How come phospholipids rarely filp from one layer to the other in most membranes?

Phospholipids can diffuse laterally in a membrane bilayer, but they rarely flip from one layer to the other in most membranes. why?





and, if you can answer this too:


cholesterol readily flips from one layer to another. why?How come phospholipids rarely filp from one layer to the other in most membranes?
Think about the phopholipid bilayer. There is a huge polar gradient from the interior to the exterior. While it's possible for longitudinal flipping to occur, it's extremely energetically unfavorable. The energy incurred to overcome the polarity is quite high, instead phospholipids move around and flip laterally.





It's a matter of kinetics and good ol' delta-G