What substance primarily composes the majority of the cell membrane?

The majority of the cell membrane is primarily composed of phospholipids, which play a critical role in its structure and function. Phospholipids have a unique molecular structure that includes a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails." This dual nature allows phospholipids to arrange themselves into a bilayer, creating the fundamental framework of the cell membrane.

In this bilayer, the hydrophilic heads face outward towards the aqueous environments both inside and outside the cell, while the hydrophobic tails face inward, shielding themselves from water. This arrangement not only provides a stable barrier that separates the cell from its external environment but also helps in regulating what enters and exits the cell.

While proteins and carbohydrates are important components of the cell membrane as well, they play more supportive or functional roles. Proteins, for instance, can act as channels, receptors, or enzymes, and carbohydrates often serve as recognition sites on the surface of the membrane. However, it is the phospholipid bilayer that serves as the structural foundation of the membrane.