Lipoproteins in bloodstream. Computer illustration of lipoprotein particles (round) inside a blood vessel. Lipoproteins are the major carriers of cholesterol in the blood and are mainly composed of lipids (fats). The complex structure includes carrier proteins known as apo-B proteins (green 'ribbons'), which assist transport in the blood. The outer coat consists of phospholipids and free cholesterol molecules. Within the core, cholesterol molecules are bound into long esters by fatty acid chains.