A system of membranes within a cell that serves as a single functional and developmental unit
The endomembrane system is a system of membranous components. It includes the membranes of nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, endosomes, vesicles, and the cell membrane. It does not include the membranes of mitochondria and/or chloroplasts.
The membranes of the organelles included in the endomembrane system are related through direct contact, e.g. the nuclear envelope being connected to the endoplasmic reticulum, and the latter to the Golgi apparatus, or via indirect contact, e.g. by the transfer of membrane segments as vesicles.
The endomembrane system is involved in the manufacturing and distributing cellular products. Nonetheless, the membranes of the organelle components vary in specific function. For instance, the nuclear envelope encases the nuclear material. The endoplasmic reticulum is associated with the synthesis of proteins and other biomolecules. The Golgi apparatus does the packaging of newly synthesized biomolecules for transport within or outside the cell. The lysosomes are vesicles containing enzymes synthesized from the endoplasmic reticulum and released from Golgi apparatus. Endosomes are compartments of the endocytic membrane transport pathway from the cell membrane to the lysosome. The cell membrane is the protective barrier that separates the interior of all cells from the outside environment.
Nuclear envelope, endoplasmic reticulum, Golgi apparatus and cell membrane are bounded by two membranes whereas lysosomes are bounded by a single membrane. Nevertheless, their membranes share a lipid bilayer structure wherein proteins traverse or attach to the lipid bilayer.