Which macromolecule serves as the main structural component of cell membranes?

Phospholipids are the primary macromolecules that form the structural basis of cell membranes. They consist of a hydrophilic "head" that is attracted to water and two hydrophobic "tails" that repel water. This unique structure allows phospholipids to arrange themselves into a bilayer, with the hydrophobic tails facing inward away from water, while the hydrophilic heads face outward towards the aqueous environment inside and outside the cell. This bilayer arrangement is crucial as it creates a selective barrier that regulates the movement of substances in and out of the cell, thus maintaining cellular integrity and contributing to the fluidity and flexibility of the membrane.

Proteins, while important for various functions in membranes such as transport and signaling, do not serve as the main structural component. Carbohydrates also play significant roles in cell recognition and signaling but are not the primary structural elements of membranes. Nucleic acids, which include DNA and RNA, are involved in genetic information storage and transfer and are not related to the structural makeup of cell membranes. Thus, the correct answer focuses on the role of phospholipids in forming the essential bilayer structure of cell membranes.