A lipid is a fatlike substance, composed of nonpolar organic molecules, which is are insoluble in water (itself made of polar molecules) but soluble in nonpolar organic solvents such as chloroform or ethanol. Lipids include fats, oils, waxes, steroids, phospholipids, glycolipids, and carotenoids.
Lipids are the major fuel reserve for humans and most mammals. These molecules are very efficient at storing needed energy. One gram of fat stores about 9 kilocalories per gram, compared to about 4 kilocalories per gram in the case of carbohydrates or proteins. For mobile animals, this means that less bulk has to be carried around and a normal sized body that is about 20% fat has enough stored energy to last about 20–30 days without eating. Fatty foods, with high lipid content, often provide more lipids than can be digested and used right away. The excess is stored in the adipose tissue.
A lipid bilayer gives rise to the structure of a cell membrane, in which two layers of phospholipids spontaneously align so that the hydrophilic head groups are exposed to water, while the hydrophobic tails (consisting of fatty acids) are pointed toward the center of the membrane. This establishes a boundary separating the contents of the cell from its surroundings, or an organelle within a cell from the general intracellular environment.