How does inositol triphosphate (IP3) contribute to calcium signaling within cells?

• A. IP3 directly activates protein kinase C, which then opens calcium channels.
• B. IP3 inhibits calcium uptake into the mitochondria, increasing cytoplasmic calcium levels.
• C. IP3 is converted into cyclic AMP, which then opens calcium channels.
• D. IP3 binds to receptors on the endoplasmic reticulum, causing calcium release into the cytoplasm.

Answer: (D)