Question

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

1) They diffuse freely into the luminal leaflet
2) There are enzymes called flippases that flip these lipids later into the opposite leaflet
3) They are disassembled on the cytoplasmic side and reassembled on the luminal side
4) They move to the cytoplasmic leaflet by osmosis
5) There are enzymes called translocases that flip these lipids later into the opposite leaflet

Answer 1

Phospholipids destined for the luminal leaflet of the ER membrane reach their destination through transport vesicles that bud from the ER membrane.

Step-by-step explanation:

Phospholipids destined for the luminal leaflet of the ER membrane are not inserted directly from the cytosol. Instead, they reach their destination through a process involving transport vesicles that bud from the ER membrane. These vesicles carry the lipids to the luminal side, allowing them to be incorporated into the luminal leaflet of the ER membrane.

This process ensures that the phospholipids are correctly localized to their intended leaflet of the ER membrane. Enzymes in the Golgi apparatus further modify lipids and proteins transported from the ER to the Golgi, adding carbohydrate components to them before they are inserted into the plasma membrane or other organelles.