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Lipids
Lipids are water-insoluble biomolecules that are highly soluble in organic solvents like chloroform.
Lipids are a heterogeneous group of compounds related to fatty acids and include fats, oils, waxes, and other related substances.
Lipids are hydrophobic in nature.
The term ‘lipid’ was first coined by a German biochemist, Bioor, in 1943.
Chemically fats are defined as the esters of glycerol and fatty acids or triglycerides of fatty acids.
Functions of lipids
A diverse range of functions in biological systems:
Important constituents of diet because of their high energy value
Serve as a source of fat-soluble vitamins and essential fatty acids in natural foodstuffs.
Serve as a stored form of energy in adipose tissues.
Serve as an insulating material in subcutaneous tissues and around certain organs.
Lipoproteins are important constituents of biological membranes.
Plays crucial roles as:
enzyme cofactors,electron carriers,light absorbing pigments,emulsifying agent in digestive tracts,hormones and intracellular messengers
May act as chaperones to help membrane proteins fold.
Advantage of energy storage as fatty acids
The carbons in the fatty acids (mostly –CH₂ group) is almost completely reduced compared to the carbon in other simple biomolecules (sugars, amino acids, etc.). Therefore, oxidation of fatty acids will yield more energy (in form of ATP) than any other form of carbon.
Fatty acids are not hydrated as mono- and polysaccharides and thus can be packed more closely in storage tissues.
Fatty acids
Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with the carboxyl group.
Fatty acids that occur in natural fats are usually monocarboxylic.
They contain an even number of carbon atoms as they are synthesized from condensation of 2 carbon units.
These range from 4 to 36 C-atoms.
The chain may be saturated (no double bonds) or unsaturated (contain one or more double bonds).
Some fatty acids may have hydroxyl groups in the chain (hydroxy-fatty acid) and still, others may possess ring structures (cyclic-fatty acids).
Alcohols in lipid molecules
Alcohols present in lipid molecules commonly include glycerol, cholesterol, and higher alcohols like cetyl alcohol and mericyl alcohol.
In the structural formula of glycerol, the C-atoms are numbered 1, 2, 3 from any end. Since C1 and C3 are identical, they are also denoted as α, β, and α’.
Nomenclature of fatty acids
The systematic nomenclature of fatty acids is based on the Genevan System under the instruction of IUPAC.
The fatty acid is named after the hydrocarbon with the same number of carbon atoms, the suffix –oic is written in place of the final letter ‘-e’ in the name of the hydrocarbon.
The names of the saturated fatty acids end with the suffix –anoic and those of unsaturated fatty acids with the suffix –enoic.
The positions of the carbon atoms are denoted by numbering (carboxyl carbon atom is C1, the adjacent carbon atom is C2 and so on) or by using Greek letters (C2 is denoted as α-carbon, C3 is denoted as β-carbon and so on).
A widely used convention is to indicate the carbon atom number followed by the number and position of double bonds in the case of unsaturated fatty acids (position of a double bond is indicated by a lower number of the two carbon atoms involved in double bonding).
In nearly all naturally occurring unsaturated fatty acids, the double bonds are in the cis configuration.
Trans fatty acids are produced by fermentation in the rumen of dairy animals and are obtained from dairy products and meat.
They are also produced during hydrogenation of fish or vegetable oils.
Diets high in trans fatty acids correlate with increased blood levels of LDL (bad cholesterol) and decreased HDL (good cholesterol).
French fries, doughnuts, fast foods, and cookies tend to be high in trans fatty acids.
Lipid classification
Bioor (1943) classified lipids on the basis of their chemical composition:
Simple lipids or homolipidsCompound lipids or heterolipidsDerived lipids
Conn and Stumpf (1976) traditionally classified lipids into the following categories:
Acyl glycerolsWaxesPhospholipidsSphingolipidsGlycolipidsTerpenoid lipids including carotenoids and steroids
Lipid types (on the basis of function)
Physical properties of lipids
State: Saturated fatty acids are solid at room temperature while unsaturated fatty acids are liquid, in general.
Colour, odour and taste: Pure fats are colourless, odourless and bland taste.
Solubility: Soluble in organic solvent. Longer the fatty acyl chain and fewer the double bonds, the lower is its solubility in water.
Specific gravity: Less than 1
Geometric isomerism: Present
Emulsification: Found
Melting point: Depends on the chain length of the constituent fatty acyl chain and the degree of unsaturation.
Chemical properties of lipids
Hydrolysis: Fats are hydrolyzed by enzyme lipase to yield fatty acids and glycerol.
Saponification: Hydrolysis of fats by alkali is called saponification which results in the formation of glycerol and salts of fatty acids (called soaps).
Hydrogenation: Unsaturated fatty acids react with gaseous hydrogen to yield saturated fatty acids.
Halogenation: Unsaturated fatty acids and their esters can take up halogens (Br2, I2, etc.) at their double bonds at room temperature in acetic acid or methanol solution. This reaction is the basis of ‘iodine number determination’.
Oxidation: Unsaturated fatty acids are susceptible to oxidation at their double bonds.
Acid number: It is the number of milligrams of KOH required to neutralize the free fatty acids present in 1 gram of fat. The acid number indicates the quantity of free fatty acid present in fat.
Saponification number: It is the number of milligrams of KOH required to saponify 1 gram of fat. The saponification number provides information on the average chain length of the fatty acids in fats.
Iodine number: It is the number of grams of iodine absorbed by 100 grams of fat. The iodine number indicates the degree of unsaturation of the fatty acids present in fat.
Acetyl number: It is the number of milligrams of KOH required to neutralize the acetic acid obtained by saponification of 1 gram of fat after it has been acetylated. The acetyl number indicates the number of –OH groups present in fat.
Rancidity: When lipid-rich foods are exposed for too long in the air (oxygen), they may spoil and become rancid. The unpleasant taste and smell associated with rancidity results from oxidative cleavage of the double bonds in unsaturated fatty acids, producing aldehydes and carboxylic acids of shorter chain length which are volatile.
Essential fatty acids
Our body can synthesize most of the fatty acids except a few. These fatty acids must be supplied through diet and are termed essential fatty acids.
Example-α-Linolenic acid (ω-3 fatty acid), Linoleic acid (ω-6 fatty acid)
It is recommended that essential fatty acids make up 3% to 6% of your daily caloric intake. Of this percentage, you should consume 2 to 4 times more omega-6 fatty acids than omega-3 fatty acids.
Omega-3 sources include:
NutsSoybeansWalnut oilCanola oilFlaxseed oilCold water fatty fish such as salmon, herring, cod, flounder, tuna, bluefish, and shrimp
Omega-6 sources include:
Leafy vegetablesSeedsNutsGrainsVegetable oils (corn, safflower, soybean, cottonseed, sesame, sunflower)
Roles of Essential fatty acids
- Help with cellular development and the formation of healthy cell membranes.
- Blocks tumor formation and growth of cancer cells
- Assist in the development and function of the brain and nervous system.
- Regulates proper thyroid and adrenal activity.
- Plays a role in thinning your blood, which prevents blood clots, heart attacks and stroke.
- Regulates blood pressure, immune responses and liver function.
- Deficiency causes skin problems, including eczema, dandruff, split nails and brittle hair.
- Forms Lipid rafts which affects cellular signalling.
- Acts on DNA, activates or inhibits transcription factors.
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