Objective

To isolate the endoplasmic reticulum from mammalian cells.

Theory

ER is a network of membranous sacs that form interconnected tubules, vesicles and cisternae.  In 1945, Ernest Fullam, Keith Porter and Albert Claude  were  observed the endoplasmic reticulum. The ER membrane is a continuation of the outer nuclear membrane.  The eukaryotic cells have ER. The ER membrane synthesizes all of  the transmembrane proteins and lipids necessary for most of the cells’ organelles. They have several functions in the cell like protein synthesis, production of steroids, sequestration of calcium, and also storage and production of glycogen.

Most of the lipids for mitochondrial and peroxisomal membranes are also made by the ER membrane. The ER are of two types- rough ER and smooth ER. The rough ER contains ribosomes and hence appears rough whereas the smooth ER does not have ribosomes. The endoplasmic reticulum is a highly versatile organelle involved in synthesis and transport of proteins, glycoproteins, and lipoproteins; synthesis of cholesterol, steroids, phospholipids, and triglycerides; it participates in degradation of glycogen, and in the metabolism of  xenobiotics.

Structure

The rough ER is associated  with ribosomes which is the site protein synthesis. The ribosome free ER is known as smooth ER which is the  centre of lipid and membrane protein synthesis.

 Rough Endoplasmic Reticulum (RER)

RER is arranged as a series of stacked membranes close to the nucleus. They appear rough as there are large number of ribosome attached to the cytoplasmic side of their membranes. The ribosomes undergo the function of protein synthesis.

Regions of cytoplasmic matrix containing RER take basic stain due to the RNA content of ribosomes. These regions are known as cergastoplasm or basophilic bodies or chromophilic substances by early cytologists. In nerve cells such regions are called nissl bodies. In RER, ribosomes are often present as polysomes held together by mRNA.

Smooth Endoplasmic Reticulum (SER)

 
SER appear smooth as ribosomes are not attached to their walls. It occurs in cells involved in the metabolism of lipids and glycogen. Muscle cells have numerous SER and are known as sarcoplasmic reticulum. Smooth ER is also abundant in hepatocytes which is the principal  site of production of lipoproteins which are carriers of lipids to various parts of the body through the bloodstream. The SER consists of tubules and vesicles that  forming a network, which increased surface area for the action and storage  of key enzymes and the product of these enzymes.

Sarcoplasmic reticulum

Sarcoplasmic reticulum is a type of smooth ER found in smooth and striated muscles. The SER synthesizes molecules while the SR stores and pumps calcium ions. Calcium was stored in the  SR, which it sequesters and then releases when the muscle cell is stimulated. The release of calcium by SR during electrical stimulation of the cell plays a important role in excitation-contraction coupling.

Functions of  ER

The functions of SER include steroid metabolism, regulation of calcium concentration, metabolism of carbohydrates, synthesis of lipids and steroids, drug detoxification. The enzyme glucose-6-phosphatasepresentin SER  which converts glucose-6-phosphate to glucose, which is a  step in gluconeogenesis.

ER also contains enzymes that catalyze the detoxification of certain drugs and harmful substances produced by metabolism.
ER also serves as an intracellular Ca 2+ store which are used in many cell signaling processes. Sarcoplasmic reticulum in muscle cells are also specialized in Ca 2+ storage.