Derived lipids
These are the derivatives obtained on the hydrolysis of group 1 and group 2 lipids, e.g., steroids, terpenes and prostaglandins.
(i) Steroids : Steroids are the compounds containing a cyclic steroid nucleus (or ring) namely cyclopentanoperhydro
- phenanthrene (CPPP). The steroids do not contain fatty acids, but are included in the lipids because they have fat-like properties.
They are not straight chain compounds rather they are composed of four fused carbon rings.
Sterols
Steroids with 8-10 carbon long aliphatic side chain at C-17,
e.g., cholesterol, ergasterol
Gonadal (Sex) hormones
Estradiol (oestrogen), testosterone, progesteroneAdrenocortical hormones
Corticosterone
Cardiac glycosides
They contain carbohydrate steroid complex with steroid nucleus
having unsaturated lactone ring attached to carbon 17 e.g.,
digitoxin.Bile acids
Steroid carboxylic acids derived from cholesterol, e.g., glycoholic acid,
taurocholic acidMajor Groups of SteroidsCholesterol is the most abundant steroid in the animal tissues.
It is present in food especially that is rich in animal fats. It is also
formed in the liver itself. Its formula is C 27 H 45 OH. The cholesterol
and its esters are insoluble in water. Therefore, they are deposited
on the lining of the blood vessels, leads to high blood pressure and
many heart diseases.
(ii) Terpenes : They are lipid like hydrocarbons formed of isoprene
(C 5 H 8 ) units. Terpenes are a major component of “essential oils”
produced by certain plants. They impart fragrance to plant parts.
(iii) Prostaglandins : The prostaglandins are hormone like compounds derived from the 20-carbon polyunsaturated fatty acids
such as arachidonic acid. Their richest sources are the secretion of seminal vesicles in males and menstrual secretion fluid in
the females. The prostaglandins have a variety of effects such as vasodilation, vasoconstriction stimulation of intestinal smooth
muscle, etc. They perhaps act by regulating the production of cyclic AMP by adenyl cyclase.
Useful roles of steroids
(i) Cholesterol is precursor of many steroid hormones like progesterone, estradiol, testosterone, cortisol.
(ii) Steroid hormones control sex expression, sexual cycle, allergies.
(iii) Cholesterol forms vitamin D on exposure to ultraviolet rays.
(iv) Cardiac glycosides tone up cardiac muscles and correct arrhythmia.
(v) Birth control pills have steroid hormone progesterone with or without estrogen.
Functions of Lipids
(i) Fats serve as food reserve in both plants and animals. Hibernating animals store extra fat prior to onset of winter.
(ii) Fats can be converted to carbohydrates. Therefore, fats stored in oil seeds (e.g., groundnut, mustard, castor, sunflower, cotton,
coconut) not only provide energy but also raw materials for growth of embryo.
(iii) In animals, fat occurs as droplets inside cells called adipocytes. Adipocytes of cold blooded or poikilothermic animals have higher
amount of unsaturated fatty acids as compared to warm blooded or homoeothermic animals. Fatty or adipose tissue forms an
insulating layer below the skin of animals for protection against low temperature. Whale has a very thick layer of subcutaneous
fat called blubber. Animals of colder regions also have a thick fatty layer for insulation, e.g., polar bear.
(iv) Subcutaneous fat rounds off the body contours of animals and human beings. In animals, the fats produce a shock absorbing
cushion around eye balls, gonads, kidneys and other vital organs.
(v) Waxes form a protective layer over the animal fur. They protect the floating leaves of aquatic plants against wetting. In land
plants, they reduce the rate of transpiration.
(vi) Myelin sheath around nerve fibres takes part in insulation.