Two cells in an animal may communicate by interaction
between molecules protruding from their surfaces.
 In
other cases, animal cells
 Communicate using local regulators
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.
Target cell
is stimulated
(b) Synaptic signaling. A nerve cell
releases neurotransmitter molecules
into a synapse, stimulating the
target cell.

In long-distance signaling
 Both plants and animals use hormones
(e.g. Insulin)
Long-distance signaling
Endocrine cell
Blood
vessel
Hormone travels
in bloodstream
to target cells
Target
cell
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.
The Three Stages of Cell
Signaling

Earl W. Sutherland
 Discovered how the hormone epinephrine
acts on cells
Sutherland’s Three Steps
 Sutherland
suggested that cells
receiving signals went through
three processes
 Reception
 Transduction
 Response
 Overview
EXTRACELLULAR
FLUID
1 Reception
of cell signaling
Plasma membrane
CYTOPLASM
2 Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Signal
molecule
Figure 11.5
Step One - Reception
 Reception
occurs when a signal
molecule binds to a receptor protein,
causing it to change shape
 Receptor protein is on the cell
surface
 The
binding between signal molecule
(ligand) and receptor is highly specific
 A conformational change in a receptor
is often the initial transduction of the
signal
Step Two - Transduction
The binding of the signal molecule alters
the receptor protein in some way
 The signal usually starts a cascade of
reactions known as a signal transduction
pathway
 Multistep pathways can amplify a signal

Step Three - Response
 Cell
signaling leads to regulation of
cytoplasmic activities or transcription
 Signaling pathways regulate a variety
of cellular activities
Example of Pathway

Steroid hormones bind to intracellular receptors
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
mRNA
NUCLEUS
Figure 11.6
CYTOPLASM
receptor complex
enters the nucleus
and binds to specific
genes.
4
New protein
The bound protein
stimulates the
transcription of
the gene into mRNA.
5 The mRNA is
translated into a
specific protein.
 Other
pathways 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
NUCLEUS
mRNA
Termination of the Signal
 Signal
response is terminated
quickly by the reversal of ligand
binding
Receptors in the Plasma
Membrane
 There
are three main types of
membrane receptors:
 G-protein-linked
 Tyrosine kinases
 Ion channel
 G-protein-linked
receptors
Signal-binding site
Figure 11.7
Segment that
interacts with
G proteins
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
Inactivate
enzyme
 Receptor
tyrosine kinases
Signal-binding site
Signal
molecule
Signal
molecule
Helix in the
Membrane
Tyr
Tyrosines
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine
kinase proteins
(inactive monomers)
CYTOPLASM
Tyr
Dimer
Figure 11.7
Activated
relay proteins
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
6
ATP
Activated tyrosinekinase regions
(unphosphorylated
dimer)
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
Signal
molecule
(ligand)
Gate closed
Ligand-gated
ion channel receptor
Ions
Plasma
Membrane
Gate open
Cellular
response
Gate close
Figure 11.7