endomembranisüsteemi

The endomembrane system is a network of membranous organelles within most eukaryotic cells that coordinate the synthesis, modification, sorting, and transport of proteins and lipids. It includes the nuclear envelope, endoplasmic reticulum (ER) (rough and smooth), Golgi apparatus, lysosomes, endosomes, vesicles, and the plasma membrane. These components are physically connected by vesicular trafficking, allowing materials to move between compartments.

Proteins and lipids enter this system through the nuclear envelope/ER. The rough ER synthesizes secretory and membrane proteins, while the smooth ER participates in lipid synthesis and metabolic processes. Transport between ER and Golgi, and within the Golgi, is mediated by vesicles coated with coat protein complexes such as COPII (ER to Golgi), COPI (Golgi to ER and intra-Golgi), and clathrin (from the trans-Golgi network to endosomes or the plasma membrane).

The Golgi apparatus further processes cargo through modification, such as glycosylation, sulfation, and sorting signals that direct cargo to their final destinations. Secretory cargo can be delivered to the plasma membrane or secreted outside the cell, whereas endosomes and lysosomes handle endocytosed material and degradation. Lysosomal enzymes are often targeted by mannose-6-phosphate tagging.

Quality control mechanisms in the ER monitor protein folding; prolonged stress can trigger the unfolded protein response. Dysfunction in endomembrane trafficking is associated with diseases including lysosomal storage disorders, congenital disorders of glycosylation, and other vesicular transport defects. The system thus plays a central role in cellular organization and intercompartmental communication.