One of a class of compounds which contains long-chain aliphatic hydrocarbons (cyclic or acyclic) and their derivatives, such as acids (fatty acids), alcohols, amines, amino alcohols, and aldehydes. The presence of the long aliphatic chain as the characteristic component of lipids confers distinct solubility properties on the simpler members of this class of naturally occurring compounds. This led to the traditional definition of lipids as substances which are insoluble in water but soluble in fat solvents such as ether, chloroform, and benzene. However, those lipids (particularly glycolipids and phospholipids) which contain polar components in the molecule may be insoluble in these solvents, and some are even soluble in water. Most of the phosphatides are good emulsifying agents.