Lipids (Greek: lipos, fat) are the fourth major group of biological molecules
found in all cells. Unlike other biopolymers, lipids are not polymeric although
they do aggregate. In living systems, lipids are rarely found as free molecules
but instead associate with other molecules, i.e., carbohydrates, proteins, and
usually with other lipids. Classification of lipids is generally based on the
following criteria: physical properties at room temperature, polarity, and
essentiality for humans or structure. Based on structure, lipids can be
classified into three groups—derived, simple, and complex lipids. The
derived lipids refer to “fatty acids” and “alcohols”, which are the building
blocks for the simple and complex lipids. Simple lipids include
triacylglycerols that are fatty acid triesters of glycerol. Examples of complex
lipids—yielding three or more different compounds on hydrolysis—are
glycerophospholipids, glycolipids, and sphingolipids. Due to their various
kinds of structures, lipids also perform diverse biological functions from
major constituents of cell membrane structure and protective surfaces in
many organisms to chief storage and transport form of metabolic energy.
They also play a key role in regulating of several intra- and intercellular
signaling events. Last but not least, there exists a group of lipids with
relatively low molecular weight such as steroids, terpenes, prostaglandins,
and quinones that have diverse and very important functions as hormones,
vitamins, and photopigments.