Cell Communication

AP Biology
 The “Cellular Internet”
• Biologists have discovered some universal
mechanisms of cellular regulation that involve
cell-to-cell communication.

•External signals are converted into responses within the cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Evolution of Cell Signaling
• Yeast cells
– Identify their mates by cell signaling
1 Exchange of  factor
mating factors. Receptor
Each cell type
secretes a
mating factor a 
that binds to
receptors on
the other cell
type. Yeast cell,  factor Yeast cell,
mating type a mating type 
2 Mating. Binding
of the factors to
    receptors
induces changes a 
     in the cells that
    lead to their
    fusion.
3 New a/ cell.
The nucleus of
the fused cell
includes all the
a/
genes from the
Figure 11.2 a and a cells.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Methods used by Cells to Communicate
• Cell-Cell communication

• Cell Signaling using chemical messengers

1. Local signaling over short distances
• Cell-Cell Recognition
• Local regulators
– Paracrine (growth factors)

– Synaptic (neurotransmitters)

2. Long distance signaling

• Hormones
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 Cell-Cell Communication
• Animal and plant cells
– Have cell junctions that directly connect the
cytoplasm of adjacent cells
Plasma membranes

Gap junctions Plasmodesmata

between animal cells 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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Cell-Cell Communication

 Animal cells use gap junctions to send

signals
Cells must be in direct contact
Protein channels connecting two
adjoining cells

Gap junctions
between animal cells
AP Biology
 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
AP Biology
 Local Signaling: Cell-Cell Recognition
• In local signaling, animal cells may communicate via direct
contact
• Membrane bound cell surface molecules

• Glycoproteins

• Glyolipids

Figure 11.3(b) Cell-cell recognition. Two cells in an animal may communicate by interaction
between molecules protruding from their surfaces.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 Local Signaling: Local Regulators
• In other cases, animal cells

– Communicate using local regulators

– Only work over a short distance

Local signaling

Target cell Electrical signal

along nerve cell
triggers release of
neurotransmitter

Neurotransmitter
Secretory diffuses across
vesicle synapse

Local regulator
diffuses through Target cell
extracellular fluid is stimulated

(a) Paracrine signaling. A secreting cell acts (b) Synaptic signaling. A nerve cell
on nearby target cells by discharging releases neurotransmitter molecules
molecules of a local regulator (a growth into a synapse, stimulating the
factor, for example) into the extracellular target cell.
fluid.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Long-distance Signaling: Hormones
• In long-distance signaling
– Both plants and animals use hormones
Long-distance signaling

Endocrine cell Blood

vessel

Hormone travels
in bloodstream
to target cells

Target
cell

(c) Hormonal signaling. Specialized

endocrine cells secrete hormones
into body fluids, often the blood.
Hormones may reach virtually all
Figure 11.4 C body cells.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 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

AP Biology
The Three Stages of Cell Signaling
• Earl W. Sutherland (1971)
– Discovered how the hormone epinephrine acts on cells
• Sutherland suggested that cells receiving signals went
through three processes
– Reception
– Transduction
– Response
• Called Signal transduction pathways
– Convert signals on a cell’s surface into cellular
responses
– Are similar in microbes and mammals, suggesting an
early origin
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 Overview of cell signaling

EXTRACELLULAR CYTOPLASM
FLUID Plasma membrane

1 Reception 2 Transduction 3 Response

Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway

Signal
molecule

Figure 11.5

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Three Stages of Cell Signaling
EXTRACELLULAR CYTOPLASM
FLUID
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 binds to the

receptor protein
AP Biology
 Three Stages of Cell Signaling
EXTRACELLULAR CYTOPLASM
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
AP Biology
 Three Stages of Cell Signaling
EXTRACELLULAR CYTOPLASM
FLUID
Plasma membrane

1 Reception 2 Transduction 3 Response

Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway

Signaling Can be catalysis, activation of a gene,

molecule
triggering apoptosis, almost anything!
 The transduced signal triggers a
cellular response
AP Biology
 Signal Transduction Animation

 http://media.pearsoncmg.com/bc/bc_
campbell_biology_7/media/interactiv
emedia/activities/load.html?11&A

 http://www.wiley.com/legacy/college/bo
yer/0470003790/animations/signal_tran
sduction/signal_transduction.htm

AP Biology
 There are three most common types of
membrane receptor proteins.

 G-protein coupled receptors

 Receptor tyrosine-kinases
 Ion channel receptors

AP Biology
 1. Reception
• A signal molecule, a ligand, binds to a receptor
protein in a lock and key fashion, causing the
receptor to change shape.

Most receptor proteins

are in the cell
membrane but some are
inside the cell.

The G-protein is a
common membrane
receptor.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 G-Protein Coupled Receptors
G-Protein Receptors are often involved in diseases
such as bacterial infections.
Inactive
Plasma
G protein-coupled enzyme
membrane Activated Signaling molecule
receptor
receptor

Enzyme

GDP
1 2
GDP
GTP
CYTOPLASM G protein
(inactive)

Activated
enzyme

GTP
GDP

P
3 4
Cellular response

AP Biology
 • Receptor tyrosine kinases
Signal Signal-binding site
molecule
Signal
Helix in the molecule
Membrane

Tyr Tyr
Tyr Tyr Tyr Tyr
Tyrosines Tyr Tyr
Tyr Tyr Tyr Tyr
Tyr Tyr
Tyr Tyr Tyr Tyr

Receptor tyrosine
CYTOPLASM kinase proteins Dimer
(inactive monomers)

Activated
relay proteins

Cellular
Tyr Tyr P Tyr Tyr P P Tyr Tyr P
Tyr P
response 1
Tyr Tyr P Tyr Tyr P P Tyr
Tyr Tyr P Tyr Tyr P P Tyr Tyr P Cellular
6 ATP 6 ADP
response 2
Activated tyrosine- Fully activated receptor
kinase regions tyrosine-kinase Inactive
(unphosphorylated (phosphorylated relay proteins
dimer) dimer)
Figure 11.7

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Gate
Ion Channel Receptors Signaling
1 closed Ions

molecule
 Very important in (ligand)

the nervous system Ligand-gated Plasma

 Signal triggers the ion channel receptor membrane

2
opening of an ion Gate open

channel
 depolarization
Cellular
Triggered by response

neurotransmitters 3 Gate closed

AP Biology
 2. Transduction
• Transduction: Cascades of molecular
interactions relay signals from receptors to
target molecules in the cell
• Multistep pathways
– Can amplify a signal (Amplifies the signal by
activating multiple copies of the next component in the
pathway)

– Provide more opportunities for coordination

and regulation
• At each step in a pathway, the signal is
transduced into a different form, commonly a
conformational change in a protein.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fig. 11-9

Signaling molecule

Receptor
Activated relay
molecule Transduction:
Inactive
protein kinase
A Phosphorylation
1 Active
protein Cascade
kinase

Ph
1

os
ph
Inactive

or
ATP

yla
protein kinase
ADP Active P

ti o
2
protein

n
ca
PP kinase

sc
Pi 2

ad
e
Inactive
protein kinase ATP
ADP Active P
3
protein
PP kinase
Pi 3
Inactive
protein ATP
ADP P
Active Cellular
protein response
PP
Pi
AP Biology
 Protein Phosphorylation and Dephosphorylation
• Many signal pathways
– Include phosphorylation cascades

– In this process, a series of protein kinases add

a phosphate to the next one in line, activating it
– Phosphatase enzymes then remove the
phosphates

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 • A phosphorylation cascade
Signal molecule

Receptor Activated relay 1 A relay molecule

molecule activates protein kinase 1.

Inactive
protein kinase 2 Active protein kinase 1
1 Active transfers a phosphate from ATP
protein to an inactive molecule of
kinase protein kinase 2, thus activating

Ph
1 this second kinase.

os
ph
Inactive

ory
protein kinase ATP

lat
2 ADP Active P 3 Active protein kinase 2

io
protein then catalyzes the phos-

nc
kinase phorylation (and activation) of

as
PP
Pi

ca
2 protein kinase 3.

de
Inactive
protein kinase ATP
3 ADP Active P 4 Finally, active protein
protein kinase 3 phosphorylates a
5 Enzymes called protein kinase protein (pink) that brings
phosphatases (PP) PP
Pi 3 about the cell’s response to
catalyze the removal of
the phosphate groups Inactive the signal.
from the proteins, protein ATP
ADP P
making them inactive
Active Cellular
and available for reuse.
protein response
PP
P  i
Figure 11.8
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 The transduction stage of signaling is often a
multistep process that amplifies the signal.

About 1%
of our
genes are
thought to
code for
kinases.

http://media.pearsoncmg.com/bc
/bc_campbell_biology_7/media/in
teractivemedia/activities/load.ht
ml?11&C

AP Biology
 Small Molecules and Ions as Second Messengers
• Secondary messengers
– Are small, nonprotein, water-soluble molecules or ions that act as
secondary messengers.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Cyclic AMP
• Many G-proteins trigger the formation of cAMP,
which then acts as a second messenger in
cellular pathways.
First messenger
(signal molecule
such as epinephrine) Adenylyl
G protein cyclase

G-protein-linked GTP
receptor
ATP
cAMP

Protein
kinase A

Cellular responses
Figure 11.10
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 Cyclic AMP
• Cyclic AMP (cAMP)
– Is made from ATP

NH2 NH2 NH2

N N N
N N N
O O N N N N O N N
O
Adenylyl cyclase Phoshodiesterase
–
O P O P O P O Ch2 HO P O CH2
CH2
O O O O O O O O O
Pyrophosphate P H2O
P Pi O 
O
OH OH OH OH OH
ATP Cyclic AMP AMP

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Fig. 11-11

First messenger

Adenylyl
G protein cyclase

G protein-coupled GTP
receptor

ATP
Second
cAMP messenger

Transduction in a Protein
kinase A
G-protein pathway

Cellular responses
AP Biology
 Calcium ions and Inositol Triphosphate (IP3)
• Calcium, when released into the cytosol of a
cell acts as a second messenger in many
different pathways Calcium is an important
EXTRACELLULAR
FLUID
Plasma
membrane
second messenger
Ca2+ because cells are able to
ATP pump
Mitochondrion regulate its concentration
in the cytosol
Nucleus

CYTOSOL
Other second
messengers such as
Ca2+
pump inositol triphosphate and
Endoplasmic
ATP Ca2+ reticulum (ER) diacylglycerol can
pump
trigger an increase in
Key High [Ca2+] Low [Ca2+]
calcium in the cytosol
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 1 A signal molecule binds 2 Phospholipase C cleaves a 3 DAG functions as
to a receptor, leading to plasma membrane phospholipid a second messenger
activation of phospholipase C. called PIP2 into DAG and IP3. in other pathways.

EXTRA-
Signal molecule
CELLULAR
(first messenger)
FLUID
G protein

DAG
GTP
G-protein-linked PIP2
receptor Phospholipase C IP3
(second messenger)

IP3-gated
calcium channel

Endoplasmic Various
Cellular
reticulum (ER) Ca 2+ proteins
response
activated
Ca2+
(second
messenger)

4 IP3 quickly diffuses through 5 Calcium ions flow out of 6 The calcium ions
the cytosol and binds to an IP3– the ER (down their con- activate the next
gated calcium channel in the ER centration gradient), raising protein in one or more
Figure 11.12 membrane, causing it to open. the Ca2+ level in the cytosol. signaling pathways.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Growth factor

3. Response Receptor Reception

 Many possible
outcomes Phosphorylation
cascade Transduction

 This example
shows a CYTOPLASM

transcription
response Inactive Active
transcription transcription
factor factor Response
P

DNA
Gene

NUCLEUS
mRNA
AP Biology
 Signaling
molecule

 Specificity of the Receptor

signal
The same signal
molecule can
trigger different Relay
molecules
responses
Many responses
can come from
one signal! Response 1 Response 2 Response 3

Cell A. Pathway leads Cell B. Pathway branches,

to a single response. leading to two responses.

AP Biology
 The signal
can also
trigger an
activator or
inhibitor
 The signal
can also Activation
trigger or inhibition
multiple
receptors and
different
responses Response 4 Response 5

Cell C. Cross-talk occurs Cell D. Different receptor

between two pathways. leads to a different response.

AP Biology
 Response- cell signaling leads to regulation
of transcription (turn genes on or off) or
cytoplasmic activities.

AP Biology
 Long-distance Signaling
Intracellular signaling includes hormones that are
hydrophobic and can cross the cell membrane.

Once inside the cell, the

hormone attaches to a
protein that takes it
into the nucleus where
transcription can be
stimulated.

Testosterone acts as a
transcription factor.
AP Biology
 • Steroid hormones
– Bind to intracellular receptors
Hormone EXTRACELLULAR
(testosterone) FLUID 1 The steroid
hormone testosterone
passes through the
plasma membrane.
Plasma
Receptor membrane
2 Testosterone binds
protein to a receptor protein
Hormone- in the cytoplasm,
receptor activating it.
complex

3 The hormone-
receptor complex
enters the nucleus
and binds to specific
DNA genes.
mRNA 4 The bound protein
stimulates the
transcription of
NUCLEUS New protein the gene into mRNA.

5 The mRNA is
translated into a
specific protein.
Figure 11.6 CYTOPLASM

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Signaling Efficiency: Scaffolding Proteins and
Signaling Complexes

• Scaffolding proteins
– Can increase the signal transduction efficiency
Signal
molecule Plasma
membrane

Receptor

Three
different
protein
Scaffolding kinases
protein

Figure 11.16
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 Termination of the Signal
• Signal response is terminated quickly
– By the reversal of ligand binding

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

 Any Questions??
Can You Hear Me Now?

AP Biology
 Two systems control all physiological processes

1. Nervous System –
neurosecretory glands in
endocrine tissues secrete hormones.

2. Endocrine System
AP Biology
 Human Endocrine System

AP Biology
Major Vertebrate Endocrine Glands Their Hormones
(Hypothalamus–Parathyroid glands)

AP Biology
AP Biology
 Figure 45.6b Hormones of the hypothalamus and pituitary glands

Neurosecretory cells in endocrine organs and tissues

secrete hormones. These hormones are excreted
into the circulatory system.
AP Biology
 Stress and the http://highered.mcgraw-
hill.com/olcweb/cgi/pluginpop.cgi?

Adrenal Gland
it=swf::535::535::/sites/dl/free/0072437316/120109/bio4
8.swf::Action%20of%20Epinephrine%20on%20a
%20Liver%20Cell

AP Biology
Figure 45.4 One chemical signal, different effects

AP Biology
 Figure 45.9 Hormonal control of calcium homeostasis in mammals

http://bcs.whfreeman.co
m/thelifewire/content/ch
p42/4202003.html

AP Biology
Figure 45.10 Glucose homeostasis maintained by insulin and glucagon

http://vcell.ndsu.nodak.edu/animations/regulatedsecre
tion/movie.htm
AP Biology
 Cellular Communication Review

Denise Green

AP Biology
 REVIEW: Signal-transduction pathway
 Definition: Signal on a cell’s surface is converted into a
specific cellular response

 Local signaling (short distance):

√ Paracrine (growth factors)
√ Synaptic (neurotransmitters)
 Long distance: hormones

AP Biology
 Stages of cell signaling

 Sutherland (‘71)
 Glycogen depolymerization by epinephrine
 3 steps:
•Reception: target cell detection
•Transduction: single-step or series of changes
•Response: triggering of a specific cellular response

AP Biology
 • G-protein-linked receptors
Signal-binding site

Segment that
interacts with
G proteins

G-protein-linked Activated Inctivate

Plasma Membrane Signal molecule
Receptor Receptor enzyme

GDP
G-protein GDP GTP
CYTOPLASM (inactive) Enzyme

Activated
enzyme

GTP
GDP
Pi

Cellular response
Figure 11.7
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 Protein phosphorylation

 Protein activity
regulation
 Adding phosphate from
ATP to a protein
(activates proteins)
 Enzyme: protein kinases
(1% of all our genes)
 Example: cell
reproduction
 Reversal enzyme:
protein phosphatases

AP Biology
 Second messengers

 Non-protein signaling
pathway
 Example: cyclic AMP
(cAMP)
 Ex: Glycogen
breakdown with
epinephrine
 Enzyme: adenylyl
cyclase
 G-protein-linked
receptor in membrane
(guanosine di- or tri-
phosphate)
AP Biology
 Cellular responses to signals

 Cytoplasmic activity
regulation
 Cell metabolism
regulation
 Nuclear
transcription
regulation

AP Biology
 2010 Free Response Question

AP Biology
 The three stage of cellular signaling:
Reception, Transduction, and Response.
 http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/interactivemedia/activities/load.html?11&A

AP Biology