A cell is a membrane bound structure containing biomolecules, such as nucleic acids, proteins, and polysaccharides. The Endoplasmic Reticulum is organized into a netlike labyrinth of branching tubules and flattened sacs extending throughout the cytosol.The ER has a central role in lipid and protein biosynthesis.The ER membrane makes a major contribution to mitochondrial and peroxisomal membranes by producing most of their lipids. The endoplasmic reticulum (ER) has many different functions. These include the translocation of proteins (such as secretory proteins) across the ER membrane; the integration of proteins into the membrane; the folding and modification of proteins in the ER lumen; the synthesis of phospholipids and steroids on the cytosolic side of the ER membrane; and the storage of calcium ions in the ER lumen and their regulated release into the cytosol.