Phospholipids form a bilayer in water due to their unique structure, which includes both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. Each phospholipid molecule has a hydrophilic ‘head’ made up of a phosphate group and two long hydrophobic ‘tails’ composed of fatty acids.
When phospholipids are placed in water, the hydrophilic heads face outward towards the water, while the hydrophobic tails point inward, away from the water. This configuration minimizes the exposure of the hydrophobic tails to water, which is energetically favorable. The resulting bilayer structure creates a barrier that separates the watery environment outside the cell from the interior, which is essential for cellular function.
Moreover, the bilayer structure allows for the formation of cell membranes, which are crucial for maintaining the integrity of cells, facilitating communication, and controlling the movement of substances in and out of the cell.