Endoplasmic reticulum

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(Redirected from Rough ER)

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).

1 Structure
2 Functions
- 2.1 Transport of proteins
- 2.2 Other functions

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Structure

Image:Nucleus ER golgi.jpg

The ER consists of an extensive membrane network of tubes and cisternae (sac-like structures) held together by the cytoskeleton. 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 but are contiguous with each other.

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Rough ER

The rough ER, also called granular ER or ergastoplasm (acronyms RER 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. Many types of specialized cells secrete proteins produced by ribosomes attached to rough ER. For example, certain cells in the pancreas secrete the protein insulin, into the bloodstream. As a polypeptide chain grows from a bound ribosome, it is threaded into the cisternal space through a pore formed by a protein in the ER membrane. As it enters the cisternal space, the new protein folds into its native conformation. Most secretory proteins are glycoproteins.

In addition to making secretory proteins, rough endoplasmic reticulum is a membrane factory that grows in place by adding proteins and phospholipids. As polypeptides destined to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and are anchored there by hydrophobic portions of the proteins. The rough ER also makes its own membrane phospholipids; enzymes built into the ER membrane assemble phospholipids from precursors in the cytosol. The ER membrane expands and can be transferred in the form of transport vesicles to other components of the endomembrane system.

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.

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Smooth ER

The smooth ER (also known as agranular ER or AER) has functions in several metabolic processes, including synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. Enzymes of the smooth ER are important to the synthesis of lipids, including phospholipids and steroids. Among the steroids produced by smooth ER are the sex hormones of vertebrates and the various steroid hormones secreted by the adrenal glands. The cells that actually synthesize and secrete these hormones - in the testes and ovaries, for example - are rich in the smooth ER.

Liver cells provide one example of the role of smooth ER in carbohydrate metabolism. Liver cells store carbohydrate in the form of glycogen, a polysaccharide. The hydrolysis of glycogen leads to the release of glucose from the liver cells, which is important in the regulation of sugar concentration in blood. However, the first product of glycogen hydrolysis is glucose phosphate, an ionic form of sugar that can not exit the cell and enter the blood. An enzyme embedded in the membrane of the liver cell's smooth ER removes the phosphate from the glucose, which can then leave the cell.

Enzymes of the smooth ER help detoxify drugs and poisons especially in the liver cells. Detoxification usually involves adding hydroxyl groups to drugs, making them more soluble and easier to flush from the body.

Muscle cells exhibit still another specialized function of the smooth ER. The ER membrane pumps calcium ions from the cytosol into the cisternal space. when a muscle cell is stimulated by a nerve impluse, calcium rushes back across the Er membrane into the cytosol and triggers contraction of the muscle cell.

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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.

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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.

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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.

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Other functions

Insertion of proteins into the ER membrane: Integral proteins must be inserted into the ER membrane after they are synthesized. Insertion into the ER membrane requires the correct topogenic sequences.
Glycosylation: Glycosylation involves the attachment of oligosaccharides.
Disulfide bond formation and rearrangement: Disulfide bonds stabilize the tertiary and quaternary structure of many proteins.

Organelles of the cell
Acrosome \| Cell wall \| Cell membrane \| Chloroplast \| Cilium/Flagellum \| Centrosome \| Cytoplasm \| Endoplasmic reticulum \| Golgi apparatus \| Lysosome \| Melanosome \| Mitochondrion \| Myofibril \| Nucleus \| Parenthesome \| Peroxisome \| Plastid \| Ribosome \| Vacuole \| Vesicle