Rough and Smooth Endoplasrnic Reticulum (RER and SER)
ER is differentiated into two regions, granular or rough endoplasrnic reticulum (RER) and agranular or smooth endoplasrnic reticulum (SER). These twb regions also differ considerably in shape: rough ER is organised in stacks of flattened sacs called cisternae and smooth ER consists of a ' A mesh work of fine tubules . In RER, the outer surface in the cytoplasmic site of the membrane is studded with small particles called ribosomes whereas in SER, the ribosomes are absent. Ribosomes are the sites of protein synthesis. Thus, it is evident that RER is more abundant in cells which are actively engaged in synthesis and export of proteins, for example, pancreatic acinar cells and plasma cells. SER is found in those cells which are specialised in lipid metabolism and which secrete steroids such as the cells of adrenal cortex, the testes and ovary. Smooth ER is also present in liver cells where it helps in detoxification of drugs and poisons.ER (both RER and SER) performs many mechanical functions of the cell by providing mechanical support. Large surface area of ER helps in the exchange of materials across the membrane by diffusion and active transport. ER may act as a kind of circulatory system for the distribution of various substances in the cell.