A phospholipid bilayer. Lipid molecules are arranged so that their heads point outward and their tails point to the center of the membrane. Embedded in the phospholipid bilayer are the following structures. A: A long branched chain of molecules attached to a lipid in the membrane. B: A large structure that spans the entire membrane with a hollow channel in the middle. C: A long branched chain of molecules attached to a protein. D: A protein with a long branched chain of molecules attached to it.