establishment of The asymmetry of the cell membrane

The asymmetry of the cell membrane is established primarily by which of the following?

• a.Membrane synthesis in the endoplasmic reticulum
• b.Membrane modification in the Golgi apparatus
• c.Presence of carbohydrates on the cytoplasmic surface
• d.The distribution of cholesterol
• e.Flipping proteins between the leaflets of the lipid bilayer

The answer is:  ( a ).
 Asymmetry of the lipid bilayer is established during membrane synthesis in the endoplasmic reticulum (answer a) before reaching the Golgi apparatus (answer b). Carbohydrates are associated with the N terminals of transmembrane proteins that extend from the extracellular surface, not the cytoplasmic surface (answer c).
Cholesterol is different from proteins and phospholipids that are asymmetrically distributed within the bilayer (answer d). Cholesterol is found on both sides of the bilayer. The small polar head group structure of cholesterol allows it to flip-flop from leaflet to leaflet and respond to changes in shape. In contrast to cholesterol, most proteins and phospholipids are capable of only rare flip-flop (answer e). For example, transbilayer movement of phospholipid is limited mostly to the endoplasmic reticulum.