endoplasmatischem

Endoplasmic reticulum (ER) is a network of membranous tubules and sacs that extends through the cytoplasm of eukaryotic cells. It is continuous with the outer membrane of the nuclear envelope, forming an interconnected transport system that contains a lumen inside. The ER is classified into two main regions: rough endoplasmic reticulum (RER), studded with ribosomes on the cytosolic surface, and smooth endoplasmic reticulum (SER), lacking ribosomes. The two regions share a common lumen but differ in function.

Rough ER is the site of synthesis for secretory and membrane proteins, which are threaded into the ER lumen during translation. These proteins may be folded with the help of chaperone proteins and undergo initial post-translational modifications, including N-linked glycosylation. Vesicles bud from the ER to transport these proteins to the Golgi apparatus for further processing.

Smooth ER is involved in lipid synthesis, including phospholipids and cholesterol, and in carbohydrate metabolism. It also plays a role in detoxification processes, especially in liver cells, and in calcium storage and release, which is particularly important in muscle cells where the ER is specialized as the sarcoplasmic reticulum.

ER functions extend beyond protein and lipid synthesis to quality control, signaling, and membrane production, and it forms contact sites with mitochondria and the Golgi. Disturbances of ER function can contribute to cellular stress and disease.