The endoplasmic reticulum or ER (endoplasmic means "within the cytoplasm," reticulum means "little net") is an organelle found in all eukaryotic cells. The ER modifies proteins, makes macromolecules, and transfers substances throughout the cell. Prokaryotic organisms do not have membranous organelles and thus do not have an ER. The basic structure and composition of the ER is similar to the plasma membrane, although it is actually an extension of the nuclear membrane. The ER is the site of the translation, folding, 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).
Structure
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 between the two layers of the nuclear envelope (the intermembrane space). 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). The rough and smooth ER differ in both appearance and function.
Rough ER
The rough ER ( a.k.a granular ER or GER) manufactures and transports proteins destined for membranes and secretion. It is called "rough" because ribosomes present on the cytosolic side of the membrane give it a rough appearance.
Minutes after proteins are synthesized, most of them leave the ER and travel to the Golgi apparatus in vesicles. The rough ER also modifies, folds, and controls the shape and quality of proteins.
Smooth ER
The smooth ER (also known as agranular ER or AER) has functions in several metabolic processes. It takes part in the synthesis of various lipids (e.g., for building membranes such as phospholipids), fatty acids and steroids (e.g., hormones), and also plays an important role in carbohydrate metabolism, detoxification of the cell (enzymes in the smooth ER detoxify chemicals), and calcium storage. It is well developed in cells that produce steroid hormones and in liver cells. It also is a large transporter of nutrient found in each cell.
Sarcoplasmic Reticulum
The sarcoplasmic reticulum is present in muscle cells and is specially adapted for calcium ion storage and release. This process is mediated by voltage gated calcium channels, and the calcium ATPase that pumps calcium ions into the SR.
In striated muscle it is specially adapted to surround the myofibrils, forming triads with invaginations of the plasma membrane called T-tubules.
Functions
The endoplasmic reticulum serves many general functions, including the facilitation of protein folding and the transport of proteins. Correct folding of newly-made proteins is made possible by several ER proteins including: PDI, Hsc70 family, calnexin, calreticulin, and the peptidylpropyl isomerase family. Only properly-folded proteins are transported from the RER to the Golgi complex.
Transport of proteins
Secretory proteins are moved across the ER membrane. Proteins that are transported by the ER and from there throughout the cell are marked with an address tag called a signal sequence. 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 toward their destination. The ER is also part of a protein sorting pathway. It is, in essence, the transportation system of the eukaryotic cell.
Other functions
