Answer: Lipid molecules.
Explanation: Biological membranes are bilayer of phospholipids that control the entrance and exit of molecules in the cells. The major components of phospholipids are the fatty acids. Phospholipids have two components: the fatty acyl chains and the phosphate group. The fatty acyl chains of the phospholipids molecules face each other at the core of the bilayer, forming a fluid hydrophobic interior while the phosphate (polar head) groups face outward. Proteins are embedded in this bilayer held by hydrophobic interactions between the membrane lipids and hydrophobic domains in the proteins. Both proteins and lipids move laterally in the plane of the bilayer but movement from one face of the bilayer to another is restricted.