Lipids may be broadly defined as hydrophobic or amphiphilic small molecules. In most
membranes, approximately 40%-50% of the mass of the membrane is composed of lipids. Lipids
provide a matrix for protein groups, acts as a barrier for ions & molecules and have the same
structure in all membranes. Lipids are a diverse group of large biological molecules that do not
include polymers and is made up primarily or exclusively of non-polar groups. They are grouped
together and have little or no affinity for water. Due to their non-polar character, lipids typically
dissolve more readily in non-polar solvents such as acetone, ether, and benzene etc. This
solubility characteristic is of extreme importance in cells because lipids act as barriers and form
boundaries between and within cells. The hydrophobic behavior of lipids is based on their
molecular structure. Although they have some polar bonds associated with oxygen, lipids consist
mostly of hydrocarbons. Lipids link covalently with carbohydrates to form glycolipids and with
proteins to form lipoproteins. Biological lipids originate entirely or in part from two distinct
types of biochemical subunits or "building blocks": ketoacyl and isoprene groups [19]. Lipids
can be divided into eight categories: fatty acyls, glycerolipids, phospholipids, sphingolipids,
saccharolipids and polyketides (derived from condensation of ketoacyl subunits); and sterol
lipids and prenol lipids (derived from condensation of isoprene subunits). However the major
part of the cell membrane is constituted of phospholipids.