Chapter 11 Quiz Review
1. The process by which a signal on a cell’s surface is changed or transduced into a specific
cellular response is a series of steps called a signal transduction pathway.
a. This process usually begins when a signaling molecule (a ligand) changes the protein
receptor in some way.
2. In cell communication, the cell targeted by a particular chemical signal has a receptor
protein on or in the target cell that recognizes the signal molecule. The signal molecule
behaves as a ligand, a small molecule that binds with specificity to a larger molecule.
3. Evidence that cell signaling evolved early in the history of life comes from the similarities in
signal transduction pathways in prokaryotes and eukaryotes. This suggests that ancestral
signaling molecules evolved long ago in ancient prokaryotes and single-celled eukaryotes
and have since been adopted for new uses by their multicellular descendents.
4. Sutherland’s work indicated that signal molecules DID NOT interact directly with a cytosol
enzyme but required an intact plasma membrane before an enzyme was activated.
5. Some transmitting cells release local regulators that influence cells in the local vicinity.
a. Ex: Blood Clotting
b. Paracrine signaling: When a cell releases a signal molecule into the environment
and a number of cells in the immediate vicinity response.
c. One class of local regulators in animals, growth factors, includes compounds that
stimulate nearby target cells to grow and multiply.
i. This is an example of paracrine signaling, which occurs when numerous cells
simultaneously receive and respond to growth factors produced by a single
cell in their vicinity.
6.
7. There are two sexes, a and , each of which secretes a specific signaling molecule, a factor
and  factor, respectively.
a. These factors each bind to receptor proteins on the other mating type. After the
mating factors have bound to the receptors, the two cells grow toward each other
and undergo other cellular changes.
8. Intracellular signal receptors are found in the cytosol or nucleus of target cells.
a. To reach these receptors, a chemical messenger passes through the target cell’s
plasma membrane.
b. Such chemical messengers are either hydrophobic enough or small enough to cross
the phospholipid interior of the plasma membrane. EX: Testosterone and estrogen.
c. The cytosol (not the plasma membrane) of target cells contains receptor molecules
that bind testosterone, activating the receptor.
d. Non-target cells lack the intracellular receptor molecules and its genes will not be
activated.
9. A G-protein-linked receptor consists of a receptor protein associated with a G protein on
the cytoplasmic side.
a. Seven  helices span the membrane.
b. G-protein-linked receptors bind many different signal molecules, including yeast
mating factors, epinephrine and many other hormones, and neurotransmitters.
c. The G protein acts as an on-off switch.
d. A G protein is active when GTP is bound to it.
e. If GDP is bound to the G protein, the G protein is inactive.
1
f. When the appropriate signal molecule binds to the extracellular side of the
receptor, the G protein binds GTP (instead of GDP) and becomes active
10. Certain microbes cause disease by disrupting G-protein signaling pathways.
a. For example, the cholera bacterium, Vibrio cholerae, may be present in water
contaminated with human feces.
b. This bacterium colonizes the small intestine and produces a toxin that modifies a G
protein that regulates salt and water secretion.
c.
11. The tyrosine-kinase receptor system is especially effective when the cell needs to trigger
several signal transduction pathways and cellular responses at once.
a. This system helps the cell regulate and coordinate many aspects of cell growth and
reproduction.
b. The activation of a tyrosine-kinase receptor involves dimerization &
phosphorylation.
c. The tyrosine-kinase receptor belongs to a major class of plasma membrane
receptors that have enzymatic activity.
d. A protein kinase is an enzyme that catalyzes the transfer of phosphate groups
e. A ligand binds to two receptors, causing the two receptors to aggregate and form a
dimer.
i. This dimerization activates the tyrosine-kinase section of the receptors, each
of which then adds phosphate from ATP to the tyrosine tail of the other
polypeptide.
ii. The fully activated receptor proteins activate a variety of specific relay
proteins that bind to specific phosphorylated tyrosine molecules.
12. A ligand-gated ion channel is a type of membrane receptor that can act as a gate when the
receptor changes shape.
a. When a signal molecule binds as a ligand to the receptor protein, the gate
opens to allow the flow of specific ions, such as Na+ or Ca2+, through a channel
in the receptor.
b. Binding by a ligand to the extracellular side changes the protein’s shape and
opens the channel. This affects Membrane potential
c. When the ligand dissociates from the receptor protein, the channel closes.
d. The change in ion concentration within the cell may directly affect the activity of
the cell.
e. Ligand-gated ion channels are very important in the nervous system.
f. For example, neurotransmitter molecules released at a synapse between two
neurons bind as ligands to ion channels on the receiving cell, causing the
channels to open.
g. Ions flow in and trigger an electrical signal that propagates down the length of
the receiving cell.
h. Calcium ions activate the next protein in a signal transduction pathway
13. The pathways leading to calcium release involve still other second messengers,
diacylglycerol (DAG) and inositol trisphosphate (IP3).
a. DAG and IP3 are created when a phospholipase cleaves a specific membrane
phospholipid.
2
b. The phospholipase may be activated by a G protein or by a tyrosine-kinase
receptor.
c. IP3 activates a gated calcium channel, releasing Ca2+ from the ER.
14. Some transmitting cells release local regulators that influence cells in the local vicinity.
a. Paracrine signaling: When a cell releases a signal molecule into the environment
and a number of cells in the immediate vicinity response.
b. One class of local regulators in animals, growth factors, includes compounds that
stimulate nearby target cells to grow and multiply.
i. This is an example of paracrine signaling, which occurs when numerous cells
simultaneously receive and respond to growth factors produced by a single
cell in their vicinity.
15. Binding of signal molecules to receptors must be reversible, allowing the receptors to
return to their inactive state when the signal is released.
16. By a variety of means, relay molecules in transduction processes return to their inactive
forms:
a. The GTPase activity intrinsic to a G protein hydrolyzes its bound GTP; the enzyme
phosphodiesterase converts cAMP to AMP;
b. protein phosphatases inactivate phosphorylated kinases and other proteins;
17. Many signaling pathways involve small, water-soluble, nonprotein molecules or ions called
second messengers.
a. The extracellular signaling molecule that binds to the membrane receptor is a
pathway’s “first messenger.”
b. Second messengers diffuse rapidly throughout the cell.
c. Second messengers participate in pathways initiated by both G-protein-linked
receptors and tyrosine-kinase receptors.
d. Two of the most widely used second messengers are cyclic AMP and Ca2+.
e. A large variety of relay proteins are sensitive to the cytosolic concentration of one
or the other of these second messengers.
18. Once Sutherland knew that epinephrine causes glycogen breakdown without entering the
cell, he looked for a second messenger that transmits the signal from the plasma
membrane to the metabolic machinery in the cytoplasm.
a. Binding by epinephrine leads to increases in the cytosolic concentration of cyclic
AMP, or cAMP.
b. This increase occurs because the activated receptor activates adenylyl cyclase,
which converts ATP to cAMP in response to epinephrine.
c. Phosphodiesterase converts cAMP back to ATP
d. Inhibition of phosphodiesterase activity would prolong the effect of epinephrine
by maintaining elevated cAMP levels in the cytoplasm
e. When epinephrine outside the cell binds to a specific receptor protein, the normal
cellular concentration of cAMP can be boosted 20-fold within seconds.
f. cAMP is short-lived because phosphodiesterase converts it to AMP.
g. Another surge of epinephrine is needed to reboost the cytosolic concentration of
cAMP
19. Rather than relying on the diffusion of large relay molecules such as proteins, many signal
pathways are linked together physically by scaffolding proteins.
a. Scaffolding proteins may themselves be large molecules to which several other
relay proteins attach to facilitate cascading effects
3
b. For example, one scaffolding protein isolated from mouse brain cells holds three
protein kinases and carries these kinases with it when it binds to an appropriately
activated membrane receptor, facilitating a specific phosphorylation cascade.
c. Phosphorylation cascades involving a series of protein kinases can amplify the
original signal many times.
20. Elaborate enzyme cascades amplify the cell’s response to a signal.
a. At each catalytic step in a cascade, the number of activated products is much
greater than in the preceding step.
b. In the epinephrine-triggered pathway, binding by a small number of epinephrine
molecules can lead to the release of hundreds of millions of glucose molecules.
21. The responsibility for turning off a signal transduction pathway belongs to protein
phosphatases.
a. These enzymes rapidly remove phosphate groups from proteins, a process called
dephosphorylation.
b. By dephosphorylating and thus inactivating protein kinases, phosphatases provide
the mechanism for turning off the signal transduction pathway when the initial
signal is no longer present.
c. Phosphatases also make the protein kinases available for reuse, enabling the cell to
respond again to a signal.
22. Study the following diagrams: Be able to ID all of the structures depicted on thse diagrams.
23.
24.
4