The endoplasmic reticulum (endoplasmic="within the cytoplasm", reticulum="little net"; short : ER) is an important organelle (a structure within the cell that is divided from the cytoplasm by a membrane) in all eukaryotic cells. Prokaryotic organisms do not have organelles, thus also no ER. It is an extension of the nuclear membrane and responsible for the folding, modification and transport of proteins that will become part of the cell membrane (e.g., receptor proteins), and for proteins that are expelled from the cell (e.g., digestive enzymes) by exocytosis.
The ER consists of an extensive membrane network of tubes and cisternae (sac-like structures). Parts of the ER membrane are continuous with the nuclear membrane, as is the cisternal space with the space between the lipid layers of the nuclear membrane. Parts of the ER is covered with ribosomes (protein production machines) and called rough ER (RER), other parts are free of ribosomes and called smooth ER (SER). The ribosomes on the surface of the rough ER insert the freshly produced proteins directly into the ER, which processes them and then passes them on to the Golgi apparatus (Fig. 1).
Figure 1 : Image of nucleus, endoplasmic reticulum and Golgi apparatus.
(1) Nucleus. (2) Nuclear pore. (3) Rough endoplasmic reticulum (RER). (4) Smooth endoplasmic reticulum (SER). (5) Ribosome on the rough ER. (6) Proteins that are transported. (7) Transport vesicle. (8) Golgi apparatus. (9) Cis face of the Golgi apparatus. (10) Trans face of the Golgi apparatus. (11) Cisternae of the Golgi apparatus.
Rough and smooth ER differ not only in appearance, but also in function. While the rough ER manufactures and transports proteins destined for membranes and secretion, the smooth ER has functions in several metabolic processes. It takes part in the synthesis of various lipids (e.g., for building membranes) and steroids (e.g., hormones), and also plays an important role in carbohydrate metabolism, detoxification of the cell, and calcium storage.
Proteins that are transported by the ER and from there throughout the cell are marked with an address tag that are called signaling sequences. Günter Blobel was awarded the 1999 Nobel prize for his discovery of these signaling sequences in 1975. The N-terminus (one end) of a polypeptide chain (e.g., a protein) contains a few amino acids that work as an address tag, which are removed when the polypeptide reaches its destination. Proteins that are destined for places outside the ER are packed into transport vesicles and moved along the cytoskeleton towards their destination.