 Welcome to Biomolecules, the lipids. Lipids are organic molecules that are insoluble in water. In biology, lipids' main functions include storing energy, signaling, and acting as structural components for cell membranes. Let's examine three types of lipids, neutral fats, phospholipids, and cholesterol. Neutral fats, also called triglycerides, are three fatty acid chains attached to a single glycerol molecule by dehydration synthesis. The fatty acid chains are the building blocks. While these chains can differ in length, each one has an even number of carbon molecules. Neutral fat stores energy fuel, insulates body tissues, and cushions and protects organs. Saturated fatty acids have single bonds between the carbon atoms. They originate from animal sources and are solid at room temperature. Other fatty acids don't bind their maximum number of hydrogens due to double bonding between carbon atoms in the chain. Fatty acids with one double bond are mono-unsaturated, while those with two or more double bonds are polyunsaturated fatty acids. Unsaturated fats originate from plants and are liquid at room temperature. Phospholipids are modified triglycerides where a phosphate group replaces one of the fatty acid chains. Phospholipids, or phosphoglycerides, have a non-polar fatty acid chain portion and a phosphate polar portion. Two layers of phospholipids are the chief component of all cell membranes. In a cell membrane, the two layers self-assemble so that their water-soluble heads, or hydrophilic heads, form the surface and interior of the membrane and the water-insoluble tails, or hydrophobic tails, face each other. Cholesterol is an organic molecule known as a sterol, or modified steroid. It helps build and maintain membranes and composes over 30% of all animal cell membranes. The cholesterol molecule has four interconnected carbon rings. Cholesterol helps to stabilize all animal cell membranes. The body uses it to synthesize steroid hormones including sex hormones and those of the adrenal cortex. It's also used during vitamin D and biosynthesis. Congratulations! You've completed biomolecules, the lipids.