The phospholipid bilayer is a fundamental structure of cell membranes, composed of two layers of phospholipid molecules. Each phospholipid has a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." This arrangement allows the heads to face outward towards the water inside and outside the cell, while the tails face inward, away from the water, creating a barrier that separates the cell from its environment. This bilayer is crucial for maintaining the integrity of the cell and regulating the movement of substances in and out. It provides a flexible yet stable structure that supports various proteins and carbohydrates, which play roles in communication and transport. The phospholipid bilayer is essential for the function of all living cells, including those in plants, animals, and microorganisms.