Lipoprotein, illustration. This is a high density lipoprotein (HDL), or 'good' cholesterol, molecule. It consists of a core of esterified cholesterol molecules surrounded by a shell (green and yellow) of unesterified cholesterol and phospholipids. The complex structure includes carrier proteins (blue) known as apolipoproteins, which assist transport in the blood. HDL cholesterol plays a role in fat metabolism and contributes to cardiovascular health.