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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

1
Yeast cells identify their mates by cell
signaling (early evidence of signaling)
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.
3 New a/ cell.
The nucleus of
the fused cell
includes all the
genes from the
a and a cells.
2
 factor
Receptor
a

Yeast cell,
mating type a
 factor
Yeast cell,
mating type 

a
a/
Hello tiger, go back to the previous slide
to answer # 2 (part 2) question!
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
CYTOPLASM
Plasma membrane
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
Organisms detect changes in their
environment and respond to these
changes in a variety of ways.
These changes may occur at the
cellular or organism level
Feedback Mechanism
These have evolved in living things as a
mechanism by which they maintain homeostasis
or dynamic equilibrium.
It occurs when the level of one substance
influences the level of another substance or
activity of another organ.
28
Feedback Mechanism
An example of a feedback mechanism in
humans would be the increase in heart rate
and respiratory rate which occurs in response
to increased exercise or other increased
muscle cell activity.
29
examples of
feedback
mechanisms
30
examples of feedback
mechanisms
The pancreas is an endocrine gland
which produces hormones which
regulate blood glucose (sugar) levels
An increase in blood sugar level
triggers the release of the hormone
insulin by the pancreas
the hormone insulin lowers blood sugar
level restoring the body to its original
blood glucose level in two major ways:
it increases the ability of body cells to
take in glucose from the blood
it converts blood glucose to the
compound glycogen -- this compound
is also called animal starch and is
stored in our liver and muscles
31
Maintenance of Water : plants need to regulate water loss and
carbon dioxide intake for photosynthesis and other life activities
when plants do not keep enough water in their cells, they wilt and die.
Stomate: a microscopic hole in a plant leaf which allows gases to enter
and leave and water vapor to leave as well. Stomata is the plural of
stomate.
Guard cells: open and close the stomate.
the ability of the guard cell to close during periods of limited water
availability for the plant allows the plant to maintain water
homeostasis
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Positive and Negative Feedback
Negative feedback occurs when the rate of
the process decreases as the concentration
of the product increases. It controls the
rate of a process to avoid accumulation of a
product.
Positive feedback occurs when the rate of a
process increases as the concentration of the
product increases. The rate of a process will
continuously accelerate under positive
feedback as long as substrate is available and
the product is not consumed by some other
process.
video
Hormonal Communication
The central nervous system can directly
release hormones, or it can signal tissues
throughout the body to release hormones to
provide rapid, short term communication
between different body regions.
 Hormones can stimulate nervous activity and
the release of hormones that can stimulate
the parasympathetic nervous system without
any input from the brain. They act more
slowly but generally have a longer effect.

video
Timing and coordination of
physiological events are
regulated by multiple
mechanisms.
What are circadian rhythms?
 They are physical, mental and behavioral
changes that follow a roughly 24-hour cycle,
responding primarily to light and darkness in
an organism’s environment.
 They are found in most living things, including
animals, plants and many tiny microbes.
 They are produced by natural factors within
the body, but they are also affected by
signals from the environment. Light is the
main cue influencing circadian rhythms,
turning on or turning off genes that control
an organism’s internal clocks.
How do circadian rhythms affect body
function and health?
 They can influence sleep-wake cycles,
hormone release, body temperature and other
important bodily functions. They have been
linked to various sleep disorders, such as
insomnia. Abnormal circadian rhythms have
also been associated with obesity, diabetes,
depression, bipolar disorder and seasonal
affective disorder.
How are circadian rhythms related to jet
lag?
 Jet lag occurs when travelers suffer from
disrupted circadian rhythms. When you pass
through different time zones, your body’s
clock will be different from your wristwatch.
For example, if you fly in an airplane from
California to New York, you “lose” 3 hours of
time. So when you wake up at 7:00 a.m., your
body still thinks it’s 4:00 a.m., making you feel
groggy and disoriented. Your body’s clock will
eventually reset itself, but this often takes a
few days.
Circadian clocks in plants
 are endogenous timekeepers that keep plant
responses synchronized with the environment.
They must continue to run:
 in absence of external inputs
 must be about 24 hours in duration
 can be reset or entrained
 can compensate for temperature differences
In plants, physiological events
involve interactions between
environmental stimuli and
internal molecular signals.
Plants and Light
Plants have three basic responses or
reactions to light. They are:
photosynthesis
Phototropism
photoperiodism
Plants and Light
 Photosynthesis is the process on which all life
on earth depends.
 Radiant energy from the sun is converted into
chemical energy.
 The energy is stored in chemical bonds in
sugars like glucose and fructose.
Plants and Light
Phototropism is the plant's movement in
response to light. All of us have seen the
houseplant that leans toward the window.
That is phototropism.
Growth hormones are produced which cause the
stem cells on the side away from the light to
multiply causing the stem to tilt.
The leaves are then closer to the light source
and aligned to intercept the most light.
Plants and Light
Photoperiodism is the plant's reaction to dark,
and it is controlled by the phytochrome
pigment in the leaves.
The pigment shifts between two forms based
on whether it receives more red or far red
light.
The reaction controls several different plant
reactions including seed germination, stem
elongation, dormancy, and blooming in day
length sensitive plants.
Plant Hormones:
 Auxin: causes stem elongation and growth formation of adventitious and lateral roots,
inhibits leaf loss, promotes cell division (with
cytokinins), increases ethylene production,
enforces dormancy of lateral buds produced
by shoot apical meristems and other immature
parts
Plant Hormones:
 Cytokinins: stimulate cell division (with auxin),
promote chloroplast development, delay leaf
aging, promote formation of buds, inhibit
formation of lateral roots produced by root
apical meristems and immature fruits
Plant Hormones
Gibberellins: promote stem elongation,
stimulate enzyme production in germinating
seeds produced by roots and shoot tips, young
leaves, seeds
Ethylene: controls shedding of leaves, flowers,
fruits, promotes fruit ripening produced by
apical meristems, leaf nodes, aging flowers,
ripening fruit