The endoplasmic reticulum or ER (endoplasmic means "within the cytoplasm," reticulum means "little net") is an organelle found in all eukaryotic cells. It is a continuation of the membrane. The ER modifies proteins, makes macromolecules, and transfers substances throughout the cell. Prokaryotic organisms do not have membrane-bound organelles, and thus do not have an ER (and other integral membrane proteins) as well as proteins that are to be secreted or "exocytosed" from the cell (e.g., digestive enzymes). In more depth, there are two different types of ER, Smooth ER (or agranular ER) and Rough ER (or granular ER). The Smooth ER synthesizes lipids, metabolizes carbohydrates and detoxifies drugs and poisons, whereas the Rough ER makes Secretory Proteins and produces membranes. The Smooth ER does not have ribosomes attached to it as the Rough ER does.
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, Hsp70 family, calnexin, calreticulin, and the peptidylpropyl isomerase family. Properly folded proteins are then transported from the ER to the Golgi complex for further maturation and sorting.
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 that are called a signal sequence. The N-terminus (one end) of a polypeptide chain 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 path.
Insertion of proteins into the ER membrane. Integral proteins need to be inserted into the ER membrane after they are synthesized. Insertion into the ER membrane requires the correct topogenic sequences (a segment of a protein that ensures it acquires the proper orientation during its insertion into the endoplasmic reticulum)
Glycosylation. Glycosylation involves the attachment of oligosaccharides.
Disulfide bond formation and rearrangement. Disulfide bonds stabilize the tertiary and quaternary structure of many proteins.