The substance that makes up the majority of the cell membrane is phospholipids. These molecules comprise hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, which arrange themselves into a bilayer, creating a semi-permeable membrane. The phospholipid bilayer is crucial for maintaining the integrity of the cell, allowing selective passage of substances, and facilitating cell communication and signaling. Proteins and cholesterol embedded in this layer further enhance membrane function.