Structurally, the channel system can be divided into rough or granu
lar endoplasmic reticulum (RER), which has ribosomes attached to its
outer, cytosolic surface, and smooth or agranular endoplasmic reticulum (SER), which lacks ribosomes. The functions of the endoplasmic
reticulum vary greatly and include: the synthesis, folding and transport
of proteins; synthesis and transport of phospholipids and steroids; and
storage of calcium within the cisternal space and regulated release into
the cytoplasm. In general, RER is well developed in cells that produce

**Fig 1 4 Smooth endoplasmic reticulum with associated vesicles The**