Lipid Bilayer

A lipid bilayer is a double-layered sheet of phospholipid molecules that forms the core structure of cell membranes. The phospholipids are arranged with their hydrophilic (water-attracted) -heads- facing the aqueous environments on each side of the membrane and their hydrophobic -tails- tucked inward, forming a stable barrier between the inside and outside of the cell.

• The hydrophobic interior of the bilayer makes the membrane selectively permeable: small nonpolar molecules can diffuse through, but charged or large polar substances are blocked without transport proteins.
• Phospholipids self-assemble into bilayers in water due to the hydrophobic effect - this spontaneous formation is fundamental to how cells form membranes.
• All biological membranes (the plasma membrane and organelle membranes) have a phospholipid bilayer foundation. The fluid mosaic model describes how proteins, cholesterol, and carbohydrates are embedded in or attached to this moving bilayer framework.

• Diffusion question - -Which type of molecule crosses a phospholipid bilayer most easily-- Answer: small, nonpolar molecules (e.g. gases like O-/CO-); ions or polar molecules cannot pass freely.
• Structural clue - A description of two layers of molecules with hydrophobic tails facing each other points to the phospholipid bilayer (basic structure of a membrane).
• Terminology trap - -Lipid bilayer- is essentially the same as -phospholipid bilayer- (sometimes just called the cell membrane structure in general). If you see either term, it refers to the double layer of phospholipids in membranes.