Endoplasmic Reticulum

The endoplasmic reticulum, or ER, is the largest of all the cell's internal membrane systems, and it was named by K. R. Porter in 1948. Structurally, it is a genuinely three-dimensional, double-membraned network rather than a single sac, built from three distinguishable kinds of component: cisternae, which are long, broad, flattened, sac-like structures that arrange themselves in parallel stacks (called lamellae) with a fluid-filled space between the membranes; vesicles, smaller oval, membrane-bound sacs; and tubules, irregularly shaped, branched, smooth-walled channels enclosing their own internal space. The ER stays physically connected both to the nuclear membrane and to the cell surface membrane, and it spreads out as a network right through the cytoplasm, giving the cell real mechanical support in the process. Its internal chemical environment is specifically suited to folding newly made proteins correctly and carrying out the further chemical modifications those proteins need before they can function - and any protein that comes out misfolded is pulled aside and degraded within the ER rather than being allowed to continue on. The ER comes in two functionally distinct forms, distinguished simply by whether ribosomes are attached to its outer surface. When ribosomes are present, it is called rough endoplasmic reticulum (RER), and its main job is prote …