The E.R. is a network of flattened, interweaving tubules, formed by membranes within the cell, which create mazes of narrow channels. Eukaryotic cells have two types of ER: rough ER & smooth ER.
Rough ER has many small ribosomes studded on its outer membrane, hence the name 'rough'. The ribosomes synthesize proteins, especially membrane proteins and thus new membranes for the ER itself, and to replace nuclear membranes or to form Golgi, lysosomes, and plasma membranes.
      Rough ER ribosomes synthesize proteins in
secretory cells destined for export, including digestive enzymes and protein hormones. These proteins are trageted for interior cisternae by a hydrophobic N-terminal amino acid sequence (signal sequence) that binds to a singal recognition particle (receptor protein). Once made these proteins move through the channels of the ER and gather up in small membrane bound transport vesicles, which are carried off to the Golgi for modification and processing by the cell.
the Rough Endoplasmic Reticulum