Lipids are a broad group and diverse group of naturally occurring biomolecules which quite differ from other bio molecules like carbohydrates and nucleic acids.
Some common types of lipids are natural fats (triglyceride), waxes, sterols, fat-soluble vitamins like vitamins A, D, E and K), mono glycerides, diglycerides and phospholipids. These compounds are related by their solubility in non polar organic solvents like ether, chloroform, acetone & benzene and general insolubility in water.Lipids are primarily involved in the formation of cell membranes which is a bilipid layer and present in all living things like bacteria and human. Lipids also present in the form of more complex structures such as cholesterol and other biologically active molecules in living systems. Lipids act as best energy storage molecules as the cleavage of fatty acids produces both energy and metabolic water which is a main component in almost all lipids.
Other examples of lipids like waxes also contains fatty acids. Another group of lipids; sterols like cholesterol is a lipid as most of us know about. Some sterols also act as hormones like estrogens, testosterone, and progesterone, cortisol which are chemical messengers enter in cells and turn on specific chemical reactions.
The classification of lipids can be structural based or based on their functions.
Mainly lipids are classified in five types.
The relation between different types of lipids is as follows.
- Fatty acyl (FA)
- Glycerolipids (GL)
- Glycerophospholipids (GP)
- Sterol lipids (ST)
- Sphengolipids (SP)
1. Fatty acyls
- Fatty acyls, a generic term are used for fatty acids and their derivatives.
- The fatty acid composed of one hydrocarbon chain which terminates with a carboxylic acid group.
- The carboxyl end is polar and soluble in water; hydrophilic.
- The long hydrocarbon chain is non-polar in nature and hydrophobic.
- The carbon chain in fatty acids can be saturated or unsaturated and can have some other functional groups.
- In case of unsaturated fatty acids, cis-trans isomers can be possible. Cis-fatty acids are found naturally while trans can be synthesized artificially by partial hydrogenation of fats and oils.
2. Glycerolipids (triglycerides)
- They are known as triglycerides or triglycrols which composed of mono-, di- and tri-substituted glycerol.
- They formed by the esterification of glycerols with different fatty acids.
- During fat metabolism, glycerolipids releases glycerol and fatty acids from adipose tissue, thus they function as a food store comprising the bulk of storage fat in animal tissues.
- Glycosylglycerols is a class of glycerolipids which composed of one or more sugar residues with glycerol through a glycosidic linkage and fatty acids.
- The plant membranes having Digalactosyldiacylglycerols and the mammalian sperm containing seminolipid are best example of glycosylglycerols.
- They are also called phospholipids, composed of fatty acids, glycerol with phosphate groups.
- Phospholidis are the main constituents of the lipid bilayer of cells and also are involved in metabolism.
- These lipids are quite different from first two lipids and more complex compared to them.
- They composed of a sphingoid base backbone which is synthesized from serine; an amino acid and a fatty acyl CoA long-chain.
- This further converted into ceramides, glycosphingolipids, phosphosphingolipids and other compounds.
- Some common examples of sphingolipids are Ceramides, sphingomyelins( a phosphosphingolipids) are found in mammals.
- Whereas, ceramide phosphoethanolamines are found in insects and phytoceramide phosphoinositols, mannose-containing headgroups are found in fungi.
- Examples of glycosphingolipids are cerebrosides and gangliosides.
5. Sterol lipids
- These lipids are main components of membrane lipids along with sphingomyelins and the glycerophospholipids.
- The best common examples of sterol are cholesterol and its derivatives.
- Steroids are composed of fused four-ring core structure but show different biological roles such as hormones like estrogen, testosterone and androsterone.
- Some vitamins like vitamin-D also composed of one of the sterol; secosteroids.
- Other examples of sterols are bile acids in mammals, phytosterols in plants like Î²-sitosterol, brassicasterol and stigmasterol.
6. Prenol lipids
- These lipids are synthesized from the 5-carbon precursor dimethylallyl diphosphate and the isopentenyl diphosphate.
- The simple isoprenoids are classified according to the terpene units number.
- Carotenoids is a terpene act as antioxidants as well as precursors of vitamin A.
- However, if there are more than 40 carbons present in lipids, they are called as polyterpenes.
- If a tail of isoprenoid gets attached to an quinonoid core of non-isoprenoid origin, they are exemplified by the quinones and hydro quinones.
Those fatty acids which are directly linked to a sugar backbone are known as saccharolipids.
- These lipids are synthesized by the polymerization method of acetyl subunit and propionyl subunit in the presence of enzymes which share mechanistic features with the fatty acid syntheses.
- Generally, these molecules are cyclic in nature whose backbones are then further modified by hydroxylation, glycosylation, methylation, and oxidation.
- Polyketides are used as anti-microbial, anti-cancer agents and anti-parasitic.
- Some common examples of polyketides are tetracyclines, avermectins, erythromycins and anti-tumor epothilones.
Lipids are naturally occurring molecules that are all soluble in organic solvents. The group of lipids encompasses compounds of large structural diversity. There are four main groups of lipids, fatty acids, glycerides, nonglyceride lipids, and complex lipids. Other systems are based on their chemical properties. In addition, physiological lipids such as phospholipids, cholesterol and triglycerides are important as major components of lipid based control drug delivery.
Out of all lipid compounds; three types of lipids are more common.
They are also called as neutral fats. The triglycerides have ester as functional group which comprised of one glycerol molecule (1,2,3-trihydroxypropane) combined with three fatty acids molecules.
Fatty acids in triglycerides can be saturated or unsecured with a long aliphatic hydrocarbon chain and one carboxyl group. The hydrocarbon chain is hydrophobic in nature due to non-polarity, while the polar carboxyl group is hydrophilic in nature and oriented towards water in aqueous solution.
Generally, fats formed from saturated fatty acids have a high melting point and exist in solid state at room temperature. However unsaturated fats exist in a liquid state with low melting point. For example; butter is an animal fat found in solid state while vegetable oils are found in liquid state.
Triglycerides molecules are considered for efficient energy storing as more energy could be stored in a pound of fat compared to a pound of carbohydrates like glycogen. Some common triglycerides with their compositions are as follow.
This class of lipids shows many similarity with triglycerides except the fact that they are hydrophobic in nature. They are metabolic derivative of terpenes with tetracyclic skeleton of three fused six-member and one five-member ring.
The presence of different functional groups on rings makes one sterol differ from other. They are generally part of cell membranes and also found as hormones. Some other examples of steroids are sex hormones like testosterone, progesterone, estrogen and cortisol.
They are phosphate derivatives of triglycerides and is composed of a glycerol molecule with a couple of fatty acids or diglyceride and a phosphate group with some polar molecule like serine, choline or ethanolamine.
They also have one hydrophilic end with hydrophobic at another end and such type of molecules are known as amphipathic. In the aqueous solution of phospholipids; the hydrophobic end will tend to aggregate together away from water and the hydrophilic part that is polar phosphate group are oriented towards the molecules of water and tend to dissolve in it.
Hence, these appear as small spherical solids in water in which the hydrophobic "tails" aggregate together in the spherical center and with the phosphate "head" which is hydrophillic oriented towards water. Such aggregations are called as liposomes formed by phospholipids in aqueous solution. They are differing from micelle in that fact they have both aqueous interiors and exteriors. These phospholipids bilayers form spontaneously in an aqueous environment and called as membrane vesicles which are used to study membrane properties experimentally.
Lecithin is the most common example of phospholipid which is a major component in the lipid bilayers of cell membranes. Another example of phospholipid is Cephalins which composed of ehtanolamine or the amino acid serine bonded with a phosphate group through phosphate ester linkage. This phospholipid is found in cell membranes of brain tissues and involve in the blood clotting process.