Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a fine network of membranous tubules and sacs, called cisternae, running through the cytosol. It is present in all eukaryotic cells except mature ova and mature red blood corpuscles, and it is extensive enough to make up more than half of the total membrane in a eukaryotic cell. The ER divides the cytoplasm into two regions -- the laminal cytoplasm, lying within the lumen of the ER, and the non-laminal cytoplasm, lying outside it. The membrane of the ER is continuous with the nuclear envelope at one end and extends all the way to the cell membrane at the other, so besides its other roles the ER also acts as an intracellular supporting framework that helps hold various organelles in position within the cytoplasm.

Rough and smooth ER

The outer surface of the ER membrane may or may not be studded with ribosomes; where ribosomes are present, it is called rough ER, and where they are absent, smooth ER, and the two differ in their functions. Smooth ER is involved in a range of processes depending on the cell type -- it helps synthesise lipids (for instance in the steroid-secreting cells of the adrenal cortex, testes and ovaries), detoxifies drugs and poisons (in liver cells), and stores calcium ions (in muscle cells). Rough ER, by contrast, is chiefly involved in protein synthesis -- for example, in pancreatic cells that secrete insulin. Proteins made on the ribosomes attached to rough ER, called secretory proteins, become wrapped in membrane that buds off from a transitional region of the ER, and depart as transport vesicles. Rough ER is also involved in building new membrane for the cell: the ER membrane grows in place by adding membrane proteins and phospholipids to itself, and portions of this expanded membrane are then passed on to other parts of the endom …