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AP Biology
Ch. 11 Cell Signaling
Overview of Cell Signaling
• Cell signaling evolved early in the history
of life. Evidence is that signaling in
microbes has much in common with the
process in multi-cellular organisms.
• The process by which a signal on a cell’s
surface is converted into a specific cellular
response is a series of steps called a
signal-transduction pathway.
Cell Signaling, cont.
• Cells communicate with one another by
means of chemical signals.
• The cell targeted by a particular signal has
a receptor molecule complementary to the
signal molecule, or ligand. The ligand fits
like a key in a lock and triggers a change
in the receptor molecule.
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Communication between mating yeast cells
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Communication between cells
• Communicating cells may be close
together or far apart. Animal cells signal
with nearby cells by secreting local
regulators, or if nerve cells,
neurotransmitters at synapses.
• Both animal and plant cells use hormones
for signaling over long distances.
• Cells can also communicate by direct
contact.
Cell Communication
Communication by direct contact
Q: Which of the following is true of intercellular junctions
such as tight junctions, desmosomes, and gap junctions?
• A) They are specialized forms of communication among
nerve cells.
• B) They are of primary importance in the contraction of
skeletal muscle cells.
• C) They are equally distributed among all tissue types.
• D) They help to integrate cells into a functioning unit.
• E) They aid in distributing DNA among cells.
Q: Which of the following provides the best evidence that
cell-signaling pathways evolved early in the history of life?
• A) They are seen in primitive cells such as yeast.
• B) Signal transduction molecules found in distantly
related organisms are similar.
• C) Signals can be sent long distance by cells.
• D) Most signals have surface receptors.
• E) Yeast signal each other for mating.
Stages of Cell Signaling
• The three stages of cell signaling are
reception, transduction, and response.
• Earl Sutherland discovered how
epinephrine binds to receptors on a cell’s
surface (reception), leading to a series of
changes in the receptor and other
molecules inside the cell (transduction)
and finally to the activation of an enzyme
that breaks down glycogen (response).
Cell Signaling Overview
Signal Reception
• A signal molecule binds to a receptor
protein, causing the protein to change
shape.
• 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.
Signal molecules
• Hormones can be classified as
polypeptides (proteins and
peptides), amines, and steroids
• Binding of water-soluble
hormones to cell-surface
receptors triggers intracellular
signal transduction, leading to
specific responses in the
cytoplasm or changes in gene
expression. Ex) epinephrine
• Lipid-soluble hormones and
intracellular receptors act in the
nucleus to regulate
transcription of genes. Ex)
testosterone and estrogen
G-protein-linked receptors
• Most signal receptors are plasma membrane
proteins.
• A G-linked receptor is a membrane receptor
that works with the help of a cytoplasmic G
protein.
• Ligand binding activates the receptor, which
then activates a specific G-protein, which
activates yet another protein in a signaltransduction pathway.
• Epinephrine uses this sort of receptor.
G-protein-linked receptor
Tyrosine-kinase receptors
• Tyrosine-kinase receptors react to the binding
of signal molecules by forming dimers and then
adding phosphate groups to tyrosines on the
cytoplasmic side of the receptor.
• Relay proteins in the cell can then be activated
by binding to different phosphorylated tyrosines,
allowing this receptor to trigger several
pathways at once.
• Growth factors commonly use tyrosine-kinase
receptors.
Ion-channel receptors
• Specific signal molecules cause ligandgated ion channels in a membrane to open
or close, regulating the flow of specific
ions, such as Na+ or Ca2+.
• Ligand-gated ion channels are very
important in the nervous system,
controlled by electrical signals.
Q: The activation of receptor tyrosine kinases is always
characterized by
•
•
•
•
•
A) dimerization and phosphorylation
B) IP3 binding
C) a phosphorylation cascade
D) GTP hydrolysis
E) channel protein conformational change
Q: Binding of a signal molecule to which type of receptor
leads directly to a change in the distribution of anions
and/or cations on opposite sides of the membrane?
•
•
•
•
•
A)
B)
C)
D)
E)
receptor tyrosine kinase
G-protein linked receptor
intracellular receptor
ligand-gated ion channel
phosphorylated kinase dimer
Intracellular receptors
• Intracellular receptors are cytosolic or
nuclear proteins.
• Signal molecules that can readily cross the
plasma membrane, such as steroid
hormones and nitric oxide, use these
receptors.
Steroid hormone interacts with
intracellular receptors
• Testosterone passes through
the plasma membrane.
• Testosterone binds to a
receptor protein in the cytosol,
activating it.
• The receptor-hormone
complex enters the nucleus
and binds to specific genes.
• The bound protein stimulates
the transcription of the gene
into mRNA.
• The mRNA is translated into a
specific protein.
Signal-transduction pathways
• Pathways relay signals from receptors to
cellular responses, usually by a multi-step
pathway.
• At each step in a pathway, the signal is
transduced into a different form, commonly
a conformational change in a protein.
Protein phosphorylation
• Protein phosphorylation, a common mode of
regulation in cells, is a major mechanism of
signal transduction.
• Many signal-transduction pathways include
phosphorylation cascades, in which a series of
protein kinases successively add phosphate
groups to the next one in line, activating it.
• Phosphatase enzymes soon remove the
phosphates.
A phosphorylation cascade
Cyclic AMP
• Certain small molecules and ions are key
components of signaling pathways as
“second messengers”.
• Second messengers, such as cyclic AMP
(cAMP) and Ca2+, diffuse readily through
the cytosol and help broadcast signals
quickly.
• Many G proteins activate adenylyl cyclase,
which makes cAMP from ATP.
Cyclic AMP: Second messengers
Q: Phosphorylation cascades involving a series of protein
kinases are useful for cellular signal transduction because
•
•
•
•
•
A)
B)
C)
D)
E)
they are species specific.
they always lead to the same cellular response.
they amplify the original signal manyfold.
they reverse the effects of phosphatases.
the number of molecules involved is small and fixed.
Calcium Ions and IP3
• Ca2+ can serve as a messenger because protein pumps
usually keep it at lower concentrations in the cytosol than
outside the cell.
• IP3 (inositol triphosphate) is the ligand for a gated
calcium channel in the membrane of the ER, which
stores Ca2+ at high concentrations.
• When IP3 binds, Ca2+ flows into the cytosol, where it
activates proteins of many signaling pathways.
• Increasing cytosolic concentration of Ca2+ causes many
responses in animal cells, including muscle contraction,
secretion of certain substances, and cell division. In
plant cells, it is involved in adaptive responses to
environmental stresses, such as drought or cold.
Calcium and IP3 in signaling pathways
Cellular Responses to Signals
• In response to a signal, a cell may
regulate activities in the cytoplasm or
transcription in the nucleus.
• For example, signaling pathways regulate
enzyme activity and cytoskeleton
rearrangement in the cytoplasm.
• Other pathways regulate genes; they do
this by activating transcription factors,
proteins that turn specific genes on or off.
Signal Amplification
• Elaborate pathways amplify and specify
the cell’s response to signals.
• Each catalytic protein in a signaling
pathway amplifies the signal by activating
multiple copies of the next component of
the pathway.
• Scaffolding proteins can increase signaltransduction efficiency by holding
components of a pathway together.
Cytoplasmic response to a signal: The stimulation of glycogen breakdown
by epinephrine
Refer to the molecules on the left for #1-4
•
•
•
•
•
A) Adenylyl cyclase
B) G-protein
C) Carrier protein
D) cAMP
E) Ion channel
receptor protein
• 1) Activated by directly
binding to GTP
• 2) Catalyzes the formation
of cytosolic secondmessenger molecules
• 3) Enhances the transport
of lipophilic signals in the
blood
• 4) Serves as an
intracellular second
messenger
Q: Which of the following is typically NOT a component of
a signal transduction pathway?
•
•
•
•
•
A)
B)
C)
D)
E)
production of more signal
production of second messengers such as cAMP
expression of specific proteins
activation of protein kinases
phosphorylation of certain transcription factors
Q: Lipid-soluble signal molecules, such as testosterone,
cross the membranes of all cells but affect only target cells
because
• A) only target cells have the appropriate DNA segments.
• B) intracellular receptors are present only in target cells.
• C) most cells lack surface receptors to inhibit lipid
soluble molecules.
• D) only target cells produce testosterone.
• E) most cells lack the enzymes to utilize testosterone.