During exocytosis cells expel a variety of materials including waste products, toxins and large molecules such as hormones, proteins and neurotransmitters into the extracellular environment. These molecules, which are either manufactured by intracellular organelles or generated as byproducts of metabolism, are transported to the cell membrane via membrane-bound secretory vesicles. Once a vesicle has reached the cell membrane,  exocytosis is initiated which enables the vesicle to fuse with the membrane and expel its contents.