- The primary structural component of biological cell membranes is phospholipids.
- Phospholipids have a unique molecular structure consisting of a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails."
- This amphipathic nature allows them to arrange themselves into a bilayer, forming the fundamental architecture of the cell membrane.

- In the phospholipid bilayer, the hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward toward the aqueous environments inside and outside the cell.
- This arrangement creates a semi-permeable membrane that controls the passage of substances in and out of the cell.

- Other lipids like glycolipids and cholesterol also contribute to the membrane's structure and function, but they are not the main components.
- Glycolipids play a role in cell recognition and signaling, whereas cholesterol helps maintain membrane fluidity and stability.
- Triglycerides, on the other hand, primarily serve as energy storage molecules and are not involved in membrane structure.

Therefore, the correct answer is phospholipids, as they form the essential structural framework of all biological membranes.

Reference: Biochemistry, 8th Edition, Chapter 12 – Biological Membranes / Page 456