There are several different lipoprotein species found in plasma but their basic structures are similar. The insoluble lipid (cholesterol ester and triglyceride) forms a central core in the form of a lipid droplet. This is surrounded by an outer monolayer of molecules such as free cholesterol, phospholipids and proteins termed apoproteins, which give the complexes their name. These molecules are able to sit at the water/fat interface because they are partly water-soluble and partly lipid-soluble.