Membranes are made of phospholipids , which are made of lipids . Lipids are the stuff of oils, fats and waxes. Unlike proteins and carbohydrates, lipids are not polymers. Lipids which store energy are triglycerides , while those which form membranes are phospholipids. Triglycerides are formed by a molecule of glycerol with three fatty acids attached. The reaction which results in triglycerides is condensation .

Glycerol (green) + 3x fatty acids (red) The fatty acids can be simplified in drawing:

The bonds formed (C-O) are called ester bonds . Fatty acids can be saturated or unsaturated (monounsaturated; polyunsaturated). Saturated fatty acids have all their carbon (C) atoms linked to hydrogen (H) atoms, hence saturated with hydrogen . If there is a carbon atom with a double bond to its neighbour carbon atom, then it will only have one bond to a hydrogen atom, hence it is unsaturated. If there is one double bond present, the fatty acid is monounsaturated . If there are multiple double bonds present, the fatty acid is polyunsaturated . In phospholipids one of the fatty acids is replaced by a phosphate group.