Which macromolecule is primarily responsible for the structure of cell membranes?

• A. Proteins
• B. Carbohydrates
• C. Lipids
• D. Nucleic acids

Answer: (C)

Cell membranes are primarily composed of lipids, specifically phospholipids, which form a bilayer. This lipid bilayer is crucial because it provides a semi-permeable barrier that separates the interior of the cell from the external environment. The hydrophobic tails of phospholipids face inward, away from water, while the hydrophilic heads face outward, towards the water inside and outside the cell. This arrangement is essential for maintaining the integrity and fluidity of the membrane, allowing for the selective transport of substances in and out of the cell.

In addition to phospholipids, cholesterol is also found within cell membranes, contributing further to their structural stability and fluidity. While proteins do play significant roles in cell membrane functions, such as acting as receptors or channels, they do not form the fundamental structure of the membrane itself, which is primarily lipid-based. Carbohydrates are primarily involved in cell recognition and signaling and do not serve as the backbone of the membrane structure. Nucleic acids are essential for genetic information storage and transmission but are not involved in forming cellular membranes. Thus, lipids are the correct macromolecule responsible for the structural composition of cell membranes.