Question

Phospholipids are made by integral ER membrane enzymes whose active sites face the cytosol and they are inserted into the outer (cytoplasmic) leaflet of the ER membrane. How then do lipids destined for the luminal leaflet of the ER membrane get there?

Answer 1

Phospholipids reach the luminal leaflet of the ER membrane through flippase-mediated lipid translocation, where integral proteins called flippases transfer specific lipids from the cytoplasmic to the luminal leaflet.

Step-by-step explanation:

Phospholipids destined for the luminal leaflet of the ER membrane reach there by a mechanism known as flippase-mediated lipid translocation. Initially, these lipids are synthesized by integral ER membrane enzymes and inserted into the outer (cytoplasmic) leaflet. Integral proteins called flippases then facilitate the transfer of specific phospholipids from the outer leaflet to the inner (luminal) leaflet, maintaining the asymmetric distribution of lipids in the bilayer. This process ensures that the composition of the cell membrane can be finely regulated, allowing it to perform its various functions effectively.