The many tubules that the ER packs into certain places in the cell also produce a large surface area. This could allow the ER to provide the space for making lipids, folding proteins and storing important molecules until they are needed.

The endoplasmic reticulum is a network of tubules and flattened sacs that serve a variety of functions in plant and animal cells. Smooth ER lacks attached ribosomes. Typically, the smooth ER is a tubule network and the rough ER is a series of flattened sacs. The space inside of the ER is called the lumen.

Cells that specialize in protein secretion contain large amounts of rough endoplasmic reticulum. For instance, cells of the pancreas that produce the protein insulin, have abundant rough endoplasmic reticulum.

The smooth endoplasmic reticulum functions in many metabolic processes. It synthesizes lipids, phospholipids as in plasma membranes, and steroids. Cells that secrete these products, such as cells of the testes, ovaries, and skin oil glands, have an excess of smooth endoplasmic reticulum.

Proteins called reticulons and DP1/Yop1p play an important role in this stabilization. These proteins are integral membrane proteins that form oligomers to shape the lipid bilayer. In addition, they also use a structural motif that gets inserted into one leaflet of the membrane and increases its curvature.

There are two types of endoplasmic reticulum: rough endoplasmic reticulum (rough ER) and smooth endoplasmic reticulum (smooth ER). Both types are present in plant and animal cells. The two types of ER often appear as if separate, but they are sub-compartments of the same organelle.

Cilium, plural cilia, short eyelashlike filament that is numerous on tissue cells of most animals and provides the means for locomotion of protozoans of the phylum Ciliophora. Cilia may be fused in short transverse rows to form membranelles or in tufts to form cirri.