Classification of lipids

Classification of lipids
Lipids
Lipids constitute a heterogeneous group of compounds of biochemical importance. Lipids may be defined as compounds which are relatively insoluble in water, but freely soluble in non-polar organic solvents, such as benzene, chloroform, ether, hot alcohol, acetone, etc.
CLASSIFICATION OF LIPIDS
Detailed classification Based on the chemical nature, lipids are classified as:
1. Simple lipids: They are esters of fatty acids with glycerol or other higher alcohols.
2. Compound lipids: They are fatty acids esterified with alcohol; but in addition they contain other groups. Depending on these extra groups, they are sub classified in
a. Phospholipids, containing phosphoric acid.
b. Non-phosphorylated lipids
3. Derived lipids: They are compounds, which are derived from lipids or precursors of lipids, e.g. fatty acids, steroids. For details of cholesterol and steroids
4. Lipids complexed to other compounds
Functions of Lipids
1.Storage form of energy (triacylglycerol)
2. Structural components of biomembranes (phospholipids and cholesterol)
3. Metabolic regulators (steroid hormones and prostaglan-dins
4.Act as surfactants, detergents and emulsifying agents amphipathic lipids)
5.Act as electric insulators in neurons
6. Provide insulation against changes in external temperature subcutaneous fat
7. Give shape and contour to the body
8. Protect internal organs by providing a cushioning effect pads of fat)
9. Help in absorption of fat soluble vitamins (A, D, E and K(
10. Improve taste and palatability of food.
FATTY ACIDS
Fatty acids, are included in the group of derived lipids. It is the most common component of lipids in the body. They are generally found in ester linkage in different classes of lipids. In the human body, free fatty acids are formed only during metabolism. Fatty acids are aliphatic carboxylic acids and have the general formula, R—CO—OH, where COOH (carboxylic group) represents the functional group. Depending on the R group (the hydrocarbon chain), the physical properties of fatty acids may vary.
SATURATED FATTY ACIDS
Swedish scientist Scheele isolated glycerol in 1779. Chevreul ME isolated oleic, stearic, butyric and caproic acids in 1823. In 1898, Edmed FG identified the structure of oleic acid. Linoleic acid was prepared from linseed oil by Sacc F in 1844. Lauric acid (12:0) was discovered in Lauraceae seeds by Marsson T in 1849. Franz Soxhlet, a German chemist, invented the Soxhlet apparatus in 1879. This apparatus was used first to separate fats from food.
i. They have the general formula CH3-(CH2)n-COOH.
For example,
Acetic acid CH3COOH
ii. They are named by adding the suffix ‘anoic after the hydrocarbon.
iii. The two carbon acetic acid and 4 carbon butyric acid are important metabolic intermediates.
iv. The C16 (palmitic acid) and C18 (stearic acid) are most abundant in body fat.
v. Each animal species will have characteristic pattern of fatty acid composition. Thus, human body fat contains 50%oleic acid, 25% palmitic acid, 10% linoleic and 5%stearic acid.
vi. The carbon atoms of fatty acids are numbered as C1, C2, etc. starting from the COOH group. Or, starting from the methyl end .
Unsaturated fatty acids
They are named by adding the suffix ‘enoic after the systematic name. They are similar to saturated fatty acids in the reaction of the carboxylic group but also show properties due to presence of the double bond. Unsaturated fatty acids exhibit geometrical isomerism at the double bonds . All the naturally occurring fatty acids have the cis configuration. However, in the body during metabolism trans fatty acids are formed
The polyunsaturated fatty acids (PUFA) exist in cis configuration in naturally occurring lipids. Clinical significance of PUFA . Many clinical and epidemiologic studies have shown positive roles for omega-3 fatty acids in infant development; cancer; cardiovascular diseases; and more recently, in various mental illnesses, including depression, attention-deficit hyperactivity disorder, and dementia. These fatty acids are known to have pleiotropic effects, including effects against inflammation, platelet aggregation, hypertension, and hyperlipidemia.
Physical Properties of Triacylglycerols
i. They are hydrophobic and insoluble in water.
ii. Oils are liquids at 20oC; they are triacylglycerols, which contain a higher proportion of unsaturated fatty acids or short chain triglycerides. Oils are generally of plant origin.
iii. Fats are solids at room temperature and contain mainly saturated long chain fatty acids. Fats are mainly of animal origin.
iv. When the constituent fatty acids have a higher chain length and are predominantly saturated, ‘hard fat is formed, e.g. pig fat.
v. Fats containing medium chain triacylglycerols or unsaturated fatty acids are soft fats, e.g. butter, coconut oil. Coconut oil contains mainly medium chain TAG, e.g. Lauric and Myristic acids.