The greater the concentration of unsaturated fatty acid residues, the more fluid the bilayer. However, phospholipid molecules rarely flip-flop from one layer to the other. The fluidity of the phospholipid bilayer allows cells to be pliable. Many transmembrane proteins are glycoproteins with a carbohydrate chain that projects externally. Some are anchored to membrane by covalently attached lipid or covalently bonded to carbohydrate chain of a glycolipid.
Chains vary in branching.
Glycolipids and glycoproteins vary from species to species, from individual to individual of same species, and even from cell to cell in same individual. Membrane proteins determine most of the membrane's functions. Small noncharged lipid molecules pass through the membrane freely. Macromolecules cannot freely cross a plasma membrane. Ions and charged molecules have difficulty crossing the membrane.