Many lipophilic substances, including fat-soluble vitamins, cholesterol, and triglycerides are essential for life. The body needs to be able to absorb and transport these substances. However, by definition, lipophilic substances are not water-soluble, and, since blood is aqueous, this presents a challenge. The body addresses this need by using carriers that can bind or sequester lipophilic molecules to "vehicles," which can then be transported through the aqueous environment of the blood. For example, small lipid-soluble hormone molecules like estrogen, testosterone or cortisone are carried through the blood by binding to carrier proteins (such as albumin or sex-hormone binding globulin).
Cholesterol and triglycerides are carried through the body in small spherical particles that trap the lipophilic molecules in their centers. These particles have an outer shell that is polar on the surface so that the particles are soluble in the blood but they have a lipophilic core which can hold fat-soluble molecules. These particles are not cells, they are just advanced micelles, specialized globules that are designed for transporting lipids to water-soluble cells.
