The enoplasmic reticulum, or ER, is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945. These structures are responsible for several specialized functions: protein translation, folding and transport of proteins to be used in the cell membrane (e.g. transmembrane receptors and other integral membrane proteins), or to be secreted (exocytosed) from the cell (e.g. digestive enzymes); sequestration of calcium; and production and storage of glycogen, steroids, and other macromolecules. The endoplasmic reticulum is part of the endomembrane system. The basic structure and composition of the ER membrane is similar to the plasma membrane.