a network of membranous tubules within the cytoplasm of a eukaryotic cell, continuous with the nuclear membrane. The endoplasmic reticulum is a type of organelle made up of two subunits – rough endoplasmic reticulum, and smooth endoplasmic reticulum. Endoplasmic reticulum (ER), in biology, a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells and serves multiple functions, being important particularly in the synthesis, folding, modification, and transport of proteins. The endoplasmic reticulum (ER) is a type of organelle made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). “Endoplasmic Reticulum is a complex network of tubular membranes exclusively present in the cytoplasm of the eukaryotic cell. The endoplasmic reticulum (ER) is a large organelle made of membranous sheets and tubules that begin near the nucleus and extend across the cell.