The secretory pathway routes proteins from ER → Golgi → plasma membrane. The Golgi apparatus (4-7 stacked cisternae) acts as the cell's sorting station — adding glycosylation modifications, then packing cargo into vesicles. SNARE proteins (v-SNAREs on vesicles, t-SNAREs on targets) form a zipper complex driving membrane fusion — botulinum toxin kills by cleaving neuronal SNAREs. Clathrin-coated pits mediate receptor-mediated endocytosis; cargo enters early endosomes, either recycled (Rab11) or sent to late endosomes→lysosomes for degradation. COP-I/COP-II vesicles handle retrograde/anterograde ER-Golgi traffic. This machinery processes ~10,000 vesicle fusion events per cell per hour.