Endomembrane

The endomembrane system is a network of intracellular membranes that divide eukaryotic cells into functional compartments and coordinate protein and lipid synthesis, modification, and transport. It includes the nuclear envelope, the endoplasmic reticulum (rough and smooth), the Golgi apparatus, lysosomes, endosomes, vesicles, and the plasma membrane. Some descriptions also include transitional organelles connected by vesicular traffic.

Proteins synthesized in the rough endoplasmic reticulum are folded and modified and packaged into vesicles that move to the Golgi apparatus for further processing, sorting, and dispatch to their final destinations: secretion, lysosomes, or the plasma membrane. Lipids are synthesized in the ER and distributed through the system to membranes elsewhere. The Golgi apparatus functions as a central sorting station, adding carbohydrate tags for targeting and directing cargo to late endosomes, lysosomes, the plasma membrane, or secretory pathways.

Trafficking between compartments occurs via vesicles that bud from one membrane and fuse with another. This vesicle transport is orchestrated by coat proteins, SNAREs, tethering factors, and cytoskeletal elements, maintaining distinct chemical environments in each compartment while allowing coordinated flow of materials.

Dysfunction of the endomembrane system can underlie various diseases, including lysosomal storage disorders and defects in protein processing that affect secretion or membrane composition.