What is the composition of a thin polar membrane that forms a barrier around cells?

• A. Protein layers
• B. Carbohydrate chains
• C. Bilayers of lipid molecules
• D. RNA strands

Answer: (C)

The correct answer is a bilayer of lipid molecules, which is foundational to the structure of cell membranes. Cell membranes, or plasma membranes, are primarily composed of a double layer of phospholipids. This bilayer provides a semi-permeable barrier that allows certain substances to pass while blocking others, thus maintaining the internal environment of the cell.

Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, which naturally arrange themselves into bilayers when exposed to an aqueous environment. This arrangement is crucial for the integrity and functionality of the cell membrane, allowing the cell to control its interactions with the extracellular environment.

While protein layers are found within and on the surface of the membrane, serving various functions such as signaling and transport, they do not form the primary barrier structure. Carbohydrate chains may be attached to proteins and lipids in the membrane, contributing to cell recognition and signaling, but they are not the main component of the membrane's structural framework. RNA strands play essential roles in protein synthesis and various cellular functions, but they do not contribute to the formation of the cell membrane itself.