Endomembrane system

The endomembrane system comprises functionally connected internal membranes such as the endoplasmic reticulum, Golgi apparatus, endosomes and lysosomes, together with their transport carriers. Connections through carrier traffic differ from direct continuity of the membrane sheets.

Ordinary vesicle budding and fusion preserve which side of a membrane a protein domain faces: a cytoplasmic domain remains cytoplasmic, and a luminal domain remains luminal. Invagination can enclose previously external space without transporting its contents through a bilayer. The endoplasmic reticulum lumen is continuous with the space between the two nuclear envelope membranes; it is not the chromosome-containing nucleoplasm. Any proposed evolutionary remodelling must account for these distinct spaces and the membrane connections between them.