Figure 5.4 The phospholipase C second messenger mechanism by which hormones act on target cells. A hormone binds to a G protein coupled receptor (GPCR) on the plasma membrane and activates a G protein. The G protein activates the effector protein phospho- lipase C, which causes phosphotidylinositol (PIP2) to be broken down into inositol triphosphate (IP) and diacylglycerol (DAG). The IP, causes release of calcium from intracellular organelles that activates the calcium-binding protein calmodulin, and the DAG activates protein kinase C to bring about additional cellular responses.