Question 1

Which of the following best describes the role of IP$$_3 ($$inositol 1,4,5-trisphosphate) in the GPCR signaling pathway shown in the image?

Accepted Answer

IP$$_3 is a $$second messenger that binds to receptors on the smooth ER, triggering Ca$$^{2+}$$ release into the cytosol.

Question 2

In the GPCR signaling pathway, what is the immediate effect of phospholipase C (PLC) activation?

Accepted Answer

PLC hydrolyzes PIP$$_2$$ into IP$$_3$$ and DAG.

Question 3

Which of the following correctly matches the molecule with its function in the pathway shown?

Accepted Answer

PIP$$_2$$: Substrate for PLC, which is cleaved to form IP$$_3$$ and DAG.

Question 4

Suppose a mutation prevents IP$$_3$$ from binding to its receptor on the smooth ER. What would be the most likely cellular consequence?

Accepted Answer

Decreased Ca$$^{2+}$$ release from the smooth ER.

Question 5

Which of the following best explains why IP$$_3 is $$considered a 'second messenger' in this pathway?

Accepted Answer

It transmits the signal from the cell surface to internal targets after being generated in response to an extracellular ligand.

Question 6

If a drug inhibits PLC activity in this pathway, which of the following would you expect to decrease as a direct result?

Accepted Answer

IP$$_3$$ production

Question 7

Which of the following sequences correctly represents the order of events in the GPCR-PLC-IP$$_3$$ signaling pathway?

Accepted Answer

Ligand binds GPCR → G-protein activation → PLC activation → PIP$$_2$$ hydrolysis → IP$$_3$$ production → Ca$$^{2+}$$ release

Question 8

A researcher adds a non-hydrolyzable analog of GTP to cells. What is the most likely effect on the GPCR signaling pathway shown?

Accepted Answer

G-proteins remain continuously active, leading to prolonged PLC activation and excessive Ca$$^{2+}$$ release.

Question 9

Which of the following best describes the role of Ca$$^{2+} in $$the pathway depicted?

Accepted Answer

Ca$$^{2+} is $$released from the smooth ER into the cytosol, where it acts as a second messenger to activate downstream targets.

Question 10

A patient has a genetic defect that causes overactive PLC in their cells. Which of the following symptoms would most likely be observed?

Accepted Answer

Excessive cytosolic Ca$$^{2+}$$ signaling, potentially leading to muscle spasms or abnormal cell activity

Question 11

Which equation best represents the hydrolysis reaction catalyzed by PLC in this pathway?

Accepted Answer

$$ \mathrm{PIP_2 \rightarrow IP_3 + DAG} $$

Question 12

In a cell where the smooth ER is depleted of Ca$$^{2+}$$, what would be the effect of stimulating the GPCR-PLC-IP$$_3$$ pathway?

Accepted Answer

IP$$_3$$ would still be produced, but there would be little to no Ca$$^{2+}$$ release into the cytosol.

Question 13

Which of the following is a real-world example of a physiological process that relies on the GPCR-PLC-IP$$_3-Ca$^{2+}$$ pathway?

Accepted Answer

Insulin secretion from pancreatic beta cells in response to glucose.

Question 14

If a toxin blocks the binding of IP$$_3 to $$its receptor, which step in the pathway would be directly affected?

Accepted Answer

Ca$$^{2+}$$ release from the smooth ER

Question 15

Which of the following statements about the GPCR-PLC-IP$$_3$$ pathway is correct?

Accepted Answer

The pathway amplifies extracellular signals by generating multiple second messengers from a single ligand-receptor interaction.

Cell Signaling: GPCR Pathways and Calcium Release

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