Which lipid type is primarily involved in forming cell membranes?

Phospholipids are primarily involved in forming cell membranes due to their unique structure, which consists of a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." This amphipathic nature allows phospholipids to arrange themselves into a bilayer, where the hydrophilic heads face outward towards the aqueous environment and the hydrophobic tails face inward, away from water. This bilayer structure is essential for creating the semi-permeable membrane characteristic of all cells, allowing for the selective passage of substances in and out of the cell while maintaining the integrity of the cell's internal environment.

The other lipid types have different roles; triglycerides primarily serve as energy storage and do not contribute significantly to membrane structure. Sterols, like cholesterol, play a role in membrane fluidity and stability but are not the main components of the membrane itself. Saturated fats refer to a type of fatty acid that can be found in various lipids but again do not specifically form cell membranes. Thus, phospholipids are the correct answer for their pivotal role in cell membrane formation.