Before describing the rough endoplasmic reticulum function and structure, it is important to have an idea about what exactly endoplasmic reticulum is. ER, short for endoplasmic reticulum, is an organelle responsible for producing and transporting lipids and proteins to various locations in the body. The term endoplasmic reticulum has been derived from the words ‘reticulum’ and ‘endo’ which mean network and inside of the cytoplasm respectively. The structure of the ER is composed of many folds, although its membrane is in the form of a thin sheet that opens into a single arc. It is a part of the eukaryotic cells which is capable of manufacturing proteins for itself and for various other cell components like the Golgi apparatus, lysosomes, cell membrane as well as the secretary vesicles. Structurally, the ER consists of a large network of sacs and long tubules to provide a lot of functions in the cells of plants and animals. The ER is separated into two regions based on the availability of ribosomes. These regions have their own structure and function, completely different from one another. The region which lacks these ribosomes is called the smooth ER and the one that has ribosomes at the membrane cytoplasm is called the rough ER. The structure of smooth ER is basically tubular networks while those of rough ER consist of flattened sacs which have an empty space called the lumen. The structure of an ER is very extensive and spreads from the cytoplasm and the cell membrane and ends near the nuclear envelope. The rough ER can be seen in the cells like hepatocytes while the smooth ER is found in abundance in the gonad cells. The rough endoplasmic reticulum function is significantly different from those of the smooth ER especially because of the lack of ribosomes in the latter. As the metabolic changes occur in the cell, the quantities of both the smooth and rough ER begin to interchange slowly. The ER works by making use of a mechanism for quality control known as the chaperones. These are basically special proteins that help in assisting the new proteins in folding at respective sites by attaching itself to them. The proteins that get folded incorrectly are detached through an ERAD process. Moreover, the chaperones sometimes get affected by diseases like fibrosis which occurs when there is huge build-up of this protein chaperone.
The structure and shape of the rough ER is somewhat branched because of the extensive production and synthesis of proteins by the cells. Sometimes, the ER behaves such that it shows branching to form large flattened sacs that fill up the whole cell. This particular ER has been named ‘rough’ mainly because of the large numbers of ribosomes that branch out of it and give it a further coarse and bumpy look when viewed under a microscope. The ribosomes that bind to this rough ER do not form a stable part of its structure as they are released from time to time by the membrane. The ribosomes help in the manufacturing of amino acids and all the information for protein synthesis is provided to the ribosomes from the DNA by RNA or the messenger RNA. Thus, as soon as the information is conveyed to the ribosomes through the RNA, it begins to produce the exact sequence of proteins and amino acids needed by the cells. The rough ER is also connected to the nucleus of the cell body through an outer envelope.
The most common rough endoplasmic reticulum function is the synthesis and manufacture of different protein molecules which are to be exported from the ribosomal sites. At the rough ER, the proteins undergo advanced processing after they have been produced by the ribosomes. The two most common types of proteins produced at rough ER sites are the water soluble proteins which move into the lumen after getting produced and the other is the type of protein that is fixed to its ER membrane and tries to toughen itself after some time. A few other rough endoplasmic reticulum functions include the production of antibodies in some leukocytes and sending the manufactured and processed protein to the Golgi apparatus using special vesicles for transportation. After rounds of processing in the Golgi apparatus, the proteins are transported to various sites in the cells. The rough endoplasmic reticulum function is not only limited to protein synthesis but also includes the synthesis of lysosomal enzymes which contain the mannose 6-phosphate that finally gets added to the Golgi apparatus. It also helps in glycosylation which is N-linked in the rough ER and O-linked in the Golgi body. In N-linked glycosylation, when the protein gets properly folded, then the Oligosaccharyltransferase identifies the AA structure and helps in the synthesizing glycoprotein.
The rough endoplasmic reticulum function is entirely different from those of the smooth ER. The smooth ER helps in various metabolic processes by synthesizing steroids, phospholipids and lipids. The smooth ER is found in abundance in the cells of the sebaceous glands, ovaries and testes where they are secreted. It is also responsible for carrying out the metabolic activities of carbohydrates, in attaining steroid metabolism, attaching receptors to the proteins in the cell membrane as well as in detoxifying alcohol, drugs and other metabolic wastes from the body. It also works to regulate the calcium ion concentration in the body and particularly those in the muscles. Unlike the rough endoplasmic reticulum function, the functions of the smooth ER are different for both the plant and animal cells. The smooth ER also helps in gluconeogenesis wherein the enzyme glucose 6-phosphate is changed to glucose using the enzyme glucose 6-phosphatase which is solely found in the smooth ER tubules. These tubules lead to an increased surface in the ER which further helps in storing all the useful enzymes. When the smooth ER is found in myocytes then it is also referred to as the sarcoplasmic reticulum. The only difference between the two is that of their structure and how they bind to their respective cell membrane.
The functions of the smooth ER depend on the cell type. Whenever the smooth ER is not busy with its metabolic activities, it acts as a storehouse for various enzymes until it can prepare itself to begin its cycle of metabolism. It also packages all the newly synthesized proteins. Its detoxifying function includes metabolizing the alcohol in the liver cells, which is a common toxin found in humans. The smooth ER’s structure is such that it appears to look like a folded layer of membrane and provides it an improved surface area. The major lipids required by our body like cholesterol and phospholipids are produced by the smooth ER at the sites of the eukaryotic membrane. These membrane lipids are highly water repellent or hydrophobic in nature and are, therefore, produced in the smooth ER membrane instead of the cytosol because of its aqueous nature. The lipids travel to their final destination via a transport mechanism of vesicles from the smooth ER. The steroid hormone is also secreted in abundance by the smooth ER which means that the cells containing these hormones will have a huge number of smooth ER. At the muscle site, the smooth ER helps in muscle cell contraction and also works in the brain cells by producing the required female and male hormones.
Apart from the absence of ribosomes, many other differences exist between the rough endoplasmic reticulum function and that of the smooth ER. While the rough ER is located near the nucleus of the cell, the smooth ER occurs near the cell membrane. The formation of rough ER occurs because of the cell membrane, but the smooth ER is formed only from the rough ER through the loss of ribosomes. Apart from this, the rough ER also varies in structure and is mainly composed of cisternae and flattened sacs that have rough ribosomes attached to it while on the other hand the smooth ER has an extensive network of tubules that work as a storage area when it is not synthesizing the lipids needed by the cells. Finally, the rough ER is found at the sites of the protein manufacturing cells like globate cells, Nissl’s granules of the brain, plasma cells and acinal cells in the pancreas. The smooth ER is also found at its own lipid forming sites like the interstitial cells, adipocytes, leucocytes, muscle cells and the cells storing glycogen in the liver.