Endoplasmic reticulum (ER), an organelle in cells is a system of membranous cisternae (flattened convoluted to sacs) linking throughout the cytoplasm. It Often constitutes more of half the total membrane in the cell.
Endoplasmic reticulum structure was first noted in late 19th century, when studies of stained cells (dying of cells) indicated the presence of some type of extensive cytoplasmic structure, it was then termed the gastroplasm considering the the linking. Electron microscope made possible the study of the morphology of this organelle in early 1940s, when it was given its present name.It serves a series of functions, which is important particularly in the synthesis, folding, modification, and transport of protein molecules . It is evident that eukaryotic cells contain an endoplasmic reticulum (ER). Animal cells usually have more than half of the membranous content of the cell. Types of endoplasmic reticulumThere are two typesRough endoplasmic reticulumIt is named from its rough appearance, which is due to the attachment of ribosomes at the outer (cytoplasmic) surface. Microscopic examination of this organelle depicts it’s structural roughness. Rough ER is found immediately adjacent to cell nucleus, and its membrane is continuous with the outer membrane of the nuclear envelope.Ribosomes on rough ER specialize mainly in synthesis of proteins that possess a coding sequence that directs them specifically to the ER for processing. (A number of other proteins in a cell, including those destined for the nucleus and mitochondria, are targeted for synthesis on free ribosomes, or those not attached to the ER membrane; the article ribosome.)Proteins that are synthesized by rough ER have specific final destinations. Some proteins, remain within the ER, where others are sent to the Golgi apparatus. Golgi apparatus is found to lie next to the ER. Proteins secreted from the Golgi apparatus are directed to lysosome or to cell membrane; still others are targeted for secretion to the cell exterior. Proteins meant for transport to the Golgi apparatus are transferred from ribosomes on rough ER into the rough ER lumen, which serves as the site of protein folding, modification, and assembly.