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Cell Communication
Types of Cell Communication
• Cells in multicellular organisms
communicate via chemical messages
• Local (cells are adjacent)
• Long-distance
• Specific target cells recognize and
respond to a specific signaling molecule
Local Cell Communication
• Cells that are adjacent
may communicate via
• Cell Junctions –
molecules pass via the
cytoplasm using cell
junctions between cells
(plant and animal cells)
• Cell-cell recognition –
Communication by
interaction of molecules
protruding from the surface
of the cells (Animal)
Local Cell Communication
• Cells that are near each
other, but not necessarily
adjacent may also
communicate via
• Paracrine Signaling – A
secreting cell acts on nearby
target cells by discharging
‘messenger’ molecules of a local
regulator into the extracellular
fluid (for example: growth
factor)
• Synaptic Signaling – A nerve
cell releases neurotransmitter
molecules into a synapse,
stimulating the target cell
Long-Distant Cell Communication
• At times a signal cell and
target cell are farther apart
• Hormonal/Endocrine
Signaling – Specialized cells
release hormone molecules that
travel via the circulatory system
to the target cells
• Plants use similar system using
vessels or diffusion as travel
• The nervous system can also be
considered a part of longdistance communication
Stages of Cell Signaling
• Reception
• Transduction
• Response
Reception
• A receptor protein on or near the target cell allows
the cell to detect and react to messages
• The signaling molecule is complimentary in shape
and site specific on the receptor molecule
• Ligand (molecule that specifically binds to another)
binding generally causes a receptor protein to
change shape and may cause activation of the
receptor
• Causes the combining of 2 or more receptor molecules,
leading to other molecular events
Types of Receptors
• Plasma Membrane Receptors
• Most are water-soluble and embedded in
the cell membrane
• Transmits info from extracellular environs
to inside the cell via shape change or
aggregation
• Examples: G protein-coupled receptors,
receptor tyrosine kinases, ion channel
receptors
G Protein-Coupled
Receptors
• Composed of seven α helices
that span the plasma
membrane with loops that act
as binding sites
• Binds energy-rich GTP
• Functions include: role in
embryonic development,
sensory reception (vision and
smell)
• Involved in many bacterial
diseases (Ex: Cholera)
• Toxins produced by these
diseases interfere with the G
protein
• Many medicines also interact
with G protein pathways
G ProteinCoupled
Receptors
• G protein functions as an ‘on/off’ switch for the molecule
depending on whether GDP (inactive) or GTP (active) is
attached
• Signaling molecule causes a shape change. The cytoplasmic
side binds to the G protein, causing a GDP to be displaced by a
GTP, activating the molecule.
• Activated G protein then binds to an enzyme, triggering the
next step in a pathway.
• GTP is hydrolyzed, creating GDP and leaving the G protein
inactive
Receptor
Tyrosine
Kinases
• Receptors begin as individual
polypeptides with ligand
binding sites
• The binding of a signaling
molecule causes the
polypeptides to associate
creating a dimer and
activating the tyrosine kinase
region
• Each tyrosine kinase adds a P
from ATP, fully activating the
receptor protein
• Specific relay proteins will
now bind to specific
phosphorylated tyrosine,
causing a structural change
activating the bound proteins
and triggering a transduction
pathway
Ion Channel Receptors
• Gate on an ion channel remains
closed until a ligand binds to
the receptor
• Specific ions can flow through
when gate opens, rapidly
changing the concentration of
those ions inside the cell
• Ligand dissociates from the
receptor closing the gate
Types of Receptors
• Intracellular Receptors
• Found in either the cytoplasm or nucleus of target
cells
• A chemical messenger will need to pass through the
plasma membrane
• Tend to be either hydrophobic or small enough to cross
• Examples: Steroid Hormones, Thyroid Hormones,
nitric oxide
Steroid Hormone
• Testosterone passes through
the cell membrane, binds with
the receptor molecule
becoming active.
• The active form then enters the
nucleus and turns on specific
genes that control male sex
characteristics
• Transcription factors – control
which genes are turned on
(transcribed into mRNA)
Transduction
• Binding of the signaling molecule changes the
receptor protein and initiates transduction
• May involve a single step, but more often is a
sequence of steps in a pathway
• For plasma membrane receptors this is usually a
multistep pathway
• May include activation of proteins by addition or removal
of phosphate groups or release of ions or small molecules
• Multiple steps allows for amplification, greater
coordination and regulation of the signal
Signal Transduction
Pathways
• The binding of a signaling
molecule begins the pathway
with each step leading to
another step and eventually
leading to a cellular response
• Protein kinase is a relay
molecule and is used to pass
a P molecule from ATP to a
protein
• Several protein kinases may act
on each other in a chain or
phosphorylation cascade
• Enzymes called protein
phosphatases rapidly remove
P groups from proteins,
inactivating proteins and
turning off the pathway
Ions and Molecules as 2nd
Messengers
• Signal transduction pathways will
sometimes involve nonprotein
molecules that are water-soluble or ions
called 2nd messengers
• Can readily spread throughout the cell
via diffusion
• Examples: Cyclic AMP, Calcium ions and
IP3
Cyclic AMP
• Cyclic adenosine
monophosphate (cAMP)
• An enzyme (adenylyl cyclase)
in the plasma membrane
converts ATP to cAMP in
response to a signal (first
messenger)
• cAMP broadcasts the signal
to the cytoplasm, usually
activating protein kinase A
which phosphorylates other
proteins
Example: Epinephrine
Calcium Ions
and IP3
• More commonly used than cAMP
• Signal induces an increase in
cytosolic concentration of Ca2+
• Concentration usually much higher
outside of cell and within certain
organelles (ER, mitochondria,
chloroplasts)
• Rise in CA2+ usually accomplished by
release of calcium from ER
• Causes muscle cell contraction, cell
division, secretion of substances
• Used by neurotransmitters, growth
factors and some hormones
• Inositol triphosphate (IP3) and
diacylglycerol (DAG) are other 2nd
messengers
Response
• The final outcome of
the message which
is the regulation of
some cellular activity
• Response may occur
in cytoplasm or
nucleus, but many
regulate protein
synthesis by turning
specific genes on
and off
Fine-Tuning the
Response
• Signal Amplification
• Specificity of Cell Signaling and
Coordination of the Response
• Pathway leads to a single response
• Pathway branches leading to two
responses
• Cross-talk occurs between 2 pathways
• Different receptor leads to a different
response
• Signaling Efficiency
• May be related to the presence of
scaffolding proteins that hold several relay
proteins which may help to hold pathways
together
• Termination of Signal
Apoptosis
• Programmed cell suicide
• Occurs when cells are infected, damaged or old/beyond cell
usefulness
• Signals for death may come from outside the cell or via alarms inside
• Alarms come from either the nucleus (DNA has suffered irreparable damage)
or the ER (excessive protein misfolds)
• DNA is chopped up, organelles are fragmented, and the cell
shrinks and goes through blebbing (becomes lobed)
• The cells parts are then packaged up into vesicles and are
digested by scavenger cells