Vesicles are used to transport materials within cells by budding off the rough endoplasmic reticulum and fusing with the Golgi apparatus, where proteins are processed as they move through; vesicles then bud off the Golgi apparatus and fuse with the plasma membrane to release their contents outside the cell through exocytosis. The fluidity of cell membranes allows them to change shape during vesicle formation and fusion by breaking apart and re-forming as needed for endocytosis and exocytosis.