Question 1

In a signal transduction pathway, the binding of epinephrine to a G protein-coupled receptor results in the activation of adenylyl cyclase. Which of the following best describes the immediate subsequent step involving a second messenger?

Accepted Answer

Adenylyl cyclase catalyzes the conversion of ATP to cAMP, which then activates protein kinase A.

Answer

cAMP binds directly to the extracellular ligand, increasing its affinity for the receptor.

Answer

Ligand-gated ion channels open to allow

Ca^{2+}

to flow out of the cell into the extracellular fluid.

Answer

Adenylyl cyclase opens calcium channels in the ER, releasing

Ca^{2+}

into the cytosol.

Question 2

Which of the following describes the mechanism by which

Ca^{2+}

ions typically function as second messengers within a cell?

Accepted Answer

Cytosolic concentration is kept low by active transport, allowing a rapid increase to trigger responses when released from the endoplasmic reticulum or imported from outside.

Answer

The concentration of

Ca^{2+}

is naturally higher in the cytosol than in the extracellular fluid, allowing it to diffuse out of the cell to signal neighboring cells.

Answer

Calcium ions act by binding to G protein-coupled receptors on the outer surface of the plasma membrane to initiate a phosphorylation cascade.

Answer

Calcium ions are synthesized from ATP by the enzyme phospholipase C in response to a signal.

Question 3

Which of the following describes a commonality between both cyclic AMP (cAMP) and calcium ions (

Ca^{2+}

) in cell signaling?

Accepted Answer

They are small, non-protein, water-soluble molecules or ions that rapidly relay signals from membrane receptors to the cell interior.

Answer

Both function by binding to transcription factors in the cytoplasm to enter the nucleus and initiate DNA replication.

Answer

Both are produced by the enzyme adenylyl cyclase upon the activation of a G protein-coupled receptor.

Answer

Both are specialized proteins that phosphorylate target enzymes to change their activity.

Question 4

The concentration of

Ca^{2+}

in the cytosol is typically much lower than the concentration in the endoplasmic reticulum (ER). Which of the following is most likely to occur immediately after a signaling molecule triggers the opening of ligand-gated

Ca^{2+}

channels on the ER membrane?

Accepted Answer

Ca^{2+}

ions flow into the cytosol, where they bind to and activate proteins such as calmodulin.

Answer

Ca^{2+}

ions are actively pumped into the ER to decrease the cytosolic concentration further.

Answer

The

Ca^{2+}

ions convert ATP into cAMP to amplify the signal within the ER.

Answer

The change in

Ca^{2+}

concentration causes the G protein-coupled receptor to detach from the plasma membrane.

Question 5

In the IP3/DAG signaling pathway, what is the specific role of inositol trisphosphate (IP3) in regulating second messengers?

Accepted Answer

It binds to ligand-gated ion channels on the endoplasmic reticulum, stimulating the release of

Ca^{2+}

into the cytosol.

Answer

It acts as an enzyme that converts cyclic AMP into AMP to terminate the signal.

Answer

It serves as a primary messenger that travels through the bloodstream to bind to extracellular receptors.

Answer

It phosphorylates adenylyl cyclase to increase the production of cAMP.

Question 6

Which of the following occurs to the signal transduction pathway when the enzyme phosphodiesterase is inhibited within a cell?

Accepted Answer

The concentration of cAMP remains elevated, leading to prolonged activation of protein kinase A.

Answer

The cell becomes unable to produce

Ca^{2+}

ions in the endoplasmic reticulum.

Answer

Adenylyl cyclase is permanently deactivated, preventing the conversion of ATP to cAMP.

Answer

The G protein-coupled receptor is internalized and degraded to stop the signal.

Question 7

Vibrio cholerae produces a toxin that modifies a G protein so that it is stuck in its active form (bound to GTP), leading to the continuous production of cAMP in intestinal cells. Which of the following is the most direct consequence of this toxin at the cellular level?

Accepted Answer

Protein kinase A remains continually active, leading to the constant phosphorylation of membrane proteins that control chloride ion secretion.

Answer

The concentration of

Ca^{2+}

in the cytosol decreases to zero as all ion channels close.

Answer

Adenylyl cyclase is inhibited, preventing the cell from responding to any extracellular hormones.

Answer

Phosphodiesterase activity increases to compensate, rapidly converting all ATP into ADP.

Question 8

Which of the following describes the correct sequence of events involving second messengers after a hormone binds to a G protein-coupled receptor (GPCR) that activates the phospholipase C pathway?

Accepted Answer

Phospholipase C cleaves a membrane phospholipid into DAG and IP3, and then IP3 triggers the release of

Ca^{2+}

from the endoplasmic reticulum.

Answer

G proteins directly transport

Ca^{2+}

ions from the extracellular fluid into the cytosol to activate cAMP.

Answer

Adenylyl cyclase activates IP3, which then binds to calmodulin to stimulate the production of

Ca^{2+}

ions.

Answer

Phospholipase C converts ATP into cAMP, which then directly opens

Ca^{2+}

channels in the plasma membrane.

Question 9

Which of the following statements best explains why signal transduction pathways often involve second messengers like cAMP or

Ca^{2+}

instead of the primary messenger entering the cell directly?

Accepted Answer

The use of second messengers allows for significant amplification of the signal, as one receptor-ligand complex can trigger the production of many second messenger molecules.

Answer

The primary messenger is usually an enzyme that requires a second messenger to provide the activation energy for catalysis.

Answer

Second messengers are larger and more complex than primary messengers, allowing them to carry more genetic information to the nucleus.

Answer

Second messengers are required because primary messengers like epinephrine are lipid-soluble and cannot pass through the plasma membrane.

Question 10

Which of the following describes the most likely impact on a cell if a mutation causes a G protein-coupled receptor to remain in its active conformation even in the absence of a signaling ligand?

Accepted Answer

Adenylyl cyclase would continuously produce cAMP, leading to the overactivation of downstream proteins regardless of external signals.

Answer

The concentration of ATP would increase significantly as cAMP is no longer being synthesized by the cell.

Answer

The cell would be unable to transport

Ca^{2+}

into the endoplasmic reticulum, causing the cytosol to become depleted of ions.

Answer

Phosphodiesterase would be inhibited, preventing the termination of all other unrelated signaling pathways in the cell.

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