Q.1- Define and classify lipids.
Answer- The lipids are a heterogeneous group of compounds, including fats, oils, steroids, waxes, and related compounds, that are related more by their physical than by their chemical properties. They have the common property of being (1) relatively insoluble in water and (2) soluble in nonpolar solvents such as ether and chloroform.
Classification- The lipids are classified as –
1. Simple lipids: Esters of fatty acids with various alcohols.
a) Fats: Esters of fatty acids with glycerol. Oils are fats in the liquid state.
b) Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols
2. Complex lipids: Esters of fatty acids containing groups in addition to an alcohol and a fatty acid
a. Phospholipids: Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue. They frequently have nitrogen-containing bases and other substituents, eg, in glycerophospholipids the alcohol is glycerol and in Sphingophospholipid the alcohol is sphingosine.
b. Glycolipids (glyco sphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate.
c. Other complex lipids: Lipids such as sulfolipids and amino lipids. Lipoproteins may also be placed in this category.
3. Precursor and derived lipids: These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid-soluble vitamins, and hormones.
Q.2- Give a brief account of the nomenclature and the biological importance of fatty acids.
Answer- Fatty acids are the key constituents of lipids. Chemically they are aliphatic carboxylic acids. Fatty acids occur mainly as esters in natural fats and oils but do occur in the unesterified form as free fatty acids, a transport form found in the plasma. Fatty acids that occur in natural fats are usually straight-chain derivatives containing an even number of carbon atoms. The chain may be saturated (containing no double bonds) or unsaturated (containing one or more double bonds).
Nomenclature- Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with carboxylic acid groups. The systematic name for a fatty acid is derived from the name of its parent hydrocarbon by the substitution of oic for the final e. For example, the C18 saturated fatty acid is called octadecanoic acid because the parent hydrocarbon is octadecane. A C18 fatty acid with one double bond is called octadecenoic acid; with two double bonds, octadecadienoic acid; and with three double bonds, octadecatrienoic acid. The notation 18:0 denotes a C18 fatty acid with no double bonds, whereas 18:2 signifies that there are two double bonds.
Carbon atoms are numbered from the carboxyl carbon (carbon No. 1). The carbon atoms adjacent to the carboxyl carbon (Nos. 2, 3, and 4) are also known as the alpha, beta, and gamma carbons, respectively, and the terminal methyl carbon is known as the ώ or n-carbon.(Figure-1)
The position of a double bond is represented by the symbol ∆ followed by a superscript number.
eg. ∆ 9 indicates a double bond between carbons 9 and 10 of the fatty acid;
For example, cis-∆9 means that there is a cis double bond between carbon atoms 9 and 10; trans-∆2 means that there is a trans double bond between carbon atoms 2 and 3. Alternatively, the position of a double bond can be denoted by counting from the distal end, with the ώ-carbon atom (the methyl carbon) as number 1.
ώ 9 indicates a double bond on the ninth carbon counting from the ώ-carbon.
Figure-1-(a) showing the ionized structure of a fatty acid and the numbering of carbon atoms in a fatty acid
(b) showing the structure of ώ3 fatty acid
In animals, additional double bonds are introduced only between the existing double bond (eg, 9, 6, or 3) and the carboxyl carbon, leading to three series of fatty acids known as the ώ9, ώ6, andώ3 families, respectively.
Examples of fatty acids-
1) Saturated fatty acids- Saturated fatty acids may be envisaged as based on acetic acid (CH3 —COOH) as the first member of the series in which —CH2 — is progressively added between the terminal CH3 — and —COOH groups. Other higher members of the series are known to occur, particularly in waxes. Fatty acids in biological systems usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon fatty acids are most common. The hydrocarbon chain is almost invariably unbranched in animal fatty acids. A few branched-chain fatty acids have also been isolated from both plant and animal sources. Some of the saturated fatty acids of biological significance are as follows-
|S.No.||Number of carbon atoms||Common Name||Systematic Name||Formula|
|1||2||Acetic acid||Ethanoic acid||CH3COOH|
|2||4||Butyric acid||Butanoic acid||CH3(CH2)2COOH|
|3||6||Caproic acid||Hexanoic acid||CH3(CH2)4COOH|
|4||8||Caprylic acid||Octanoic acid||CH3(CH2)6COOH|
|5||10||Capric acid||Decanoic acid||CH3(CH2)8COOH|
|6||12||Lauric acid||Dodecanoic acid||CH3(CH2)10COOH|
|7||14||Myristic acid||Tetradecanoic acid||CH3(CH2)12COOH|
|8||16||Palmitic acid||Hexadecanoic acid||CH3(CH2)14COOH|
|9||18||Stearic acid||Octadecanoic acid||CH3(CH2)16COOH|
|10||20||Arachidic acid||Eicosanoic acid||CH3(CH2)18COOH|
|12||22||Behenic acid||Docosanoic acid||CH3(CH2)20COOH|
Palmitic acid is present both in plant and animal fat, while butyric acid is abundantly present in butter.
2) Unsaturated fatty acids
The alkyl chain of a fatty acid may contain one or more double bonds. The configuration of the double bonds in most unsaturated fatty acids is cis. The double bonds in polyunsaturated fatty acids are separated by at least one methylene group.
Unsaturated fatty acids may further be divided as follows-
(1) Monounsaturated (monoethenoid, monoenoic) acids, containing one double bond.
(2) Polyunsaturated (polyethenoid, polyenoic) acids, containing two or more double bonds.
|S.No.||Number of carbon atomsNumber and Location of double bond||Family||Common Name||Systematic Name||Formula|
|(A)||Monoenoic acids (one double bond)|
|9||Elaidic acid||trans 9- Octadecanoic||CH3(CH2)7CH=CH(CH2)7COOH|
|(B)||Dienoic acids (two double bonds)|
|1||18:2;9,12||6||Linoleic||all-cis-9,12-Octadecadienoic||CH3(CH2)4 CH=CH CH2CH=CH(CH2)7COOH|
|(C)||Trienoic acids (three double bonds)|
|1||18:3;6,9,12||6||Gamma-Linolenic||all-cis-6,9,12-Octadecatrienoic||CH3(CH2)4 CH=CHCH2 CH=CHCH2CH=CH (CH2)4COOH|
|2||18:3;9,12,15||3||Alpha-Linolenic||all-cis-9,12,15Octadecatrienoic||CH3(CH2) CH=CH CH2CH=CH CH2 CH=CH(CH2)7COOH|
|(D)||Tetraenoic acids (four double bonds)|
|20:4;5,8,11,14||6||Arachidonic||all-cis-5,8,11,14-Eicosatetraenoic||CH3(CH2)3CH2CH=CH CH2CH=CH CH2 CH=CH CH2CH=CH(CH2)3COOH|
|(E)||Pentaenoic acids (five double bonds)|
|20:5;5,8,11,14,17||3||Timnodonic||all-cis-5,8,11,14,17-Eicosapentaenoic||CH3 CH2 CH=CH CH2CH=CH CH2 CH=CH CH2 CH=CH CH2CH=CH(CH2)3COOH|
|(F)||Hexaenoic acids (six double bonds)|
|22:6;4,7,10,13,16,19||3||Cervonic acid||all-cis-4,7,10,13,16,19-Docosahexaenoic||CH3 CH2 CH=CH CH2 CH=CH CH2CH=CH CH2 CH=CH CH2 CH=CH CH2CH=CH(CH2)2COOH|
Biological Importance of fatty acids-
1-Fatty acids are the building blocks of dietary fats. The human body stores such fats in the form of triglycerides.
2)- Fatty acids are also required for the formation of membrane lipids such as phospholipids and glycolipids.
3) -They are required for the esterificaton of cholesterol to form cholesteryl esters.
4) They act as fuel molecules and are oxidized to produce energy.
Q.3-What are Essential fatty acids ? Discuss the significance of essential fatty acids.
Answer-Essential fatty acids-
Although animals and humans are not able to produce them naturally, some polyunsaturated fatty acids such as Linoleic and Linolenic acids are essential for normal life functions. They are therefore characterized as essential fatty acids. Arachidonic acid is considered as semi essential fatty acid since it can be synthesized from Linoleic acid .
Polyunsaturated fatty acids are produced by various plants. They reach man through the food chain, either directly through the consumption of fruit and vegetables, or by eating the flesh or eggs of animals, birds or fish that have eaten plants containing the polyunsaturated fatty acids.
Essential polyunsaturated fatty acids can be classified as belonging to one of two “families”, the omega-6 family or the omega-3 family. Fatty acids belonging to these two families differ not only in their chemistry, but also in their natural occurrence and biological functions. Omega-6 oils are found in cooking oils such as corn oil and soy bean oil. Omega-3, on the other hand, is heavily represented in the marine food chain. The most important source is fat fish.
Significance of essential fatty acids
1) Components of cell membranes- Deficiencies of essential polyunsaturated fatty acids may cause a wide variety of symptoms, including retarded growth in children, reduced fertility and pathologic changes in the skin. The reason behind these apparently unrelated symptoms is the central role that polyunsaturated fatty acids play in the composition and structure of the cell membrane, which is very important for maintaining normal cell functions.
2) Precursors of Eicosanoids- Another important function of polyunsaturated fatty acids containing 20 carbon atoms (C20) and particularly Arachidonic acid (omega-6) and eicosapentaenoic acid (omega-3) is that they can be converted to locally functioning transmitter substances like prostaglandins and leukotrienes that are important for biological processes such as blood clotting, inflammatory reactions and muscle contractions.
3) Brain growth- Another important function of polyunsaturated fatty acids is that they are vital components of brain tissue and other nerves. The omega-3 fatty acid, docosahexaenoic acid (DHA), is particularly important. A normal adult brain contains more than 20 grams of DHA.
4) Role in vision-DHA also plays an important role in the composition of the retina of the eye. It is therefore also of major importance for vision. Healthy adults have a certain ability to metabolize alfa-Linolenic acid to EPA and DHA. Children do not have this ability and are entirely dependent on receiving these essential elements through diet. The ability to metabolize EPA and DHA may also be reduced in elderly people.
5) Cardioprotective role- Marine omega-3 fatty acids lower serum triglyceride levels, reduces blood pressure and stabilizes the rhythm of the heart. They are off use both as a prophylaxis and as an adjunct treatment for cardiovascular diseases.
6) Rheumatoid Arthritis-There is also growing appreciation for clinical documentation on the reduction of symptoms in patients suffering from rheumatoid arthritis.
7) Dementia and Depression- A high dietary intake of marine omega-3 has been linked to a delay in the development of senile dementia and possibly to reducing symptoms that have already manifested themselves. There have also been reports of positive effects from the dietary intake of omega-3 fatty acids in patients suffering from depression. Dietary marine omega-3 has also been associated to delay the development of cerebral dementia in elderly people.
8) Other diseases
Recent scientific publications have reported positive effects on a number of clinical conditions, including migraine, heart arrhythmia, mental cognition in adults and attention deficit/ hyperactivity disorder in children.
Q.4- What is the relationship between melting point and
(i) degree of unsaturation and
(ii) the hydrophobic chain length of fatty acids?
Answer- Melting point is affected by the chain length and the degree of unsaturation.
1) Degree of unsaturation-Unsaturated fatty acids have lower melting points than saturated fatty acids of the same length. For example, the melting point of Stearic acid (C18- saturated) is 69.6°C, whereas that of oleic acid (which contains one cis double bond) is 13.4°C.The melting points of polyunsaturated fatty acids of the C18 series are even lower.
2) Chain length-Chain length also affects the melting point, as illustrated by the fact that the melting temperature of Palmitic acid (C16) is 6.5 degrees lower than that of Stearic acid (C18). Thus, short chain length and unsaturation enhance the fluidity of fatty acids and of their derivatives.
The hydrocarbon chains in saturated fatty acids are, fairly straight and can pack closely together, making these fats solid at room temperature. (Figure-2) Oils, mostly from plant sources, have some double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks” in the shape of the molecules.
A type of geometric isomerism occurs in unsaturated fatty acids, depending on the orientation of atoms or groups around the axes of double bonds, which do not allow rotation. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid; if on opposite sides, it is trans-, as in Elaidic acid, the trans isomer of oleic acid (Figure-2). Naturally occurring unsaturated long-chain fatty acids are nearly all of the cis configuration, the molecules being “bent” 120 degrees at the double bond. Thus, oleic acid has an L shape, whereas Elaidic acid remains “straight.” Increase in the number of cis double bonds in a fatty acid leads to a variety of possible spatial configurations of the molecule—eg, Arachidonic acid, with four cis double bonds, has “kinks” or a U shape.
This has profound significance for molecular packing in membranes and on the positions occupied by fatty acids in more complex molecules such as phospholipids. Because of the kinks in the hydrocarbon tails, unsaturated fats can’t pack as closely together, making them liquid at room temperature.
The membrane lipids, which must be fluid at all environmental temperatures, are more unsaturated than storage lipids. Lipids in tissues that are subject to cooling, eg, in hibernators or in the extremities of animals, are more unsaturated. The carbon chains of saturated fatty acids form a zigzag pattern when extended, as at low temperatures. At higher temperatures, some bonds rotate, causing chain shortening, which explains why biomembranes become thinner with increases in temperature.
Trans fatty acids are present in certain foods, arising as a by-product of the saturation of fatty acids during hydrogenation, or “hardening,” of natural oils in the manufacture of margarine. An additional small contribution comes from the ingestion of ruminant fat that contains trans fatty acids arising from the action of micro-organisms in the rumen.
Naturally-occurring unsaturated vegetable oils have almost all cis bonds, but using oil for frying causes some of the cis bonds to convert to trans bonds. If oil is used only once only a few of the bonds do this so it’s not too bad. However, if oil is constantly reused, more and more of the cis bonds are changed to trans until significant numbers of fatty acids with trans bonds build up. The reason this is of concern is that fatty acids with trans bonds are carcinogenic. The levels of trans fatty acids in highly-processed, lipid-containing products such as margarine are quite high,
Figure-2- showing the zigzag pattern of extended fatty acids. The cis double bond produces a kink in the chain as shown above.
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