1. Forms cell membranes and maintains metabolism. The cell membrane thickness is generally 7-8nm, mainly composed of lipids (50%), protein (40%) and carbohydrates (2-10%). Among them, lipids are mainly composed of phospholipids (>70%), cholesterol (<30%) and a small amount of glycolipids (<10%). The cell membrane structure composed of phospholipid bilayer has specific permeability and plays an important role in the metabolism of the human body.