Lipids are the one class of biomolecules that are hydrophobic in nature and dissolve in non polar solvents, such as chloroform and diethyl ether. Lipids are fatty acids and their derivatives. They includes fats, oil, steroidal hormones, wax and certain pigments.
Fats and oils:
Fats and oils are composed of two types of smaller molecules fatty acids and glycerols and hence are called triglycerols. Fats are
the triglycerols that are solids at room temperature (fats contain saturated fatty acids and have high melting points.),
whereas oils are liquids at room temperature. Triglycerols can either be simple triglycerols in which all three
acyl groups are the same or the mixed triglycerols in which the acyl groups are different Most naturally
occuring fatty acids have unbranched chains. These compounds can either be saturated or unsaturated. Saturated fatty acids packed
tightly into crystals and hence have high melting points. Whereas presence of double bonds in unsaturated fatty acids interferes
with crystal packing and therefore, these compound have lower melting points.
Paclitaxel is a terpene containing antineoplastic drug derived from yew plant bark. It is a mitotic
inhibitor and is used in cancer chemotherapy.
Terpenes and terpenoids:
The mixture of odoriferous compounds, known as essential oils, have been isolated from plants since antiquity. Terpenes are built
up from two or more C5 units known as isoprene units. Many terpenes have isoprene units
linked in rings. Terpenoids are oxygen–containing terpenes. These compounds have vital significance in the field of
medicine.
Steroids are the derivatives of perhydrocyclopentanophenanthrene ring system. Cholesterol is one of the
most widely occurring steroids. It is the main intermediate in the biosynthesis of all of the steroids of the human body. It is
also present in plasma membrane and maintains its fluidity.
Steroids:
This is the another important category of lipids that exerts potent physiological effects on living organisms. Male and female
sex hormones, adrenocortical hormones, vitamin D, bile acids etc. are the few examples of steroids. Testosterone is the male
hormones, secreted by testes. It promotes the development of secondary sex characteristics, i.e. the growth of facial and body
hair, muscular development and maturation of sex organs. Estrogen is the sex hormone that promotes the secondary sex
characteristics in female, such as the development of breasts and beginning of the menstrual cycle.
Like soaps and detergents, phospholipids dissolve in aqueous media by forming micelles.
Phospholipids and cell membrane:
Phospholipids are the derivatives of phosphatidic acids
and are the major component of cell membrane. They have both polar and nonpolar groups. In cell membrane they are arranged in
lipid bilayer with their polar heads interacting with the surrounding aqueous environment. The presence of both hydrophobic
and hydrophilic portion help phospholipids to maintain an interface between an organic and an aqueous environment.