endomembransystems

The endomembrane system is a group of interconnected membranous organelles that coordinate the synthesis, modification, and transport of proteins and lipids within the cell. It includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, endosomes, secretory vesicles, and the plasma membrane. In plant and fungal cells, vacuoles also function within this network. Peroxisomes and mitochondria are not generally considered part of the endomembrane system, which is distinct from these other organelles.

The nuclear envelope is continuous with the ER, which forms the main site of synthesis for proteins destined for export or membranes. Rough ER carries ribosomes that synthesize secreted and membrane proteins, which are delivered to the Golgi apparatus via vesicles. The Golgi modifies, sorts, and packages these cargos for delivery to lysosomes, the plasma membrane, or for secretion. Endosomes receive material brought into the cell by endocytosis and route it to lysosomes or recycle it back to the membrane. Lysosomes and plant vacuoles contain hydrolytic enzymes for degradation and recycling. The endomembrane system relies on vesicle budding and fusion, guided by SNARE proteins and small GTPases, and works in concert with cytoskeletal elements for transport.

A key function is the coordinated production and trafficking of proteins and lipids, along with membrane biogenesis and quality control, such as proper protein folding in the ER. Disruptions in trafficking or folding can contribute to disease, reflecting the system’s central role in cellular physiology.