ENDOPLASMIC RETICULUM (ER)Eucaryotic celIs have two major compartments- nucleus and cytoplasm. Cytoplasm was known to have no structure until the discovery of electron micrsscope. That cytoplasm is permeated by a membranous network called endoplasmic reticulum (ER), was revealed only after the introduction of electron microscope. Endoplasmic reticulum is a three dimensional network of membrane channels which constitutes more than half of the total membrane of the cell, ER is a highly folded and convoluted structure and forms a single continuous sheet enclosing one continuous sac. The interior of the sac is called "cisternal space" or "ER lumen". This is separated from the cytoplasm also called cytosol by a single membrane which mediates the communication between these two compartments. ER provides the cell with a compartment for storage of substances to be kept separate from cytosol. In addition, it has a key role in the biosynthesis of macromolecules. ER is a double membranous organelle which consists of interconnecting flattened sacs called cisternae or interconnected tubules or vesicles. Cisternae are temporary storage sites for nutrients (Figure shown below).