Any molecules dissolved in the fluid contents of these vesicles are discharged into the extracellular fluid - this is called secretion . Any integral membrane proteins exposed to the interior surface of the vesicles will now be displayed at the cell surface because the vesicles turn inside out as they fuse with the plasma membrane. Thus exocytosis does not simply replace plasma membrane but ensures that the plasma membrane will display its characteristic cell-surface proteins . Others are pinched off from endosomes before they fuse with lysosomes. Others bud off from the endoplasmic reticulum and Golgi apparatus taking their products to the surface of the cell.
Some cells specialize in secretion.
In cells that secrete large amounts of protein, for example, the protein accumulates in specialized secretory granules formed by the Golgi apparatus. These move to the cell surface and discharge their contents to the outside. Exocrine cells in the pancreas synthesize and secrete pancreatic digestive enzymes. The lumen where their apical surfaces meet leads eventually to the pancreatic duct draining into the small intestine.