Lipidos

Lipids
Philip L Yeagle, Rutgers University, Newark, New Jersey, USA
Lipids are molecular building blocks for the most fundamental structures in biology, including the serum lipoproteins, the membranes of cells and cellular organelles and the membranes of enveloped viruses. Lipids provide a form of energy storage, like an electrical battery for an organism. Lipids are the metabolic precursors ofhormones and signalling molecules in cells. Lipids regulate, often in specific ways, the functionality of biological membranes. A nearly uncountable catalogue of lipid molecules is found in nature, their individual structures imparting a wide variety of properties essential to the structures and functions of lipids in life.

Introductory article
Article Contents
. Introduction . PropertiesArising from Lipid Structures . Overview of Lipid Classes . Structural Roles of Lipids in Cells . Biological Roles of Lipids in Cells . Summary

Online posting date: 15th September 2009

Introduction
The lipids of life govern many biological structures and functions through their wide array of chemical structures and corresponding physical properties. Although the catalogue of chemicalstructures is vast, unifying principles assist the student of lipids in organizing lipid properties, and their corresponding roles, into an understandable text. In the following, the relationship between lipid chemical structure and lipid fundamental properties will be explored. With this information in hand, many of the roles of lipids in biology can be more readily understood (History of lipid science,http://www.cyberlipid.org/history/ history1.htm).

Properties Arising from Lipid Structures
The structures of lipids in large part determine their properties. For example, the lipids of cell membranes are amphipathic: one portion is largely hydrocarbon, and thus hydrophobic, and another portion is polar and hydrophilic. Lipids in the interior of lipoproteins are nearly entirely hydrocarbon andthus nearly fully hydrophobic. Because these properties, hydrophobicity, hydrophilicity and amphipathicity, are critical to an understanding of lipids in nature, they deserve some consideration before moving on.

The hydrophobic effect
The hydrophobic effect is the most important influence in the structures of biology outside the covalent bond. It

ELS subject area: Biochemistry How to cite:Yeagle, Philip L (September 2009) Lipids. In: Encyclopedia of Life Sciences (ELS). John Wiley & Sons, Ltd: Chichester. DOI: 10.1002/9780470015902.a0000711.pub2

reflects the thermodynamically unfavourable encounter between water and hydrocarbon. Hydrocarbons are built around carbon backbones, with direct bonds from carbons to either hydrogen or another carbon. Since the electronegativity differencebetween hydrogen and carbon is small, the electrons in these bonds of the hydrocarbon are not polarized and there is little if any separation of charge under most circumstances. Hydrocarbons consequently cannot form hydrogen bonds. Consequently, hydrocarbon structures cannot participate in the structure of the water because to do so, an ability to form hydrogen bonds is required. Therefore watermust become more structured in response to the intrusion of a hydrocarbon molecule, forming, for example, a clathrate-type structure of hydrogen-bonded water molecules around the hydrocarbon. This ordering of the water molecules is achieved through an entropy cost and is thermodynamically unfavourable. Therefore oil and water do not mix because the water must become structured to accommodate ahydrocarbon molecule (of the oil). Interestingly, one of the first experiments recorded that revealed this principle is the famous calming of the waters by Benjamin Franklin. In 1773, Mr. Franklin, using an English pond, observed the ability of a small amount of oil to coat the surface of a pond, probably with a molecular monolayer of the oil, but not to penetrate the water phase (A Letter from...