Membrane Flow Revisited

iving cells are defined by their limiting membrane, known as the cell surface membrane or plasma membrane. Without exception, plasma membranes contain myriad lipid and protein molecules that are synthesized and assembled within the interior of the cell, primarily in the endoplasmic reticulum and Golgi apparatus. The general outlines of the pathway that proteins and lipids follow as they are translocated from their sites of synthesis and assembly within the cell to the cell surface were elucidated from several decades of pioneering studies that culminated in those of Palade and colleagues (Palade 1975). Proteins destined for export are synthesized on ribosomes attached to the endoplasmic reticulum. From there, these proteins are transported to the Golgi apparatus, where they are packaged into secretion granules that detach from the Golgi apparatus and migrate to the cell surface. Secretion granules then fuse with the plasma membrane, discharging their contents to the exterior compartment. Several considerations suggested that anterograde (forward) transport of membrane material probably parallels the movement of secretory proteins toward the cell surface. First,