Page 14 - 11-Bio-2 Biological Molecules

2. Biological Molecules eLearn.Punjab
LIPIDS

The lipids are a heterogenous group of compounds related to fatty acids. They are insoluble in water
but soluble in organic solvents such as ether, alcohol, chloroform and benzene. Lipids include fats,
oils, waxes, cholesterol, and related compounds.

Lipids as hydrophobic compounds, are components of cellular membranes. Lipids are also used to
store energy. Because of higher proportion of C-H bonds and very low proportion of oxygen, lipids
store double the amount of energy as compared to the same amount of any carbohydrate. Some
lipids provide insulation against atmospheric heat and cold and also act as water proof material.
Waxes, in the exoskeleton of insects, and cutin, an additional protective layer on the cuticle of
epidermis of some plant organs e.g. leaves, fruits, seeds etc., are some of the main examples.

Lipids have been classified as acylglycerols, waxes, phospholipids, sphingolipids, glycolipids and
terpenoid lipids including carotenoids and steroids. The structure of some of these lipids is given
below.

Acylglycerols

Acylglycerols are composed of glycerol and fatty acids (Fig. 2.7). The most widely spread acyl glycerol
is triacyl glycerol, also called triglycerides or neutral lipids. Chemically, acylglycerols can be defined
as esters of fatty acids and alcohol. An ester is the compound produced as the result of a chemical
reaction of an alcohol with an acid and a water molecule is released as shown below:

C2H5OH + HOOCCH3 8 C2H5OCOCH3 + H20

alcohol acetic acid an ester

As indicated by dotted squares, OH is released from alcohol and H from an acid.
H and OH combine and form a water molecule. Fatty acids are one of the most important components
of triglycerides.

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