Your body is a complicated machine, so you may be surprised to learn that important biological molecules that make up living things fall into only four main classes: carbohydrates, lipids, proteins, and nucleic acids.Your cells are surrounded by a very important type of lipid, calledphospholipids. Phospholipids consist of a hydrophilic (or 'water loving') head and a hydrophobic (or 'water fearing') tail. Phospholipids like to line up and arrange themselves into two parallel layers, called a phospholipid bilayer. This layer makes up your cell membranes and is critical to a cell's ability to function.