Final answer:
Inositol trisphosphate (IP3) acts as a signaling molecule that causes the release of calcium ions (Ca2+) from the endoplasmic reticulum, leading to various cellular responses.
Step-by-step explanation:
Inositol trisphosphate's (IP3) major action in the cytoplasm is as a signaling molecule that initiates the release of calcium ions (Ca2+) from intracellular stores, specifically the endoplasmic reticulum. Upon its formation by the cleavage of a phospholipid called phosphatidylinositol bisphosphate (PIP2) through the action of the enzyme phospholipase C, IP3 diffuses within the cytoplasm. It binds to ligand-gated calcium channels on the endoplasmic reticulum, causing the channels to open and release Ca2+ into the cytoplasm. This increase in intracytoplasmic calcium levels acts as a part of a signal cascade, activating various cellular responses including metabolic pathways, gene expression, and muscle contraction processes.