Raven/Johnson Biology 8e
Chapter 09- Answers
1. What is a ligand?
a. An integral membrane protein associated with G proteins
b. A DNA-binding protein that alters gene expression
c. A cytoplasmic second-messenger molecule
d. A molecule or protein that can bind to a receptor
The correct answer is d—
A. Answer a is incorrect. Receptor proteins are integral membrane proteins. G proteincoupled receptors function through their association with G proteins.
The correct answer is d—
B. Answer b is incorrect. Transcription factors are proteins that bind DNA and trigger a
change in gene expression.
The correct answer is d—
C. Answer c is incorrect. Second-messenger molecules are produced as a result of an initial
ligand-receptor signaling event.
The correct answer is d—A molecule or protein that can bind to a receptor
D. Answer d is correct. Ligands are typically small molecules or proteins that travel through
the extracellular environment and interact with cell-surface receptor proteins.
2. In the case of paracrine signaling the ligand is—
a. produced by the cell itself
b. secreted by neighboring cells
c. present on the plasma membrane of neighboring cells
d. secreted by distant cells
The correct answer is b—
A. Answer a is incorrect. A signal produced by the cell itself is an example of autocrine
signaling.
The correct answer is b—secreted by neighboring cells
B. Answer b is correct. Paracrine signaling is limited to the immediate cellular environment.
The correct answer is b—
C. Answer c is incorrect. Signaling through the interaction of membrane proteins is an
example of direct contact signaling.
The correct answer is b—
D. Answer d is incorrect. When the origin of a ligand is distant from the site receptor
binding, the signaling is said to be endocrine.
3. A neurotransmitter functions as a ligand in which type of signaling?
a. Direct contact
b. Endocrine
c. Synaptic signaling
d. Autocrine
Raven/Johnson Biology 8e
Chapter 09- Answers
The correct answer is c—
A. Answer a is incorrect. Direct contact signaling requires that proteins on the surface of the
two neighboring cells form a physical connection.
The correct answer is c—
B. Answer b is incorrect. Endocrine signaling requires that a physical distance exist between
the cellular source of a ligand and the eventual site of ligand–receptor binding.
The correct answer is c—Synaptic signaling
C. Answer c is correct. Synaptic signaling is a special type of signaling associated with the
cells of the nervous system. A synapse is the small gap that exists between two nerve
cells. Neurotransmitters diffuse across this small space.
The correct answer is c—
D. Answer d is incorrect. Autocrine signaling occurs when the cell creates its own signals.
4. The function of a ________ is to add phosphates to proteins, whereas a _________
functions to remove the phosphates.
a. tyrosine; serine
b. protein phosphatase; protein dephosphatase
c. protein kinase; protein phosphatase
d. receptor; ligand
The correct answer is c—
A. Answer a is incorrect. The amino acids tyrosine and serine are capable of being
phosphorylated. They receive the phosphate group, but do not function in the process of
phosphorylation.
The correct answer is c—
B. Answer b is incorrect. A phosphatase is an enzyme that removes phosphates.
The correct answer is c—protein kinase; protein phosphatase
C. Answer c is correct. A kinase is an enzyme that attaches a phosphate group to a protein,
and a phosphatase removes phosphate groups.
The correct answer is c—
D. Answer d is incorrect. Some receptors can function to phosphorylate proteins; however,
ligand molecules are not involved in the removal of phosphate group.
5. Which of the following type(s) of membrane receptors functions by changing the
phosphorylation state of proteins in the cell?
a. Channel-linked receptor
b. Enzymatic receptor
c. G protein-coupled receptor
d. Both b and c
The correct answer is d—
A. Answer a is incorrect. A channel-linked receptor functions to allow the movement of ions
into or out of a cell.
Raven/Johnson Biology 8e
Chapter 09- Answers
The correct answer is d—
B. Answer b is incorrect. Enzymatic receptors can function as kinases, phosphorylating
themselves and surrounding protein substrates. However, this is not the only true option.
The correct answer is d—
C. Answer c is incorrect. G protein-coupled receptors can activate signal transduction
pathways that activate protein kinases. They activate PKA, PKC, and also can stimulate
MAP kinase cascade. However, this is not the only true option.
The correct answer is d—Both b and c
D. Answer d is correct. Both enzymatic receptors and G protein-coupled receptors can
activate protein kinases or phosphatases, and thus the phosphorylation state of a cell.
6. How does the function of an intracellular receptor differ from that of a membrane
receptor?
a. The intracellular receptor binds a ligand.
b. The intracellular receptor binds DNA.
c. The intracellular receptor activates a kinase.
d. The intracellular receptor functions as a second messenger.
The correct answer is b—
A. Answer a is incorrect. All receptors must bind a ligand in order to function.
The correct answer is b—The intracellular receptor binds DNA.
B. Answer b is correct. Only the intracellular type of receptors is capable of directly binding
to DNA in response to ligand binding.
The correct answer is b—
C. Answer c is incorrect. Enzymatic receptors present on the plasma membrane can function
as kinases themselves, or can activate a signal cascade involving kinases.
The correct answer is b—
D. Answer d is incorrect. G protein-coupled receptors located in the plasma membrane of a
cell function to trigger the production of second-messenger molecules.
7. During a protein kinase signal cascade—
a. sequential phosphorylation of different kinases leads to a change in gene
expression
b. multiple G proteins become activated
c. phosphorylation of adapter proteins leads to the formation of second messengers
d. the number of MAP kinase proteins present in the cytoplasm is amplified
The correct answer is a—sequential phosphorylation of different kinases leads to a change in
gene expression
A. Answer a is correct. A signal cascade involves many different proteins that interact in a
step-by-step pathway.
The correct answer is a—
Raven/Johnson Biology 8e
Chapter 09- Answers
B. Answer b is incorrect. G proteins are associated with enzymes that catalyze the
production of second-messenger molecules, not enzymes that phosphorylate proteins.
The correct answer is a—
C. Answer c is incorrect. Adaptor proteins are an important part of a protein kinase signal
cascade, but they do not function to synthesize second-messenger molecules.
The correct answer is a—
D. Answer d is incorrect. The activity of MAP kinase proteins changes as part of the
cascade; however, the number of proteins present in the cytoplasm does not.
8. What is the function of Ras during tyrosine kinase cell signaling?
a. It activates the opening of channel-linked receptors.
b. It synthesizes the formation of second messengers.
c. It phosphorylates other enzymes as part of a pathway.
d. It links the receptor protein to the MAP kinase pathway.
The correct answer is d—
A. Answer a is incorrect. The opening of a channel-linked receptor is controlled by ligand
binding.
The correct answer is d—
B. Answer b is incorrect. G protein-regulated enzymes such as adenylyl cyclase are
responsible for the synthesis of second-messenger molecules.
The correct answer is d—
C. Answer c is incorrect. Ras is not a kinase protein.
The correct answer is d—It links the receptor protein to the MAP kinase pathway.
D. Answer d is correct. Ras functions to activate the first kinase in the MAP kinase pathway.
The activity of Ras is regulated by the tyrosine kinase receptor.
9. Which of the following best describes the immediate effect of ligand binding to a G
protein-coupled receptor?
a. The G protein trimer releases a GDP and binds a GTP.
b. The G protein trimer dissociates from the receptor.
c. The G protein trimer interacts with an effector protein.
d. The α subunit of the G protein becomes phosphorylated.
The correct answer a—The G protein trimer releases a GDP and binds a GTP.
A. Answer a is correct. Ligand binding to a G protein-coupled receptor results in the
activation of the G protein. G proteins are only active when they bind a GTP molecule.
The correct answer a—
B. Answer b is incorrect. The G protein trimer must be released from the receptor in order to
function, but it first must bind a molecule of GTP.
The correct answer a—
Raven/Johnson Biology 8e
Chapter 09- Answers
C. Answer c is incorrect. G proteins can only interact with their effector proteins after they
become activated and dissociate from the G protein-coupled receptor.
The correct answer a—
D. Answer d is incorrect. The α subunit of the G protein can bind GTP; it does not become
phosphorylated.
10. The amplification of a cellular signal requires all of the following except—
a. a ligand
b. DNA
c. a second messenger
d. a protein kinase
The correct answer is b—
A. Answer a is incorrect. All signaling pathways require a ligand-binding event to start.
Amplification cannot happen without ligand binding first.
The correct answer is b—DNA
B. Answer b is correct. DNA and changes in gene expression are typically the final step in a
signaling pathway and not part of the amplification process.
The correct answer is b—
C. Answer c is incorrect. Second messengers are important for amplification because a single
signaling event can result in the production of many second-messenger molecules.
The correct answer is b—
D. Answer d is incorrect. Protein kinases are involved in signal amplification through their
ability to phosphorylate more than one substrate protein.
11. Adenylyl cyclase is responsible for the production of which second-messenger molecule?
a. Cyclic AMP
b. Calcium
c. IP3
d. Calmodulin
The correct answer is a—Cyclic AMP
A. Answer a is correct. Adenylyl cyclase is the enzyme that produces cAMP when activated
by a G protein.
The correct answer is a—
B. Answer b is incorrect. Calcium is an important second messenger; however, it is not the
product of an enzyme.
The correct answer is a—
C. Answer c is incorrect. IP3 is an important second-messenger molecule; however, it is
produced when the enzyme phospholipase C is activated by a G protein.
The correct answer is a—
Raven/Johnson Biology 8e
Chapter 09- Answers
D. Answer d is incorrect. Calmodulin is involved in mediating the influence of the calcium
in the cell.
12. The response to signaling through G protein-coupled receptors can vary in different cells
because—
a. all receptors act through the same G protein
b. different isoforms of a receptor bind the same ligand but activate different
effectors
c. the amount of receptors in the membrane differs in different cell types
d. different receptors can activate the same effector
The correct answer is b—
A. Answer a is incorrect. All receptors do not activate the same G protein. There are a
limited number of G proteins, but not a single G protein for all receptors.
The correct answer is b—different isoforms of a receptor bind the same ligand but activate
different effectors
A. Answer b is correct. A single receptor type can have different isoforms in different cells
that bind the same ligand but activate different signaling pathways.
The correct answer is b—
A. Answer c is incorrect. The number of receptors in the membrane does not determine
specificity of signaling.
The correct answer is b—
A. Answer d is incorrect. This would produce the opposite effect; that is, two different
signals producing the same response, not the same signal producing a different response.
13. What is the function of the tight junctions in the formation of a tissue?
a. Tight junctions connect one cell to the next, creating a barrier between the cells.
b. Tight junctions form a strong anchor between two cells.
c. Tight junctions allow for the movement of small molecules between cells.
d. Tight junctions connect the cell to the extracellular matrix.
The correct answer is a—Tight junctions connect one cell to the next, creating a barrier between
the cells.
A. Answer a is correct. A tight junction creates a seal between two cells that prevents the
movement of molecules between them.
The correct answer is a—
B. Answer b is incorrect. Tight junctions are formed between cells; however, they are not
involved in anchoring the cell. Desmosomes act as anchors between cells.
The correct answer is a—
C. Answer c is incorrect. Gap junctions form “tunnels” that connect the cytoplasm of two
neighboring cells.
The correct answer is a—
Raven/Johnson Biology 8e
Chapter 09- Answers
D. Answer d is incorrect. Anchoring junctions form connections between the extracellular
matrix and the cell’s cytoskeleton.
14. Cadherins and intermediate filament proteins are associated with __________, whereas
connexons are associated with ____________
a. tight junctions; anchoring junctions
b. cell surface markers; tight junctions
c. desmosomes; gap junctions
d. adherens junctions; plasmodesmata
The correct answer is c—
A. Answer a is incorrect. Tight junction proteins are associated with tight junctions while
integrins are associated with the anchoring junctions.
The correct answer is c—
B. Answer b is incorrect. Cell-surface markers are associated with recognition of self.
Connexons are not part of tight junctions.
The correct answer is c—desmosomes; gap junctions
C. Answer c is correct. Desmosomes are formed by the binding between cadherin proteins
on the surface of the cell and stabilized by intermediate filament proteins in the
cytoplasm. Gap junctions are formed by connexon proteins embedded in the cell’s plasma
membrane.
The correct answer is c—
D. Answer d is incorrect. Adherens junctions are formed by the interaction of integrin
proteins and the extracellular matrix. Plasmodesmata are extensions of plasma membrane
that connect the cytoplasm of multiple plant cells through openings in the cell wall.
15. Cells are able to anchor themselves to the extracellular matrix through the activity of—
a. connexon proteins
b. MHC proteins
c. cadherin proteins
d. integrin proteins
The correct answer is d—
A. Answer a is incorrect. Connexon proteins are involved in the formation of gap junctions.
The correct answer is d—
B. Answer b is incorrect. The MHC proteins are involved in the immune system’s ability to
distinguish “self” from “nonself.”
The correct answer is d—
C. Answer c is incorrect. Cadherin proteins participate in the formation of desmosomes that
anchor cells together.
The correct answer is d—integrin proteins
D. Answer d is correct. Integrin proteins bind to the extracellular matrix outside a cell and to
the cell’s cytoskeleton within the cytoplasm.
Raven/Johnson Biology 8e
Chapter 09- Answers
Challenge Questions
1. Describe the common features found in all examples of cellular signaling discussed in this
chapter. Provide examples to illustrate your answer.
Answer—All signaling events start with a ligand binding to a receptor. The receptor initiates a
chain of events that ultimately leads to a change in cellular behavior. In some cases the change is
immediate—such as when ligand binding results in the opening of a channel. In other cases the
change requires more time before it occurs—such as when the MAP kinase pathway becomes
activated and multiple different kinases become activated and deactivated or when G proteins
interact with effector proteins, leading to the production of second messengers. Some signals
only affect a cell for a short time (such as the channel example), but other signals can
permanently change the cell by changing gene expression, and therefore the number and kind of
proteins found in the cytoplasm of the cell.
2. The sheet of cells that form the gut epithelium folds into peaks called villi and valleys
called crypts. The cells within the crypt region secrete a protein, Netrin-1, that becomes
concentrated within the crypts. Netrin-1 is the ligand for a receptor protein that is found
on the surface of all gut epithelial cells. Netrin-1 binding triggers a signal pathway that
promotes cell growth. Gut epithelial cells undergo apoptosis (cell death) in the absence of
Netrin-1 ligand binding.
a. How would you characterize the type of signaling (autocrine, paracrine,
endocrine) found in this system?
b. Predict where the greatest amount of cell growth and cell death would occur in the
epithelium.
c. The loss of the Netrin-1 receptor is associated with some types of colon cancer.
Suggest an explanation for the link between this signaling pathway and tumor
formation.
Answer—a. This system involves both autocrine and paracrine signaling because Netrin-1 can
influence the cells within the crypt that are responsible for its production and the neighboring
cells.
b. The binding of Netrtin-1 to its receptor produces the signal for cell growth. This signal
would be strongest in the regions of the tissue with the greatest amount of Netrin-1—that is, in
the crypts. A concentration gradient of Netrin-1 exists such that the levels of this ligand are
lowest at the tips of the villi. Consequently, the greatest amount of cell death would occur at the
villi tips.
c. Tumors occur when cell growth goes on unregulated. In the absence of Netrin-1, the
Netrin-1 receptor can trigger cell death—controlling the number of cells that make up the
epithelial tissue. Without this mechanism for controlling cell number, tumor formation is more
likely.