Lipids are the fourth major group of molecules found in all the cells. Lipids are insoluble biomolecules that can be extracted from cells and tissues by non-polar solvents viz. chloroform ether, benzene. Unlike nucleic acids, proteins and polysaccharides, lipids are not polymeric. However, they aggregate, and it is in this state they perform their most important function as the structural matrix of biological membranes. They occur in oilseeds as a stored energy, required during germination. In animals, they are stored as fat granules in specialized cells of adipose tissues. Lipids have several biological functions:
- Structural component of membranes (cell or organelles membranes)
- As storage and transport form of metabolic fuel
- As protective coating on surfaces of many organisms (leaf top and insects)
- As cell surface component concerned in cell recognition, species specificity, and tissue immunity.
- As insulation subcutaneously in some of the warm-blooded animals required during winter seasons, also engulfing some of the essential organs as protecting against any external injury.
- Some have intensive biological activity viz. vitamins and hormones.
Classification of lipids with examples
The most satisfactory classification divides lipids into two classes. Complex lipids (saponifiable lipids) yield soaps (sodium salt of fatty acid) on alkali hydrolysis. They include acylglycerol, phospholipids, sphingolipids, and waxes. They differ in their backbone structure to which the fatty acids are covalently joined. Simple lipids (non-saponifiable lipids) do not contain fatty acids and hence are nonsaponifiable, e.g. terpenoids, steroids, and prostaglandins.
Complex Lipids Backbone
Waxes Non-polar alcohols of high mol. wt.
Fatty acids are carboxylic acids with long-chain hydrocarbon as the side group. They are mostly in esterified form; traces occur in free form in cell/tissues. Over 100 different fatty acids have been identified in nature in various plants and animals and microorganisms. The predominant fatty acid occurring in animals and plants are of C16 and C18 family, i.e., palmitic, oleic, linoleic and stearic acids. A fatty acid with <14 and >18 carbons are uncommon. Mostly they have carbon atoms in the even number, as they are synthesized in plants or animals by the catenation of C2 units. Over half of the fatty acids residues of plants and animals, lipids are unsaturated (contains double bond) and are often polyunsaturated (contains more than one double bond). Bacterial fatty acids are rarely polyunsaturated but are commonly branched, hydroxylated or contain cyclopropane ring.
Some generalization of fatty acids:
- The most abundant fatty acids are even numbered than odd numbered
- Chain length between C14 and C18 is predominant
- The most common saturated fatty acid is palmitic and stearic, whereas unsaturated is oleic.
- Unsaturated fatty acids (UFA) predominate over the saturated ones in plants and animals whose habitat is at lower ambient temperatures.
- UFA has the lower melting point than the saturated ones with the same chain length.
- The methylene group separates the double bonds and are rarely conjugated as shown:
- Double bonds of nearly all the fatty acids are cis type rather than trans, although the cis is less stable than trans ones. The cis double bond makes a king of 30 degrees and hence are not tightly packed. The saturated fatty acid has free rotation around their carbon-carbon bond gives the hydrocarbon chain great flexibility and thus are tightly packed. The number of saturated fatty acids in the same volume are more in number than the unsaturated ones as shown. This is due to kinks in the unsaturated fatty acids.
Also Read:- Hormones and their categories
- Triple bond rarely occur fatty acids
- Some unusual acids are 18:1 (Elaidic acid)
Essential Fatty acids
Fatty acids which are required by mammals from the external source/ in the diet are termed as essential fatty acids. Linoleic and lemma-linolenic acid cannot be synthesized by mammals and must be obtained from the plant source. Linoleic acid is a necessary precursor for the biosynthesis of arachidonic acid, which is not present in plants. They are precursors of prostaglandins and like hormones have the profound effect on the physiological activity. Linoleic acid makes 10-20% of total fatty acid of the triacylglycerol and phospholipids.
The fat and oils that occur in plants and animals consist primarily of the mixture of triacylglycerols (also called triglycerides). These nonpolar, water-insoluble substances are fatty acid triester of polyol, i.e., glycerol. Triacylglycerols that are solids at room temperature are termed as fats, whereas liquids are termed as oils. Diacylglycerol and cottonseed oil, groundnut oil, mustard oil, etc. are all plant lipids. The animal fat is not used in the diets of Indians but is used for making soaps.
Glycerol molecule itself is not having any asymmetric carbon atom, and hence their ambiguity in naming it as shown below
When one of the primary alcohol groups is modified, the molecule becomes asymmetric, i.e., it can be written as L-glycerol-3-phosphate or D-glycerol 1-phosphate. Hence IUB (The International Union of Biochemists) have recommended that glycerol written in Fisher’s formula with the hydroxyl group on the central carbon if placed on the left, the top carbon is numbered as one and the bottom carbon as number three. This system of numbering is stereospecific numbering (Sn), and this prefix is used in naming asymmetric glycerol derivatives.
Hydrolysis and Saponification:
Triacylglycerols are split into glycerol and fatty acids by enzymes (lipases) and by alkali as shown below
This process of breaking ester linkage between fatty acid and glycerol by the addition of three water molecules is termed as hydrolysis, whereas splitting triglycerides by alkali is called saponification. The product formed is glycerol and sodium salt of fatty acids (soaps).
When butter or fats are stored, they often become rancid, give the foul smell. This is caused by the presence of proteins (enzymes, lipases) in them. The presence of catalysts may be due to impurities in fat or due to microorganism growth. This happens typically when oils are not well purified and are devoid of water in them. In this process low mol. Wt (4-10 carbon) aldehydes are released, which give the foul smell. To escape rancidity either:
- The fats are stored at low temperature
- Refined to the extent of purest form (with no water and proteins)
- Solidified by hydrogenation
Hydrogenation changes the chemical structure of fatty acid present in fats. The process not only helps in removing rancidification but also makes transport more accessible. As hydrogenated fats are thought to cause atherosclerosis, in the present day world refined oils are being used for a useful purpose.