What role do phospholipids play in cells?

Phospholipids are primarily known for their function as structural components of cell membranes. They have a unique molecular structure that includes a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." This dual characteristic allows them to form a bilayer, which is essential in creating the cell membrane. The phospholipid bilayer serves as a barrier that defines the cell's boundary, allowing it to maintain a distinct internal environment, separate from the external one. This structure is vital for the proper functioning of cells, as it regulates what enters and exits the cell, facilitating communication and signaling processes.

Other roles mentioned in the options, while important in their own right, do not pertain directly to the primary role of phospholipids. Energy sources typically involve carbohydrates and fats, so phospholipids do not function as an energy source. Though transport of nutrients is a critical cellular function, it is mainly carried out by proteins and other molecules. Lastly, while some lipids can have enzymatic functions, phospholipids themselves are not primarily associated with enzymatic activity.