The molecules with two fatty acid chains and a phosphate group that form biological membranes are called phospholipids. They are the primary components of the lipid bilayer, a fundamental component of all cell membranes. Phospholipids consist of a hydrophilic (water-loving) head, which is formed by the phosphate group and a glycerol molecule, and two hydrophobic (water-hating) tails, which are composed of the fatty acid chains. This amphipathic nature of phospholipids allows them to form a bilayer structure in water, with the hydrophobic tails facing inwards and the hydrophilic heads facing outwards, interacting with the surrounding aqueous environment. This structure provides a barrier that separates the inside of the cell from the outside environment while maintaining the cell's structural integrity and regulating the movement of substances into and out of the cell.