How does inositol triphosphate (IP3) function as a secondary messenger in the cytoplasm?

• A. It converts PIP2 into DAG.
• B. It remains in the plasma membrane to activate protein kinase C.
• C. It directly phosphorylates target proteins.
• D. It binds to calcium ion channels on the endoplasmic reticulum, increasing cytoplasmic calcium levels.

Answer: (D)