The main barrier to the movement of molecules across a cell membrane is the phospholipid bilayer, which is selectively permeable. This bilayer is composed of two layers of phospholipids that create a hydrophobic interior, preventing the free passage of polar and charged substances. Instead, molecules must utilize specific transport proteins or channels to cross the membrane. This selectivity is crucial for maintaining cellular homeostasis by regulating the exchange of ions, nutrients, and waste products.