Endoplasmic reticulum

Endoplasmic reticulum (ER) is a cellular organelle. It is the transport network for molecules going to specific places, as compared to molecules that float freely in the cytoplasm. The endoplasmic reticulum is in cells that have a nucleus: in eukaryote cells but not in prokaryote cells. It takes these forms:
Similar to the ER is the sarcoplasmic reticulum (SR) found only in muscle cells. The SR stores and pumps calcium ions. The SR contains large stores of calcium, which it releases when the muscle cell is stimulated. Another type of cytoplasmic network is the plate-like Golgi apparatus.
The lacey membranes of the endoplasmic reticulum were first seen in 1945 by scientists using an electron microscope.