Endoplasmic reticulum

The endoplasmic reticulum (endoplasmic meaning "within the cytoplasm," reticulum meaning "little net" in Latin) or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions: Protein translation, folding, and transport (e.g., transmembrane receptors and other integral membrane proteins) of proteins to be used in the cell membrane, or to be secreted (exocytosed) from the cell (e.g., digestive enzymes); sequestation of calcium; production and storage of glycogen, steroids, and other macromolecules. The endoplasmic reticulum is part of the endomembrane system. The basic structure and composition of the ER membrane is similar to the plasma membrane.

The endoplasmic reticulum is called sarcoplasmic reticulum in both smooth and striated muscle.

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) held together by the cytoskeleton. The membrane encloses a space, the cisternal space (or internal lumen) from the cytosol. 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 microscope 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 ER

The rough ER contains protein-manufacturing ribosomes (the ribosomes on its surface are responsible for its being named "rough") and transports proteins. Rough ER is connected to the nuclear envelope as well as linked to the cis cisternae of the Golgi Apparatus by vesicles that shuttle between the two compartments. The rough ER works in concert with the Golgi apparatus to target new proteins to their proper destinations.

Smooth ER

The smooth ER has functions in several metabolic processes, including synthesis of lipids, metabolism of carbohydrates and calcium concentration. It is connected to the nuclear envelope. Smooth ER is found in a variety of cell types and it serves different functions in each. It consists of tubules and vesicles that branch forming a network. In some cells there are dilated areas like the sacs of rough ER. The network of smooth endoplasmic reticulum allows increased surface area for the action or storage of key enzymes and the products of these enzymes. The smooth ER is known for its storage of calcium ions in muscle cells.

Sarcoplasmic Reticulum

The Sarcoplasmic reticulum is a special type of ER found in striated muscle, which is specialized for calcium storage and release during muscle contraction.

Functions

The endoplasmic reticulum serves many general functions, including the facilitation of protein folding and the transport of synthesized proteins in sacs called cisternae.

Correct folding of newly-made proteins is made possible by several ER proteins including: Protein Disulfide Isomerase (PDI), ERp29, Hsc70 family, calnexin, calreticulin, and the peptidylpropyl isomerase family. Only properly-folded proteins are transported from the rER to the Golgi complex.