Lipids are the fourth major group of molecules found in all the cells. Lipids are insoluble biiomolecules that can be extracted fromc ells and tissues by non-polar solvents viz. choloroform, ether, benzene etc. 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 oil seeds as a stored energy, required during germination. In animals they are stored as fat granules in specialized cells of adipole 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 important 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 eg. 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. Fatty acid with <14 and >18 carbons are uncommon. Mostly they have carbon atoms in even number, as they are synthesized in plants or in 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 ambiet temperatures.
- UFA have lower melting point than the saturated ones with the same chain length.
- The double bonds are separated by methylene group and are rarely conjugated as shown:
- Double bonds of nearly all the fatty acids are cis type rather than trans, although the cis are less stable than trans ones. The cis double bonnd makes a king of 30 degree 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 hence are tightly packed. The number of saturated fatty acids in 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 unsual acids are 18:1 (Elaidic acid)
Essential Fatty acids
Fatty acids which are required by mammals from external source/ in diet are termed as essential fatty acids. Linoleic and lemda-linolenic acid can not be synthesized by mammals and must be obtained from plant source. Linoleic acids is an essential precursor for the biosynthesis of arachidonic acid, which is not present in plants. They are precursors of prostagliandins and like horones have 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 largely of mixture of triacylglycerols (also called triglycerides). These non polar, 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 cotton seed oil, groundnut oil, mustard oil, etc are all plant lipids. The animal fat is not used in the diets of Indians but are 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 (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:
Triaclyglycerols 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 foul smell. This is caused by the presence of proteins (enzymes, lipases) in them. The presence of enzymes may be due to impurities in fat or due to microoranism growth. This normally happens 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 foul smell. In order to escape rancidity either:
- The fats are stored at low temperature
- Refined to an 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 easier. As hydrogenated fats are thought to cause atherosclerosis, in the present day world refined oils are being used for edible purpose.