Final answer:
Phosphatidyl-inositol (PI) is a phospholipid vital to cellular signaling, activated by phosphorylation, leading to second messengers that propagate signal cascades within the cell.
Step-by-step explanation:
Phosphatidyl-inositol (PI) is a phospholipid located in the plasma membrane that plays a pivotal role in cellular signaling. It is part of an array of lipids that can be modified into second messengers, essential for intracellular communication. PI is activated when enzymes known as kinases add phosphate groups to it, creating PI-phosphate (PIP) and PI-bisphosphate (PIP₂).
The structure of PI includes a glycerol backbone connected by ester bonds to two fatty acid chains, constituting the hydrophobic tails. The third position on the glycerol is linked to a phosphate group which, in turn, is connected to an inositol ring forming the hydrophilic head.
Upon activation by external signals, PI is further phosphorylated by kinases, or cleaved by the enzyme phospholipase C. This cleavage results in the production of diacylglycerol (DAG) and inositol triphosphate (IP3), both of which function as second messengers. DAG stays in the membrane, activating protein kinase C (PKC), while IP3 diffuses into the cytoplasm, triggering calcium release from the endoplasmic reticulum, and propagating the signaling cascade.