What is the endoplasmic reticulum?

The endoplasmic reticulum (ER) is a large, complex organelle found within the cytoplasm of the eukaryotic cell. It consists of a highly convoluted tubular network of membranes that are organized into netlike labyrinth of branching tubules and flattened sacs. This increases the internal cellular surface about 30-40 times more than the external cellular surface. The extent of ER varies from one cell to another.

The membrane of the endoplasmic reticulum forms a continuous sheet, dividing the intracellular space into two compartments – the lumen, which is enclosed within the ER membrane and the cytosol, which comprises the rest of the cytoplasm outside the membrane. The ER membrane regulates the selective transfer of molecules between these two compartments.

The endoplasmic reticulum plays a critical role in protein synthesis and transportation, protein folding, calcium storage, and lipid metabolism.