The inner leaflet of the human erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatidylserine. the outer leaflet predominantly of phosphatidylcholine and sphingomyelin. although the phospholipids components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. how?

-Flippases: membrane enzymes that move PE and PS to internal side against their gradient

-energetically difficult for flipping, hydrophilic head has to move through hydrophobic center segment of membrane

-Main reasons: energetic barrier, other enzymes work to maintain the sidedness

-PS on surface tags for apoptosis, want to avoid

not only is it colder, it also becomes darker, less salty, and more dense.