The ER of a cell acts as the transportation system of the cell. It can be either rough or smooth ER depending on the number of ribosomes inside of the cell.
http://micro.magnet.fsu.edu/cells/endoplasmicreticulum/endoplasmicreticulum.html
http://www.biology4kids.com/files/cell_er.html
http://micro.magnet.fsu.edu/cells/endoplasmicreticulum/endoplasmicreticulum.html
http://www.biology4kids.com/files/cell_er.html