Eukaryotic cells contain numerous distinct types of membrane-bound compartment. Transport vesicles move proteins and other molecules between the compartments.
Proteins contain signalling sequences or patches that specify their destination compartment.
Proteins destined for lysosomes, secretion or the plasma membrane are synthesised in the ER, transported to the cis Golgi, modified in the Golgi apparatus, and sorted and packaged in vesicles at the trans Golgi for onward delivery to endosomes or secretory vesicles. The pH and the proteins in each compartment of the ER and Golgi are different. Misplaced proteins are returned to the ER. Proteins are sent from the Golgi to the basolateral and apical regions of the plasma membrane (if separate) in different vesicles.
Molecules are internalised either by non-specific or receptor-mediated systems. Receptors may be recycled to the plasma membrane or routed for breakdown in lysosomes, depending on the receptor and the requirements of the cell.
Molecules can be transported across cells either in solution in a non-specific manner, or by receptor-mediated mechanisms.