A lipid bilayer is a thin membrane made up of two layers of lipid molecules. These lipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. The arrangement allows the hydrophilic heads to face outward towards the water inside and outside the cell, while the hydrophobic tails face each other, creating a barrier that separates the cell's interior from its environment. Lipid bilayers are fundamental components of cell membranes, providing structure and protection. They also play a crucial role in regulating the movement of substances in and out of cells, allowing essential molecules to pass while keeping harmful substances out. This selective permeability is vital for maintaining cellular function and homeostasis.