2.5. Endoplasmic Reticulum

The endoplasmic reticulum (ER) is part of a cell’s endomembrane system. It is an organelle that forms an interconnected network of flattened, membrane-enclosed tubular structures called cisternae contiguous with the nuclear membrane. First observed by Keith R. Porter, Albert Claude, and Ernest F. Fallum under the electron microscope in 1945 as a network (hence the name “reticulum”) of membranes, the endoplasmic reticulum and are of two types: rough and smooth. The Rough Endoplasmic Reticulum is speckled with ribosomes that are the sites of protein synthesis, and the Smooth Endoplasmic Reticulum lack ribosomes and are the sites of lipid synthesis. The many intricate folds of the endoplasmic reticulum give it an incredibly extensive surface area, up to 10 square meters of endoplasmic reticulum in a milliliter of plant cell cytoplasm. The large, convoluted area allows for a high amount of biochemical activity in a small space. Proteins that are assembled on the ribosomes become either inserted into the ER membrane or injected into the ER lumen.