Prokaryotic organisms do not have organelles and thus do not have an ER. It's base structure and composition is similar to the plasma membrane, though it is an extension of the nuclear membrane. The ER is the site of the translation and folding of and transport of proteins that are to become part of the cell membrane (e.g., transmembrane receptors and other integral membrane proteins) as well as proteins that are to be secreted or "exocytosed" from the cell (e.g., digestive enzymes).
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.
The ER consists of an extensive membrane network of tubes and cisternae (sac-like structures). The membrane encloses a space, the cisternal space (or internal lumen) from the cytosol. This space is acting as a gateway. Parts of the ER membrane are continuous with the outer membrane of the nuclear envelope, and the cisternal space of the ER is continuous with the space in between the two layers of the nuclear envelope.
Parts of the ER are covered with ribosomes (which assemble amino acids into proteins based on instructions from the nucleus). Their rough appearance under electron microscopy led to their being called rough ER (RER), other parts are free of ribosomes and are 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).
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 a signal sequence. GŁnter Blobel was awarded the 1999 Nobel Prize in Physiology or Medicine for his discovery of these signal 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.
The endoplasmic reticulum found in muscle fibers is called sarcoplasmic reticulum.