Carotenoids are biosynthesized by all photosynthetic plants, protists, and bacteria, as well as some heterotrophic bacteria, some fungi, and some invertebrates. Retinoids are formally diterpenes (C20) because they are composed of four isoprene (C5) units joined in a head-to-tail manner, but they are biosynthetically derived from the oxidative cleavage of carotenoids. Several prefixes, preceded by the number of the carbon atom(s) that contain the modification, are used for the systematic nomenclature of modified retinoids and carotenoids. Mobilization of retinol to extrahepatic tissues requires the secretion of serum retinol binding protein (RBP4), which is the main carrier in blood. In the blood, free sRBP interacts with a sRBP receptor (RBPR) in the plasma membrane of cells, which is followed by the release of retinol and its uptake into the cells. Alternatively, oxidation of all-trans-retinal to all-transretinoic acid is carried out by aldehyde dehydrogenases (ALDH or RALDH), which are also cytosolic proteins.