Proteins:
* Newly synthesized proteins: The RER is the site of protein synthesis, so you'll find ribosomes attached to its surface actively translating mRNA into polypeptide chains. These newly synthesized proteins are then translocated into the cisternal lumen.
* Chaperone proteins: These proteins help newly synthesized proteins fold correctly and prevent aggregation. Examples include BiP (binding immunoglobulin protein), calnexin, and calreticulin.
* Enzymes involved in protein modification: The RER cisternae contain a variety of enzymes responsible for modifications like glycosylation (addition of sugar chains), phosphorylation (addition of phosphate groups), and disulfide bond formation.
Other molecules:
* Lipids: The RER also synthesizes lipids, particularly phospholipids, which are essential components of cell membranes.
* Calcium ions: The RER can store and release calcium ions, playing a role in various cellular processes.
* Other molecules involved in protein transport: The RER contains specific proteins involved in the transport of proteins from the ribosomes to the Golgi apparatus.
Structures:
* Ribosomes: While not strictly within the cisternae, ribosomes are attached to the surface of the RER, actively translating mRNA into proteins.
* Translocation channels: These channels embedded in the RER membrane allow newly synthesized proteins to enter the cisternal lumen.
* Transport vesicles: Small membrane-bound sacs that bud off from the RER and carry proteins to the Golgi apparatus.
Overall, the RER cisternae are bustling factories where newly synthesized proteins undergo a series of modifications and quality control checks before being packaged and transported to their final destinations within the cell or secreted outside the cell.
