The role of phospholipids in membrane structure is foundational to cellular function. These molecules form the bilayer of cell membranes, providing fluidity and a barrier that separates the interior of the cell from the external environment. Their hydrophobic tails and hydrophilic heads create a semi-permeable membrane, allowing selective passage of substances. Phospholipids also play a role in cellular signaling, membrane fusion, and the formation of lipid rafts, which are essential for various cellular processes.