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Cell Communication
• Cells communicate by generating,
transmitting, and receiving chemical
signals.
Signal transduction pathway
• Process by which a signal on a cell’s
surface is converted into a specific cellular
response
– Communication involves transduction of
stimulatory or inhibitory signals from other
cells, organisms or the environment
– Occurs in both unicellular and multicellular
organisms
• Cell communication processes share
common features that reflect a shared
evolutionary history.
– Evidence from yeast and animals suggest that
early versions of cell signaling mechanisms
used today evolved from bacteria
Unicellular organisms
• Signal transduction pathways influence
how the cell responds to its environment
– Examples:
• Use of chemical messengers by microbes
to communicate with other nearby cells
and to regulate specific pathways in
response to population density (quorum
sensing)
• Use of pheromones to trigger reproduction
and developmental pathways
– Example: mating yeast cells
• Response to external signals by bacteria
that influences cell movement
Multicellular Organisms
• Signal transduction pathways coordinate
the activities within individual cells that
support the function of the organism as a
whole
• Epinephrine
stimulation of
glycogen
breakdown in
mammals
• Temperature determination of sex in some
vertebrate organisms
• DNA repair mechanisms
• Cells communicate with each other
through direct contact with other cells or
from a distance via chemical signaling
Signal transduction pathways
– Occurs in 3 steps
• Reception: target cell detects signal coming from
outside of cell (binds with protein usually on cell
surface)
• Transduction: binding of signal molecule changes
protein in some way causing a relay of message
inside cell
• Response: transduced signal triggers a specific
cellular response
Direct Contact Cell to Cell
Communication
• Cells can communicate by transferring
chemical messengers that target cells in
close vicinity
Example of Direct contact
– Immune cells interact by cell-cell contact,
antigen presenting helpers (APCs), helper T
cells and killer T Cells
• T cells and antigens circulate within blood and
detect specific invaders, they produce chemical
signals to tell each other that the cells belong
• Plasmodesmata (gap junctions) between
plant cells that allow material to be
transported from cell to cell
• Cells communicate over short distances
by using local regulators that target cells in
the vicinity of the emitting cell
• Plant immune responses
– invading pathogen are deprived of nutrients
because the plant sends signals to kill cell
surrounding the region where the pathogen infected
the plant. Plant also induces the production of
antimicrobial proteins.
– When herbivores start eating plant, cell sends
chemical signals to other regions of the plant
• Some chemicals can inhibit leaf digestion in the insect’s
gut
Quorum Sensing
• System of stimulae and response
correlated to population density
– In bacteria, allows one organism to
influence the gene expression of other
bacteria in close proximity
• Regulates: biofilm formation, virulence, and
antibiotic resistance
– Social insects use it to determine where to
nest
QUORUM SENSING
• Morphogens in
embryonic
development
APOPTOSIS
“Cell suicide”
Why are “Death proteins“ present in inactive form?
Neurons (Nerves)
• Specialized cells to transmit
nerve impulses from one part
of body to another
• 3 main parts
– Dendrite (signal receiving end)
– Cell body
– Axon (signal sending end)
• Many axons have a myelin sheath that
acts as an electrical insulator.
• Schwann cells, which form the myelin
sheath, are separated by gaps of
unsheathed axon over which the impulse
travels as the signal propagates along the
neuron
Synapses
• junction between two neurons where
messages are passed on by
neurotransmitters
Resting neuron structure
• Contain potassium (K+) ions inside neuron
• Sodium (Na+) ions are outside plasma
membrane and don’t normally pass inward
Polarized neuron
• Inactive (not sending an impulse)
• Inside of neuron negatively charged to
compared to outside
Stimulated neuronDepolarization
• Na+ gates in membrane open and Na+
rushes into cell causing inside to be more
positive than outside of cell which
activates neuron to transmit an action
potential
Action potential
• nerve impulse which causes the axon to
release a neurotransmitter into synapse
that binds to next neuron stimulating it
Cell Repolarization
• After action potential
• K+ rushes out of cell causing the inside to
become negative again
• Must occur before neuron can send
another nerve impulse
After repolarization
• Na+/K+ pump
– Pumps K+ into cell and Na+ out to restore cell
to make it polarized again
• Transmission of information via
neurotransmitters between neurons occur
across synapses and result in responses
– Responses can be stimulatory or inhibitory
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Epinephrine
Acetylcholine
Norepinephrine
Dopamine
Serotonin
GABA
• YOUTUBE Video
The L
• Signals released by one cell type can travel long
distances to target cells of another type
– Endocrine signals (hormones) are produced by
endocrine cells that release signaling molecules,
which are specific and can travel long distances
through the blood to reach all parts of the body
– Examples:
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Insulin
Human growth hormone
Thyroid hormone
Testerone
Estrogen
Insulin
• Signal transduction pathways link signal
reception with cellular response
– Signaling begins with the recognition of a chemical
messenger, a ligand, by a receptor protein
• Different receptors recognize different chemical messengers,
which can be peptides, small chemicals or proteins, in a
specific one to one relationship
• A receptor protein recognizes signal molecules, causing
receptor protein’s shape to change, which initiates
transduction of the signal
G protein linked receptors
• Plasma membrane receptor
• When GDP is bound, G protein is inactive
– When GTP is bound, G protein is active
Ligand gated ion channels
• Incoming IONS trigger the response
Receptor tyrosine kinases
• Receptor for growth factor
• KINASE: Protein that
“phosphorylates” (adds a
phosphate to) another
molecule
• TYROSINE KINASES:
Proteins that form dimers.
Tyrosine amino acid
residues are active in
transfer of phosphates to
relay proteins
• Remain ACTIVE as long as
LIGAND is attached