The two-layer fat membrane that surrounds every cell—it controls what gets in and out.The lipid bilayer is the fundamental structural component of all biological membranes, composed of amphipathic phospholipids arranged with hydrophilic heads facing aqueous environments and hydrophobic tails forming the membrane interior. Bacterial and mammalian membranes differ in lipid composition—bacterial membranes are enriched in negatively charged phospholipids, which is why cationic antimicrobial peptides preferentially target bacteria over host cells. This selectivity is the basis for the therapeutic window of antimicrobial peptides like magainins, temporins, and defensins.