Which boundary of the cell is primarily made of phospholipids?

• A. Phospholipids
• B. Proteins
• C. Carbohydrates
• D. Nucleic Acids

Answer: (A)

The boundary is built mainly from phospholipids that form a bilayer. Phospholipids have a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. In water, they arrange themselves with the heads facing the watery interior and exterior of the cell and the tails tucked away from water in the interior of the layer. This creates a stable, flexible barrier that controls what can move into and out of the cell.

Proteins sit in or on this lipid matrix and carry out many functions, such as transport and signaling, but they are not the primary component of the boundary itself. Carbohydrates are usually attached to lipids or proteins on the outer surface, contributing to recognition and communication, not forming the main boundary. Nucleic acids are not part of membranes.