What role do phospholipids play in biological membranes?

Phospholipids are essential components of biological membranes, primarily forming the phospholipid bilayer. In this structure, the hydrophilic (water-attracting) phosphate "heads" face the aqueous environment both inside and outside the cell, while the hydrophobic (water-repelling) fatty acid "tails" are oriented inward, away from water. This arrangement creates a semi-permeable barrier that not only protects the internal environment of the cell but also regulates the movement of substances in and out of the cell.

The bilayer structure is critical for membrane fluidity and flexibility, allowing for various functions such as cell signaling, facilitated diffusion, and membrane fusion. This unique arrangement enables the compartmentalization of cellular processes, which is vital for the proper functioning of the cell. Thus, phospholipids play a foundational role in maintaining the structural integrity and functionality of biological membranes.