Which macromolecule is primarily responsible for the structure and function of a cell's membrane?

The correct answer is that lipids are primarily responsible for the structure and function of a cell's membrane. Cell membranes are primarily composed of a phospholipid bilayer, which provides both structural integrity and a barrier that separates the internal environment of the cell from the outside. The hydrophobic (water-fearing) tails of the phospholipids face inward, while the hydrophilic (water-loving) heads face outward, creating a semi-permeable membrane. This structure is essential for the membrane's function, which includes protecting the cell, providing support, and facilitating communication and transport of molecules in and out of the cell.

Proteins embedded within the membrane also play crucial roles, such as aiding in transport and acting as receptors. However, the foundational structure of the membrane is attributed to lipids, as they form the basic configuration that allows for these protein functions to occur effectively. Carbohydrates, while attached to proteins and lipids on the extracellular surface for cell recognition and signaling, do not form the structural component of the membrane itself. Nucleic acids are primarily involved in genetic information storage and transfer, making them less relevant to the structural aspects of membranes. Thus, lipids are the key macromolecules in defining the cell membrane's