The word endoplasmic means “within” the cytoplasm. The word “reticulum” means the “network”. The Endoplasmic reticulum consists of a network of membranous tubules and sacs called cisternae. The ER are extensive in the cell. They are composed of more than half of the total membrane in many eukaryotic cells. The ER membrane separates the cisternal space from the cytosol. The ER membrane is continuous with the nuclear envelop at one end. The space between the two membranes of the nuclear envelope is continuous with the cisternal sace of the ER. There are two distinct regions of ER. These regions differ in structure and function.
Smooth Endoplasmic Reticulum:
The cytoplasmic space of SER lacks ribosomes. So, they are known as SER. SER of various cell types functions in diverse metabolic processes like synthesis of lipids, carbohydrate metabolism, the detoxification of drug and other poisons. These functions are as follow:
Synthesis of Lipids: Enzymes of the smooth ER are important for the synthesis of fats, phospholipids, steroids and other lipids, they produce steroids. The steroids are the sex hormones. The cells of the testes and ovaries are rich in smooth ER. These cells synthesis and secrete steroid hormones.
Carbohydrate Metabolism: Liver stores carbohydrate in the form of glycogen. The hydrolysis of glycogen releases glucose from the liver. The hydrolysis is important in the regulation of sugar concentration in the blood. All these reactions took place in SER.
Detoxification of Drugs: The enzymes of the smooth ER detoxify the drugs and other poisons in liver cells. Hydroxyl group is added during detoxification. It increases their solubility.
Stimulation of Muscle Contraction: The ER membrane pumps calcium ions from the cytosol into the cisternal space. The muscle cell is stimulated by a nerve impulse. Calcium leaks back across the ER membrane into the cytosol. It triggers contraction of the muscle cell.
Rough Endoplasmic Reticulum:
Ribosomes are attached on the cytoplasmic surface of the membrane of RER. So, RER appears rough through electron microscope. Ribosomes are also attached to the cytoplasmic side of outer membrane of nuclear of envelop. This membrane is in continuous with the rough ER. The RER performs following functions in the cell:
Protein Synthesis: Many types of specialized cells secrete proteins produced by the rough ER. For example, white blood cells in humans and other vertebrates secrete antibodies. Proteins for secretion are synthesized by ribosomes attached to the rough ER. The polypeptide chain grows and enters through the ER membrane into the cisternal space. The protein enters the cisternal space and it is folded. Most secretory proteins are glycoproteins. In this case proteins are covalently bonded to carbohydrates in SER. The secretory proteins are wrapped in the membranes of vesicles. They are budded from a specialized region called transitional ER. Such vesicles are called transport vesicles.
Membrane Production: Rough ER is a membrane factory. It grows by adding proteins and phospholipids. They are inserted into the ER membrane itself. The enzymes present in the RER synthesize their membranes from these phospholipids and proteins. The ER membrane can be transferred into the cell membrane in the form of vesicles.