Lipids
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Lipids are a class of biomolecules, which have diverse structures and, therefore, can not be defined based on structural features. Instead, lipids are defined based on a physical property, namely the solubility. Lipids are biomolecules that are insoluble in water but soluble in nonpolar solvents, such as ether, chloroform, and dichloromethane.
see also saponifiable lipid, nonsaponifiable lipid