Lipoproteins are complexes of lipids and proteins that transport triglycerides, cholesterol, and fatty acids in the bloodstream. They are composed of a hydrophobic core of lipids surrounded by a hydrophilic coat of apolipoproteins and phospholipids. There are four main classes of lipoproteins - chylomicrons, HDL, LDL, and VLDL - which differ in their protein and lipid components and function in lipid transport. Apolipoproteins are the protein component of lipoproteins that help maintain their structure, act as enzymes, and transport and distribute lipids to tissues via the four major pathways of lipoprotein metabolism.