Endoplasmic Reticulum. Endoplasmic reticulum is a continuous membrane, which is present in both plant cells, animal cells and absent in prokaryotic cells. It is the membrane of network tubules and flattened sacs, which serves a variety of functions within the cell.ne region is called as rough Endoplasmic reticulum, as it contains ribosome attached to the cytoplasmic side of the membrane and they are the series of flattened sacs. The other region is called as smooth Endoplasmic reticulum as it lacks the attached ribosome and they are tubule network.