Which organic compound is primarily responsible for the structure of cell membranes?

The compound primarily responsible for the structure of cell membranes is lipids. Cell membranes are constructed mainly from a bilayer of phospholipids, which have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This unique structure allows cell membranes to form a semi-permeable barrier that regulates the movement of substances in and out of the cell.

In addition to phospholipids, other lipids such as cholesterol are also important as they help to stabilize the membrane's fluidity and integrity. The lipid bilayer creates an environment that facilitates the embedding of proteins, which serve various functions such as transport, signaling, and acting as receptors. While proteins, nucleic acids, and carbohydrates are crucial in various cell functions, they do not play the central role in forming the membrane’s basic structure like lipids do. Thus, lipids are the fundamental building blocks of cell membranes, enabling their structure and function.