In fact, certain lipids, such as Glycolipids that contain very large components that are polar, cannot actually rotate because of those polar interactions, because these polar interactions will be unstabilized when they interact with that core hydrophobic region. And so, just like proteins, glycolipids cannot actually flip flop or rotate. Now, nevertheless, we see that lipids such as glycolipids and spingolipids can and do lead to asymmetry of the membrane. So basically, because the flip flopping process takes place so long, takes place not so long, but takes place so slow, and because certain lipids, like Glycolipids can have actually rotate, we see that lipids also, just like proteins, actually contribute to the asymmetric nature of membranes. In fact, if we study the membranes of red blood cells, we'll see that the outside layer of the red blood cell membrane consists of these two lipids. So, these two lipids are found in high proportion on the outside of the membrane, while the inside contains these two lipids in high proportion.