Interior of membrane-bound organelles such as the endoplasmic reticulum, nuclear membrane, vesicles, Golgi apparatus, etc.
The various lumens of the endomembrane system represent volumes, or compartments, that are cut off by lipid bilayer from cytoplasm of eukaryotic cells.
The orientation of the endomembrane lumen is essentially equivalent to the outside of the cell. That is, movement from the cytoplasm to the lumen of these organelles must cross a lipid bilayer and that lipid bilayer is one that is in communication with the plasma membrane of the cell via vesicles (which also are members of the endomembrane system).
Furthermore, when a eukaryotic cell secretes something it does so by placing or synhtesizing that something within the lumen of the an endomembrane system member which then, via vesicles, fuses with the plasma membrane, releasing its contents extracellularly, i.e., via exocytosis. Similarly, when a cell takes up something by endocytosis that something, formerly found external to the cell, is now found within the lumen of a member of the endomembrane system.
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