A phospholipid is a type of lipid molecule that is a key component of cell membranes. It consists of two fatty acid tails and a phosphate group attached to a glycerol backbone. The unique structure allows phospholipids to form a bilayer, where the hydrophilic (water-attracting) heads face outward and the hydrophobic (water-repelling) tails face inward, creating a barrier that separates the inside of the cell from the outside environment. Phospholipids play a crucial role in maintaining the integrity and fluidity of cell membranes. They also facilitate the movement of substances in and out of cells, contributing to various cellular functions. In addition to their structural role, phospholipids are involved in signaling pathways and can be found in various biological systems, including bacteria, plants, and animals.