Fats and Lipids
The term lipidAn organic compound found in tissue and that is soluble in nonpolar solvents. applies to any water-insolubleUnable to dissolve appreciably in a solvent. substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. which can be extracted from cells by organicRefers to the branch of chemistry that studies compounds containing carbon, usually in combination with hydrogen and other elements such as O, N, S, and P. Certain small ions and compounds containing carbon (such as carbonate ions and carbon dioxide) are not considered to be organic, but rather are classed as inorganic. solvents such as chloroform, etherAn organic compound containing the functional group R-O-R'., or benzene. Two major categories may be identified. Nonpolar lipids have molecular structures which contain no electrically charged sites, few polar groups, and large amounts of carbon and hydrogen. They are similar to hydrocarbons in being almost completely insoluble in water, and so they are said to be hydrophobicWater-hating; not attracted to water molecules or polar molecules. (from the Greek, meaning water-hater). On the other hand, polar lipids consist of molecules which have polar groups (such as —OH) or electrically charged sites at one end, and hydrocarbon chains at the other. Since polar or charged groups can hydrogen bondAn attractive force, either intramolecular or intermolecular, between an electronegative atom and a hydrogen atom attached to another electronegative atom. to or electrostatically attract water molecules, one end of a polar lipid molecule is said to be hydrophilicWater-loving; attracted to water molecules and polar molecules. (water-loving). Lipids with a polar and nonpolar end are sometimes called amphipathic lipids, because one end is hydrophilic, while the other is hydrophobic. Such substances often form structures which bury hydrophobic surface, while exposing hydrophilic surface to water. Some typical structures of both types of lipids are shown in Fig. 1.