Lipids PDF

LIPIDS  The lipids constitute a very important heterogenous group of organic substances in plant and animal tissues, and related either actually or potentially to the fatty acids. Chemically they are various types of esters of different alcohols Lipids are a diverse group of organic molecu...

LIPIDS  The lipids constitute a very important heterogenous group of organic substances in plant and animal tissues, and related either actually or potentially to the fatty acids. Chemically they are various types of esters of different alcohols Lipids are a diverse group of organic molecules that are primarily hydrophobic (water-repelling) or amphipathic (having both hydrophobic and hydrophilic parts). They are an essential component of all living cells and play key roles in energy storage, structural integrity, and signaling. Lipids including fats, oils, steroids, waxes They have the common property of being (1) relatively insoluble in water and (2) soluble in nonpolar solvents such as ether and chloroform.  They are important dietary constituents not only because of their high energy value but also be cause of the fat-soluble vitamins and the essential fatty acids contained in the fat of natural foods One gram of fat supplies about 9 kcal, which is more than double the energy provided by carbohydrates or proteins (both of which supply 4 kcal per gram) Combinations of lipid and protein (lipoproteins) are important cellular constituents, occurring both in the cell membrane and in the mitochondria, and serving also as the means of transporting lipids in the blood. LIPIDS ARE CLASSIFIED AS SIMPLE OR COMPLEX 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. (alcohols containing a single hydroxyl group). Like fats (triglycerides), waxes are formed by an esterification reaction, but the key difference lies in the types of alcohols and fatty acids involved. (Glycerol (a three-carbon alcohol) bonds with fatty acids (long chains of hydrocarbons with a carboxyl group at one end) through a dehydration reaction (water is removed in the process). The resulting compound is called a triglyceride (or triacylglycerol), and it is the most common type of fat found in both food and the human body)  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, e. g, in glycerophospholipids the alcohol is glycerol and in sphingophospholipids the alcohol is sphingosine. b. Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate  c. Other complex lipids: Lipids such as sulfolipids and aminolipids. Lipoproteins may also be placed in this category. 3. Derived lipids: Derivatives obtained by hydrolysis of those given in group A and B, and C which still possess the general characteristics of lipids. These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid-soluble vitamins, and hormones. NEUTRAL LIPIDS : The lipids which are uncharged are referred to as neutral lipids. These are mono-, di-, and triacylglycerols, cholesterol and cholesteryl esters. Miscellaneous lipids : These include a large number of compounds possessing the characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane (in bees wax), terpenes etc. FATTY ACIDS ARE ALIPHATIC CARBOXYLIC ACIDS Fatty acids all contain a carboxyl group (–COOH) at one end of the molecule, which is the defining characteristic of carboxylic acids. The carboxyl group consists of a carbonyl group (C=O) attached to a hydroxyl group (–OH). This carboxyl group is polar and can easily donate a hydrogen ion (H⁺), giving fatty acids their acidic nature. The other part of the fatty acid is a long hydrocarbon chain (a chain of carbon atoms bonded to hydrogen atoms), which is typically non-polar and hydrophobic (water-fearing). The aliphatic nature of fatty acids means that their carbon chain is open and not aromatic (i.e., it does not contain a ring structure like benzene). The chain can be either saturated (with single bonds between the carbon atoms) or unsaturated (with one or more double bonds between the carbon atoms).  FATTY ACID Fatty acids are carboxylic acids with long-chain of hydrocarbon. They usually occur in esterified form as major components of the various lipids. The Properties of Fatty Acids Depend on Their Hydrocarbon Chains In higher plants and animals, the predominant fatty acid residues are those of the C16 and C18 species: palmitic, oleic, linoleic, and stearic acids. Fatty acids with 20 carbon atoms are uncommon. The more common biological fatty acids are listed in Table.  Fatty Acids Are Named After Corresponding Hydrocarbons The most frequently used systematic nomenclature names the fatty acid after the hydrocarbon with the same number and arrangement of carbon atoms, with -oic being substituted for the final -e. For example: Hexane (a six-carbon alkane) becomes hexanoic acid (a six-carbon fatty acid). Octane (an eight-carbon alkane) becomes octanoic acid (an eight-carbon fatty acid). Decane (a ten-carbon alkane) becomes decanoic acid (a ten-carbon fatty acid). Thus, saturated acids end in -anoic, eg, octanoic acid, and unsaturated acids with double bonds end in -enoic, eg, octadecenoic acid (oleic acid). Carbon atoms are numbered from the carboxyl car bon (carbon No. 1). The carbon atoms adjacent to the carboxyl carbon (Nos. 2, 3, and 4) are also known as the α, β, and γ carbons, respectively, and the terminal methyl carbon is known as the ω or n- carbon.  Various conventions use ∆ for indicating the number and position of the double bonds (Figure ); eg, ∆9 indicates a double bond between carbons 9 and 10 of the fatty acid; ω9 indicates a double bond on the ninth carbon counting from the ω- carbon Saturated Fatty Acids Contain No Double Bonds Saturated fatty acids (which are fully reduced or “saturated” with hydrogen) are highly flexible molecules that can assume a wide range of conformations because there is relatively free rotation around each of their C—C bonds Unsaturated Fatty Acids Unsaturated Fatty Acids Contain One or More Double Bonds Fatty acids may be further subdivided as follows: (1) Monounsaturated acids, containing one double bond. (2) Polyunsaturated acids, containing two or more double bonds. (3) Eicosanoids: These compounds, derived from eicosa- (20-carbon) polyenoic fatty acids, comprise the prostanoids, leukotrienes (LTs), and lipoxins (LXs). Prostanoids include prostaglandins (PGs), prostacyclins (PGIs), and thromboxanes (TXs).  Prostaglandins exist in virtually every mammalian tissue, acting as local hormones;. A related series of compounds, the thromboxanes, have the cyclopentane ring interrupted with an oxygen atom (oxane ring). The leukotrienes and lipoxins are a third group of eicosanoid derivatives. They are characterized by the presence of three or four conjugated double bonds, respectively. Leukotrienes cause bronchoconstriction as well as being potent proinflammatory agents and play a part in asthma.  Triacylglycerols Triacylglycerols Contain Three Esterified Fatty Acids. The fats and oils that occur in plants and animals consist largely of mixtures of triacylglycerols (also called triglycerides). These nonpolar, water-insoluble substances are fatty acid triesters of glycerol (at left). Triacylglycerols function as energy reservoirs in animals and are therefore their most abundant class of lipids even though they are not components of cellular membranes.  Triacylglycerols differ according to the identity and placement of their three fatty acid residues. Most triacylglycerols contain two or three different types of fatty acid residues and are named according to their placement on the glycerol moiety, for example, 1-palmitoleoyl-2- linoleoyl-3-stearoylglycerol (at left). Note that the -ate ending of the name of the fatty acid becomes -oyl in the fatty acid ester.  Fats and oils (which differ only in that fats are solid and oils are liquid at room temperature) are complex mixtures of triacylglycerols whose fatty acid compositions vary with the organism that produced them. Plant oils are usually richer in unsaturated fatty acid residues than animal fats, as the lower melting points of oils. Triacylglycerols Function as Energy Reserves Fats are a highly efficient form in which to store metabolic energy. Triacylglycerols, as compared to carbohydrates or proteins yield significantly more energy per unit mass on complete oxidation Furthermore, triacylglycerols, which are nonpolar, are stored in anhydrous form, whereas glycogen, for example, binds about twice its weight of water under physiological conditions. Fats therefore provide about six times the metabolic energy of an equal weight of hydrated glycogen  1. Saturated fat: Triacylglycerols composed primarily of fatty acids whose hydrocarbon chains do not contain any double bonds are referred to as saturated fats. Consumption of saturated fats is positively associated with high levels of total plasma cholesterol and LDL cholesterol, and an increased risk of CHD. The main sources of saturated fatty acids are dairy and meat products and some vegetable oils, such as coconut and palm oils  Monounsaturated fats: Triacylglycerols containing primarily fatty acids with one double bond are referred to as monounsaturated fat. Unsaturated fatty acids are generally derived from vegetables and fish. When substituted for saturated fatty acids in the diet, monounsaturated fats lower both total plasma cholesterol and LDL cholesterol, but maintain or increase HDL cholesterol. Polyunsaturated fats Triacylglycerols containing primarily fatty acids with more than one double bond are referred to as polyunsaturated fats. The effects of polyunsaturated fatty acids (PUFAs) on cardiovascular disease is influenced by the location of the double bonds within the molecule.  ω-6 Fatty acids These are long-chain, PUFAs, with the first double bond beginning at the sixth bond position when starting from the methyl end of the fatty acid molecule ω-3 Fatty acids These are long-chain, PUFAs, with the first double bond beginning at the third bond position from the methyl end.  Consumption of fats containing ω-6 PUFAs, principally linoleic acid, obtained from vegetable oils, lowers plasma cholesterol. Nuts, avocados, olives, soybeans, and various oils, including sesame, cotton - seed, and corn oil, are common sources of these fatty acid Linoleic acid, along with α-linolenic acid, an ω-3 fatty acid, are essential fatty acids required for fluidity of membrane structure. Two fatty fish (for example, salmon) meals per week are recommended. [Note: The ω-3 long-chain PUFAs are included in infant formulas to promote brain development  Cis or Trans Fatty Acids The term "cis" and "trans" describe the positions of the two hydrogen atoms located next to the carbon atoms where the double bond exists. Cis fatty acid has both hydrogen atoms located on the same side. On the contrary, trans fatty acid has the two hydrogen atoms on opposite sides.  Most of the trans fatty acids are formed during the process of hydrogenation of vegetable oils Trans fatty acids are a major component of many commercial baked goods, such as cookies and cakes, and most deep-fried foods. Many manufacturers have reformulated their products to be free of trans fats. In recent decades, it has become increasingly clear that the consumption of significant amounts of trans fats causes a large increase in the incidence of cardiovascular disease by increasing the level of cholesterol in the blood. As a consequence, several European countries have banned foods containing trans fats. In the United States, the trans fat level must be stated on a food’s Nutritional Facts label.