The endoplasmic reticulum is the term which describes an extensive network of tubes, sacs, and vesicles throughout a cell that provides transport as its main function. The endoplasmic reticulum is a small organelle and is found in most Eukaryotic cells. It is not found in red blood cells. The endoplasmic reticulum is a network in the core of the cell and occurs within the nuclear membrane and is involved in the synthesis of lipids and proteins. The endoplasmic reticulum also usually has ribosomes attached to it.