Cell Membranes and Signalinggilsonscience.weebly.com/uploads/2/1/1/4/21140528/... · Concept 5.5...
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Cell Membranes and
Signaling
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Cells can respond to many signals if they
have a specific receptor for that signal.
A signal transduction pathway is a
sequence of molecular events and
chemical reactions that lead to a cellular
response, following the receptor’s
activation by a signal.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Cells are exposed to many signals and may have different responses:
•Autocrine signals affect the same cells that release them.
• Paracrine signals diffuse to and affect nearby cells.
•Hormones travel to distant cells.
Figure 5.10 Chemical Signaling Concepts
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Only cells with the necessary receptors
can respond to a signal—the target cell
must be able to sense it and respond to
it.
A signal transduction pathway involves a
signal, a receptor, and a response.
Figure 5.11 Signal Transduction Concepts
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
A common mechanism of signal
transduction is allosteric regulation.
This involves an alteration in a protein’s
shape as a result of a molecule binding
to it.
A signal transduction pathway may
produce short or long term responses.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
A signal molecule, or ligand, fits into a
three-dimensional site on the receptor
protein.
Binding of the ligand causes the receptor
to change its three-dimensional shape.
The change in shape initiates a cellular
response.
Figure 5.12 A Signal Binds to Its Receptor
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Ligands are generally not metabolized
further, but their binding may expose an
active site on the receptor.
Binding is reversible and the ligand can
be released, to end stimulation.
An inhibitor, or antagonist, can bind in
place of the normal ligand.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Receptors can be classified by their
location in the cell.
This is determined by whether or not their
ligand can diffuse through the
membrane.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Cytoplasmic receptors have ligands, such
as estrogen, that are small or nonpolar
and can diffuse across the membrane.
Membrane receptors have large or polar
ligands, such as insulin, that cannot
diffuse and must bind to a
transmembrane receptor at an
extracellular site.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Receptors are also classified by their
activity:
• Ion channel receptors
• Protein kinase receptors
• G protein–linked receptors
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Ion channel receptors, or gated ion
channels, change their three-
dimensional shape when a ligand binds.
The acetylcholine receptor, a ligand-
gated sodium channel, binds
acetylcholine to open the channel and
allow Na+ to diffuse into the cell.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Protein kinase receptors change their
shape when a ligand binds.
The new shape exposes or activates a
cytoplasmic domain that has catalytic
(protein kinase) activity.
Figure 5.13 A Protein Kinase Receptor
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Protein kinases catalyze the following
reaction:
ATP + protein → ADP + phosphorylated
protein
Each protein kinase has a specific target
protein, whose activity is changed when
it is phosphorylated.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Ligands binding to G protein–linked
receptors expose a site that can bind to
a membrane protein, a G protein.
The G protein is partially inserted in the
lipid bilayer, and partially exposed on
the cytoplasmic surface.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
Many G proteins have three subunits and
can bind three molecules:
• The receptor
• GDP and GTP, used for energy transfer
• An effector protein to cause an effect in
the cell
Concept 5.5 The Membrane Plays a Key Role in a Cell’s
Response to Environmental Signals
The activated G protein–linked receptor
exchanges a GDP nucleotide bound to
the G protein for a higher energy GTP.
The activated G protein activates the
effector protein, leading to signal
amplification.
Figure 5.14 A G Protein–Linked Receptor
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Signal activation of a specific receptor
leads to a cellular response, which is
mediated by a signal transduction
pathway.
Signaling can initiate a cascade of protein
interactions—the signal can then be
amplified and distributed to cause
different responses.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
A second messenger is an intermediary between the receptor and the cascade of responses.
In the fight-or-flight response, epinephrine (adrenaline) activates the liver enzyme glycogen phosphorylase.
The enzyme catalyzes the breakdown of glycogen to provide quick energy.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Researchers found that the cytoplasmic
enzyme could be activated by the
membrane-bound epinephrine in broken
cells, as long as all parts were present.
They discovered that another molecule
delivered the message from the “first
messenger,” epinephrine, to the
enzyme.
Figure 5.15 The Discovery of a Second Messenger (Part 1)
Figure 5.15 The Discovery of a Second Messenger (Part 2)
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
The second messenger was later discovered to be cyclic AMP (cAMP).
Second messengers allow the cell to respond to a single membrane event with many events inside the cell—they distribute the signal.
They amplify the signal by activating more than one enzyme target.
Figure 5.16 The Formation of Cyclic AMP
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Signal transduction pathways involve
multiple steps—enzymes may be either
activated or inhibited by other enzymes.
In liver cells, a signal cascade begins
when epinephrine stimulates a G
protein–mediated protein kinase
pathway.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Epinephrine binds to its receptor and activates a G protein.
cAMP is produced and activates protein kinase A—it phosphorylates two other enzymes, with opposite effects:
• Inhibition
• Activation
Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 1)
Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 2)
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
• Inhibition—protein kinase A inactivates
glycogen synthase through
phosphorylation, and prevents glucose
storage.
• Activation—Phosphorylase kinase is
activated when phosphorylated and is
part of a cascade that results in the
liberation of glucose molecules.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Signal transduction ends after the cell
responds—enzymes convert each
transducer back to its inactive precursor.
The balance between the regulating
enzymes and the signal enzymes
determines the cell’s response.
Figure 5.18 Signal Transduction Regulatory Mechanisms
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Cells can alter the balance of enzymes in
two ways:
• Synthesis or breakdown of the enzyme
• Activation or inhibition of the enzymes
by other molecules
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its
Environment
Cell functions change in response to
environmental signals:
• Opening of ion channels
• Alterations in gene expression
• Alteration of enzyme activities
Answer to Opening Question
Caffeine is a large, polar molecule that
binds to receptors on nerve cells in the
brain.
Its structure is similar to adenosine, which
binds to receptors after activity or stress
and results in drowsiness.
Caffeine binds to the same receptor, but
does not activate it—the result is that
the person remains alert.
Figure 5.19 Caffeine and the Cell Membrane (Part 1)
Figure 5.19 Caffeine and the Cell Membrane (Part 2)