Carbonyl compounds. Structure and chemical properties of carboxylic acids. Lipids.

 

Lipids form large class of relatively water-insoluble bioorganic compounds. In humans and many animals, excess carbohydrates and other energy-yielding foods are converted to, and stored in the body in the form of, lipids called fats. These fat reservoirs constitute major way of storing chemical energy and carbon atoms in the body. Fats and other lipids also surround and insulate vital body organs, providing protection from mechanical shock and helping to maintain correct body temperature. Lipids function as coverings for nerve fibers and as the basic structural components of all cell membranes. Many chemical messengers in the human body, substances called hormones, are lipids.

Lipids, unlike carbohydrates and most other classes of compounds, cannot be defined from structural viewpoint. variety of functional groups and structural features are found in molecules classified as lipids. What lipids share are their solubility properties. Lipids are a structurally heterogeneous group of substances of biological origin that are only sparingly soluble, if at all, in water but are soluble in nonpolar organic solvents. When biological material (animal or plant tissue) is homogenized in blender and mixed with nonpolar organic solvent, the substances that dissolve in the solvent are the lipids.

Functions. 1. The most important role of lipids is as fuel. Much of the carbohydrates of the diet is converted to fat which is stored in various tissues and utilised at the time of requirement. Thus fat may be the major source of energy for many tissues. Actually, in some respects lipids (fats) are even superior to carbohydrates as source of energy.

(1) 1t yields more heat per gm. when burn

(2) It can be stored in practically anhydrous condition and in almost unlimited quantities. Thus fat is the most concentrated form in which potential energy can be stored.

(3) Fatty acids with their fiexible backbones can be stored in much more compact form than the highly spatially oriented and rigid glycogen structure. Thus fat storage provides economy in both weight and space. In addition to the above three reasons there are two other reasons for fat storage as an excellent form of energy.

(4) As it is iasoluble in water, n it has been carried to the fat depots by the specialised transport proteins in the plasma. It is unlikely to break loose and go in the watery body fluids which bath the adipose tissue.

(5) It remains as stable and fixed reserve of energy until mobilized by enzymes which hydrolyse it to glycerol and fatty acids The enzymes are under the control of various hormones and are activated under conditions where the body is involved in increased energy expenditure.

In spite of the above advantages, carbohydrate, and not fat, is the preferred fuel of the body; and any attempt to oxidize appreciable quantities of fat without concomitant degradation of adequate amount of carbohydrate can lead serious consequences.

2. Since fat is bad conductor of heat, it provides excellent insulation. Thus in cold conditions, in which heat is lost to the environment, it provides both an insulating blanket and an extra energy source.

3. Fat may also provide padding to protect the internal organs. Brain and nervous tissue are rich in certain lipids, fact which indicates the importance of these compounds to life.

4. Some compounds derived from lipids are important building blocks of biologically active materials; .g. acetic acid can be used by the body to synthesize cholesterol and related compounds (hormones).

5. Lipoproteins are constitueats of cell walls. he lipids present in lipoproteins constituting the cell walls are of the types of phospholipids. Since lipids are water msoluble they act as ideal barrier for preventiag water soluble materials from passing freely between the intra- and extra-cellular fluids.

6. One more important function of dietary lipids is that of supplying the s-called essential fatty acids Although there are several functions (essential fatty acids (EFA), none of them is well defined.

(a) EFA are found to be the constituents of structural lipids of the cell and mitochondrial membrane.

(b)These are also found in the reproductive organs in high concentration.

(c) EFA are also present in phospholipids in the 2 - position.

(d) Arachidonic acid and related 21 fatty acids give rise to group of pharmacologicaiiy active compounds known as piostaglandins.

(e) EFA are also involved in the genesis of fatty livers and in the metabolism of cholesterol.

(f) In infants certain types of eczema have been cured by feeding fats containing the essential fatty acids.

7. Dietary fat is also found to be necessary for the sufficient absorption of the essential fatty acids and fat-soluble vitamins from the gastro intestinal tract.

However, it is important to note that deficiency of an essential fatty acid has not been observed in man, because the amounts required are very small and an absolutely fat-free diet is practically unknown.

Classification: Lipids can be divided into two major classes on the basis of whether they undergo hydrolysis reactions in alkaline (basic) solution. Saponifiable lipids can be hydrolyzed under alkaline conditions to yield salts of fatty acids. Nonsaponifiable lipids do not undergo hydrolysis reactions in alkaline solution.

We begin our consideration of lipids with an in-depth discussion of saponifiable lipids. Information about nonsaponifiable lipids follows.

The basis of the nature of the products obtained on hydrolysis lipids are mainly divided into three type, viz. simple, compound and derived lipids.

1. Simple lipids. These are esters of fatty acids and alcohols and thus on hydrolysis give fatty acids and alcohols. They may be of two types.

) Fats and oils. These are esters of fatty acids and glycerol ( trihydric alcohol). These are also known as glycerides.

b) Waxes. These are esters of long-chain fatty acids and long-chain monohydric alcohols or sterols.

2 Compound lipids. Compound lipids are esters of fatty acids and alcohols in combination with other compound and thus on hydrolysis give fatty acids, alcohol and other compounds. On the basis of the nature of the other group, compound lipids may again be of following types.

) Phospholipids. These are fat like compounds containing phosphoric acid and nitrogen base.

b) Glycolipids. These are compounds containing fatty acid, carbohydrate, complex alcohol, and nitrogen. but n phosphorus.

3. Derived lipids. These compounds although do not contain an ester linkage but are obtained by the hydrolysis of simple and compound lipids. They may be fatty acids, alcohols and sterols,

Fatty acids are saponifiable lipid building blocks. We begin our discussion of saponifiable lipids by considering fatty acids, compounds that are building block in all saponifiable lipid structures.

Fatty acids are naturally occurring carboxylic acids with an unbranched carbon chain and an even number of carbon atoms. They are rarely found free in nature but rather occur mostly in esterified form in the structures of saponifiable lipids. Because of the pathway by which fatty acids are biosynthesized they almost always contain an even number of carbon atoms. Long-chain fatty acids (12 to 26 carbon atoms) are found in meats and fish; medium-chain fatty acids (6 to 10 carbon atoms) and short-chain fatty acids (fewer than carbon atoms) occur primarily in dairy products.

(a) almost all of the saturated acids with an even number of carbon atoms in the range from 4 to 30 carbons. However, among the several saturated fatty acids, palmitic and stearic acids occur most widely.

(b) unsaturated fatty acids' (with 1, 2, 3 or 4 double bonds) having 18 and 20 carbon atoms. However, among unsaturated acids, oleic acid is the most important and makes up as much as 45 per cent of fatty acid content of mammalian body fat.

(c) various hydroxy acids, .g. ricinoleic acid from castor oil.

(d) few cyclic fatty acids, .g. chaulmoogric and hydnocarpic acids fin chaulmoogra oil.

Almost all have an even number of carbon atoms, and although carboxylic acids withmore than 80 carbon atoms have been isolated from natural sources, those with 16-18 (or 16-22) carbon atoms are most common. Unsaturated acids preferentially exist as cis-isomers with the double bond in one of relatively few preferred locations.

Where there is more than one double bond in an unsaturated carboxylic acid, the double bonds usually occur in a methylene-interrupted pattern:

(-HC = CH- CH2- HC = CH- CH2-)n

The most widespread fatty acids in natural olia and fats:

Acid

Common name

Source

Saturated acids

H3C(CH2)14COOH

palmitic acid

palm kernel oil

H3C(CH2)16COOH

stearic acid

beef fat

Unsaturated acids

H3C(CH2)5CH=CH(CH2)7COOH

palmitoleic acid

palm oil

H3C(CH2)7 CH=CH (CH2)7COOH

oleic acid

olive oil

H3C(CH2)3 (CH2CH=CH) (CH2)7COOH

linoleic acid

linseed oil

H3C (CH2CH=CH)3 (CH2)7COOH

linolenic acid

linseed oil

Saturated and unsaturated fatty acids. Fatty acids are further classified as saturated, monounsaturated, or polyunsaturated. Saturated fatty acids have carbon chain in which all carbon carbon bonds are single bonds. An example is hexadecanoic acid, 16-carbon acid whose common name is palmitic acid.

Hexadecanoic acid (palmitic acid)

The structural formulas of fatty acids are usually written in more condensed form than the preceding structural formula. Two alternative notations for palmitic acids structure are

Monounsaturated fatty acids have carbon chain in which one carbon carbon double bond is present. In naturally occurring and biologically important unsaturated fatty acids, the configuration about the double bond is almost always cis. Different ways of depicting the structure of monounsaturated fatty acid are as follows; the molecule shown is the 18-carbon acid with one double bond (cis-9-octadecenoic acid, or oleic acid).

More than 500 different fatty acids have been isolated from the lipids of microorganisms, plants, animals, and humans. These fatty acids differ from one another in the length of their carbon chains, their degree of unsaturation (number of double bonds), and the positions of the double bonds in the chains.

The first of these structures correctly emphasizes that the presence of cis double bond in the carbon chain puts rigid 300 bend in the chain. Such bend affects the physical properties of fatty acid.

Polyunsaturated fatty acids have carbon chain in which two or more carbon carbon double bonds are present. Up to six double bonds are found in biologically important unsaturated fatty acids.

As was previously described, fatty acids are monocarboxylic acids that contain hydrocarbon chains of variable lengths. Fatty acids are impotant components of several types of lipid molecules. They occur primarily in triacylglycerols and several types of membrane-bound lipid molecules.

Double bonds are rigid structures, unsaturared acid molecules that contain them can occur in two isomeric forms: cis and trans. In cis-isomers, for example, similar or identical groups are on the same side of double bond (a). When such groups are on opposite stdes of a double bond, the molecule is said to be a trans-isomer (b):

The double bond in most naturally occurring fatty acids are in a cis-configuration. The presence of a cis-double bond causes an inflexible kink in a fatty acid chain. Because this structural feature, unsaturated fatty acids do not pack as closely together as saturated fatty acids. This property is responsible for the lower melting points of unsaturated fatty acids, which are liquids at room temperature. For example, a sample of palmitic acid (16:0), a saturated fatty acid, melts at 630C, while palmitoleic acid (16:1D9) melts at 00C.It is interesting to note that fatty acid with trans double bonds have three-dimensional structures similar to those of saturated fatty acids.

Fatty acids with one double bond are referred to as monounsaturated molecules. When two or more double bonds occur in fatty acids, usually separated by methylene groups (- CH2 -), they are referred to as polyunsaturated. The monounsaturated fatty acid oleic acid (18:1D9) and the polyunsaturatedlinoleic acid (18:2D9,12) are among the most abundant fatty acids in living organisms.

Omega-3 and Omega-6 Fatty Acids. Not only do the fatty acids come in three categories (saturated, monounsaturated, and polyunsaturated), but the polyunsaturates also belong to families, two of which are particularly important in human body chemistry. These two important families are the omega-6 and the omega-3 fatty acids.

Fish that live in deep, cold water- mackerel, herring, tuna, salmon, and bluefish - are better sources of omega-3 fatty acids than other fish.

The basis for the omega classification system involves the following considerations. A fatty acid has two ends, designated as the methyl (CH3) end and the carboxyl (COOH) end

Methyl end arb1 end

In the omega classification system, the carbon chain is numbered beginning at the methyl end, which is the reverse of the usual way carbon chains are numbered in the naming of simple acids. The "reverse" numbering system is used because of the mechanism by which fatty acid carbon chains are lengthened during biotransformations within the body. Lengthening involves adding carbon atoms, two at time, at the carboxyl end of the chain. Thus, by numbering polyunsaturated fatty acid chains from the methyl end, which does not change, chemists ease the task of keeping track of fatty acid identities. When omega-3 acid is lengthened, the new acid is still an omega-3 acid.

Organisms such as plants and bacteria can synthesize all the fatty acids they require from acetyl-CoA. Humans contain most of their fatty acids from dietary sources. However, these organisms are able to synthesize saturated fatty acids and some monounsaturated acids by two-carbon units and introducing some double bonds. Fatty acids that can be synthesized are called nonessential fatty acids. Because mammals do not possess the enzymes required to synthesize linoleic (18:2D9,12) and linolenic (18:2D9,12,15) acids, these essential fatty acids or vitamia F. must be obtained from the diet. (Rich sources of these molecules, which have several critical physiological functions, include some vegetable oils, nuts, seeds.) In addition to contributing to proper membrane structure, linoleic and linolenic acids are precursors of several important metabolites. The most researched example of fatty acid derivatives are the eicosanoids.

However, most of the animal systems can interconvert them (i.. linolerc, linolenic and arachidonic acids) and therefore, the diet need to contain only one of these. The actual functioning acid is unknown; there is some evidence that it is arachidonic acid.

Essential fatty acids are fatty acids that are needed by the human body and must be other substances. There are two essential fatty acids: linoleic acid and linolenic acid. Both are 18-carbon polyunsaturated acids; the former is an omega-6 acid, and the latter is an omega-3 acid.

Both of the essential fatty acids contribute to proper membrane structure. When linoleic acid is missing from the diet, the skin reddens and becomes irritated, infections and dehydration are likely to occur, and the liver may develop abnormalities. If the fatty acid is restored, then the conditions reverse themselves. Infants are especially in need of linoleic acid for their growth. Human breast milk has much higher percentage of it than cow's milk.

Linoleic acid and linlin acid furnished by the diet are also the starting points for the synthesis of variety of other longer-chain polyunsaturated acids, including arachidonic acid, EPA, and DHA. Arachidonic acid is the major precursor of eicosanoids, substances that help regulate blood pressure, clotting, and several other important body functions. EPA and DHA are important constituents of the communication membranes of the brain and are necessary for normal brain development. EPA and DHA are also active in the retina of the eye.

Physical properties of fatty acids. Fatty acids from butyric (C4) to capronic acid (C10) are liquids at ordinary temperature, while the higher members above 10 are solids.

The melting points of fatty acids depend on both the length of their hydrocarbon chains and their degree of unsaturation (number of double bonds per molecule). trend of particular significance is that saturated acids have higher melting points than unsaturated acids with the same number of carbon atoms. The greater the degree of unsaturation, the greater the reduction in melting point.

The decreasing melting point associated with increasing degree of unsaturation in fatty acids is explained by decreased molecular attractions between carbon chains. The double bonds in unsaturated fatty acids, which generally have the cis configuration, produce "bends" in the carbon chains of these molecules. These "bends" prevent unsaturated fatty acids from packing together as tightly as fully saturated fatty acids. The greater the number of double bonds, the less efficient the packing. As result, unsaturated fatty acids always have fewer intermolecular attractions and therefore lower melting points than their saturated counterparts.

1 Solubility in water decreases with chain length. Butyric acid is soluble in water in all proportions. Caproic acid is moderately soluble in water. The higher fatty acids are all insoluble in water.

2 The short chain fatty acids are steam volatile, the volatility decreases with chain length. The volatility of the lower fatty acids is utilised in testing purity of butter fat, which is characterised by its high percentage of volatile fatty acids.

3 Most of the fatty acids are soluble in hot alcohol.

4 With NaOH, or KOH fatty acids form salts known as soaps' which are excellent emulsifying and cleansing agents.

5 fatty acid can be reduced to the corresponding alcohol by hydrogen at 200 atmospheres and 320 in presence of nickel as catalyst. The alcohol can be converted to sulphuric acid derivative on treatment with fuming sulphuric acid. The sodium salts of such derivatives are used as synthetic detergents.

6 Reactions due to doable bond of unsaturated fatty acids:

a) Hydrogenation. Under suitable conditions, unsaturated acids take up hydrogen to form the corresponding saturated fatty acid.

b) Addition of halogen. Unsaturated fatty acids can also take up Cl2 Br2 and I2 forming corresponding dihalide.

c) Oxidation. The unsaturated fatty acids can be oxidized at the double bond by alkaline KMn0s to give dihydroxy fatty acids which are later broken down to simpler fatty acids and dicarboxylic acids. For example,

Fats and oils.

The most abundant fatty-acid-containing (saponifiable) lipids are the fats and oils. Because these two types of compounds have the same general chemical structure, we will consider them at the same time.

In terms of functional groups present, fats and oi1s are esters. Esters are produced from the reaction of an alcohol with carboxylic acid. For fats and oils, the alcohol involved is always glycerol, an alcohol with three hydroxyl groups.

Glycerol

Fatty acids are the carboxylic acids involved. In ester formation, single molecule of glycerol can react with three fatty acid molecules because of the three hydroxyl groups present,

Two general representations for the structure of fats and oils are

Ester linkage

The first representation, block diagram, shows that four structural components are present in the structure: glycerol and three fatty acids. The second representation, general structural formula, shows the triester nature of fats and oils. Each of the fatty acids is attached to glycerol through an ester linkage.

Fats and oils are formally called triacylglycerols. triacylglycerol is compound formed by esterification of three fatty acids to glycerol. An acyl group is the group that remains after the - group is removed from fatty acid. Thus, as the name implies, triacylglycerols contain three fatty acid residues (acyl groups) esterified to glycerol. An older name that is still frequently used for triacylglycerol is triglyceride.

The triacylglycerol produced from glycerol and three molecules of stearic acid is an example of simple triacylglycerol. simple triacylglycerol is triester formed the reaction of glycerol with three identical fatty acid molecules.

If the reacting fatty acid molecules are not all identical, then the result is mixed triacylglycerol. mixed triacylglycerol is triester formd from the reaction of glycerol with more than one kind of fatty acid molecule.

In nature, simple triacylglycerols are . Most naturally occurring triacylglycerols are mixed triacylglycerols. Biologically important fats and oils are complex mixtures of mixed triacylglycerols. They cannot be represented by single specific chemical formula because many different fatty acids are present in the mixed triacylglycerol molecules. The variation is wide because the composition of any given fat or oil depends not only on the species involved but also on dietary and climatic factors. For example, fat from corn-fed hogs has different overall composition than fat from peanut-fed hogs. Flax seed grown in warm climates gives oil with different composition than that from flax seed grown in colder climates.

Fats are mixtures of triacylglycerols. Oils are mixtures of triacylglycerols. The answer is physical state at room temperature. Fats are tiracylglycerol mixtures that are solids or semi-solids at room temperature (25 0C). Oils are trtacylglycerol mixtures that are liquids at room temperature. Here are some broad generalizations relative to this distinction, in terms of physical state, between fats and oils:

1. The melting points of triacylglycerols are directly related to the identity of the fatty acids present within them. Fats are composed largely of triacylglycerols in which saturated fatty acids predominate, although some unsaturated fatty acids are present. Such triacylglycerols can pack closely together because of the "linearity" of their fatty acid chains, causing the higher melting points associated with fats. Oils contain triacylglycerols with larger amounts of mono- and polyunsaturated fatty acids than those in fats. Such triacylglycerols cannot pack as tightly together because of the "bends" in their fatty acid chains. The result is lower melting points.

1. gives the percentages of saturated, monounsaturated, and polyunsaturated fatty acids found in common dietary oils and fats. In general, higher degree of fatty acid unsaturation is associated with oils than with fats. notable exception to this generalization is coconut oil, which is highly saturated. This oil is liquid not because it contains many double bonds within the fatty acids but because it is rich in shorter-chain fatty acids, particularly lauric acid (12:0).

2. Fats are generally obtained from animals; hence the term animal fat. Although fats are solids at room temperature, the warmer body temperature of the living animal keeps the fat somewhat liquid (semi-solid) and thus allows for movement. Oils typically come from plants, although there are also fish oils. fish would have some serious problems if its triacylglycerols "solidified" "when it encountered cold waters.

3. Pure fats and pure oils are colorless, odorless, and tasteless. The tastes, odors, and colors associated with dietary plant oils are caused by small amounts of other naturally occurring substances present in the plant that have been carried along during processing. The presence of these "other" compounds is usually considered desirable.

Percentages of saturated, monounsaturated, and polyunsaturated fatty acids in the triacylglycerols of various dietary fats and oils.

Oil or fat

Percentages of saturated fatty acids, %

Percentages of monounsaturated fatty acids, %

Percentages of polyunsaturated fatty acids, %

Canola oil

6

58

36

Safflower oil

9

13

78

Sunflower oil

11

20

69

Corn oil

13

25

62

Olive oil

14

77

9

Soybean oil

15

24

61

Peanut oil

18

48

34

Cottonseed oil

27

19

54

Lard

41

47

12

Palm oil

51

39

10

Beef tallow

52

44

4

Butterfat

66

30

4

Coconut oil

99

6

2

When person consumes too much food, much of the excess energy (calories) is stored as fat. This fat is concentrated in special cells (adipocytes) that are nearly filled with large fat droplets. Adipose tissue containing these cells is found in various parts of the body -under the skin, in the abdominal cavity, in the mammary glands, and around various organs.

In the past decade, considerable attention has been paid to the role of dietary factors as cause of disease (obesity, diabetes, cancer, hypertension, and atherosclerosis). Several organizations have proposed new dietary guidelines on the basis of these studies. The guidelines strongly suggest that total fat intake should be reduced, with particular emphasis on reduction of saturated fatty acid intake.

new development involving fats is t he creation, by food scientists, of fat substitutes called artificial fats. Chemical reactions of triacylglycerols

Nomenclature of fats. For simple glycerides the name is made up of the name of the alcohol (glycerol) or its radical (glyceryl) and the name of the acid; or the name of the acid concerned is changed to suffix in.

Glycerol tributyrate Glycerol tristearate

or tributyrin or tristearin

For mixed glycerides, the positions and the names of the acid groups are specified by the Greek letters a, b, a or by the numerals, 1, 2 and 3.

Glycerol oleobutyropalmitate b-Palmito-a, a-disteain

(found in butter rat)

All natural fats and oils invariably consist of mixture of mixed glycerides, .g. lard is mixture of the oleo-palmitostearin, palmitodistearin, stearo-dipahnitin and palmito-diolein.

The chemical properties of triacylglycerols (fats and oils) are typical of esters and alkenes because these are the two functional groups present in triacylglycerols. Four important triacylglycerol reactions are hydrolysis, saponification, hydrogenation, and rancidity.

Hydrolysis of triacylglycerol is the reverse of the esterification reaction by which it wet formed. Complete hydrolysis of triacylglycerol molecule always gives one glycerol molecule and three fatty acid molecules as products.

Note that All the three alkyl groups (i.. R, R' and R') can be identical, only two of the three alkyl groups may be identical and the third different, or all the three alkyl groups may be different. fat may be composed of triglycerides having as many as 10. or more fatty acid constituents. For example, the approximate composition of the butter fat is as below: Butyric acid 4%; Palmitic acid 30%; Caproic acid 2%; Stearic acid 10%; Caprylic acid 0.5%; Pa1mitoleic acid 5%; Lauric acid 2.5%; Oleic acid 30%; Myristic acid 10%; Linoleic acid 3. 6%

Triacylglycerol hydrolysis within the human body requires the help of enzymes (protein catalysts) produced by the pancreas. These enzymes cause the triacylglycerol to be hydrolyzed in stepwise fashion. First one of the outer fatty acids is removed, then the other outer one, leaving monoacylglycerol. In most cases this is the end product of the initial hydrolysis (digestion) of the triacylglycerol. Sometimes, enzymes remove all three fatty acids, leaving free molecule of glycerol.

Saponification is hydrolysis reaction carried out in an alkaline (basic) solution. For fats and oils, the products of saponification are glycerol and fatty acid salts. The overall reaction of triacylglycerol saponification can be thought of as occurring in two steps. The first step is the hydrolysis of the ester linkages to produce glycerol end three fatty acid molecules:

Fat or oil + 2O == 3fatty acids + glycerol

The second step involves reaction between the acid molecules and the base (usually NaOH) in the alkaline solution. This is an acid - base reaction that produces water pl salts:

3 Fatty acids + NaOH == 3 fatty acid salts + 2

The Phoenicians, a group of seafaring people who dominated trade in the Mediterranean Sea about 3000 years ago, are believed to have been the first to manufacture soap. Traditionally, soap has been made by heating animal fat with potash. (Potash is a mixture of potassium hydroxide (KOH) and potassium carbonate (K2CO3) that is obtained by mixing wood ash with water). Currently, soap is made by heating beef tallow or coconut oil with sodium or potassium hydroxide. During this reaction, which is referred to as a saponification, triacylglycerol molecules are hydrolyzed to give glycerol and sodium or potassium salts of fatty acids:

Soap

Fatty acid salts (soaps) are amphipathic molecules; that is spontaneously form into micelles. Soap micelles have negatively charged surfaces that repel each other. The cleansing properties of soap are dye to its capacity to act an emulsifying agent. (Emulsifying agents promote the formation of an emulsion, that is, the dispersal of on substance in another.)

Hydrogenation. It involves hydrogen addition across carbon - carbon multiple bonds, which increases the degree of saturation as some double bonds are converted to single bonds. With this change, there is corresponding increase in the melting point of the substance.

The carbon - carbon double bonds in vegetable oils are partially hydrogenated (some, but not all, of the double bonds are converted to single bonds) to produce semi-solid rather than liquid products. Many food products are produced in this way. The peanut oil in many popular brands of peanut butter has been partially hydrogenated to convert the oil into solid that does not separate out of the mixture. Hydrogenation is used to produce solid cooking shortenings or margarines from liquid vegetable oils.

Soft-spread margarines are partially hydrogenated oils. The extent of hydrogenation is carefully controlled to make the margarine soft at refrigerated temperatures (40). Recently, health concern has developed about the hydrogenation of food products.

Rancidity. Fats and oils often develop unpleasant odors and/or flavors upon exposure to moist air at room temperature. When this occurs, fats and oils are said to have become rancid. Rancidity results from two kinds of unwanted reactions: hydrolysis of triacylglycerol ester linkages and oxidation of carbon - carbon double bonds in the fatty acid chains of triacylglycerols.

Hydrolytic rancidity results from the exposure of fats and oils to moist air. (All air contains some moisture.) Microorganisms in the air supply necessary enzymes to catalyze the hydrolysis of ester bonds within triacyglycerols. Volatile short-chain fatty acid molecules are freed as the result of such triacylglycerol hydrolysis, and they contribute to the disagreeable odors of rancid foods. For example, butyric acid is the source of the characteristic odor of rancid butter.

Oxidative rancidity, in most cases, is more important reaction than hydrolytic rancidity. In oxidative rancidity, double bonds in the unsaturated fatty acid components of triacylglycerols are cleaved, producing low-molecular-mass aldehydes with objectionable odors. In addition, these aldehydes can be further oxidized to give equally offensive-smelling, low-molecular-mass carboxylic acids. Warmth and exposure to atmospheric oxygen induce oxidative rancidity. To avoid this unwanted oxidation, the food industry adds antioxidants to foods. Two naturally occurring antioxidants are vitamin (ascorbic acid) and vitamin (n-tocopherol). Two synthetic oxidation inhibitors are and . In the presence of air, antioxidants, rather than food, are oxidized.

Oxidation. Oils rich in unsaturated fatty acids such as linseed oil undergo spontaneous oxidation at the double bond forming aldehydes, ketones and resins; the latter form thin transparent coating on the surface to which the oil is applied. Such oils are known as drying oils and are used in the manufacture of paints and of varnishes.

Characterization of fats. The composition, quality and purity of given oil or fat is determined by means of number of physical and chemical tests. The usual physical tests include determination of m, ., specific gravity, viscosity, etc. while the chemical tests include determination of certain values discussed below.

1. Acid number. It is the number of milligrams of potassium hydroxide required to neutralise the free fatty acids in 1 gm. of the oil or fat. Thus it indicates the amount of free fatty acids present in an oil or fat. high acid value indicates stale oil or fat stored under improper conditions,

2. Saponification number. It is number of milligrams of potassium hydroxide required t completely saponify l gtn. of the oil or fat. Thus it is measure of fatty acids present as esters in given oil or fat. The sponification value gives an idea about the molecular weight of fat or oil. The saponification number and molecular weight of an oil are inversely proportional to each other; thus high saponification number indicates that the fat is made up of low molecular weight fatty acids and vice versa. It is also helpful in detecting adulteration of given fat by one of the lower or higher saponfication value.

3. Iodine number. It is the number of grams of iodine that combine with 100 gm. of oil or fat. It is measure of the degree of unsaturation of fat or oil; high iodine number indicates high degree of unsaturation of the fatty acids of the fat.

4. Reichert -Meissl number. (R. M. number). It is the number of millilitres of N/10 potassium hydroxide required to neutralise the distillate (obtained by saponification, acidification and steam distillation of the fat) f 5 gm. of the fat. It is measure of steam volatile fatty acids present as esters in oil or fat. It is used for determining the purity of butter or ghee (R.. number 20 30)

Waxes.

Wax is mixture of esters of high molecular weight alcohols and high molecular weight fatty acids.

Waxes are sainfied with great difficulty than fats and are not attacked by lipase. Although waxes may be saponified by prolonged boiling with alcoholic KOH, they are more easily saponified by treating solution of the wax in petroleum ether with absolute alcohol and metallic sodium, with sodium ethoxide. The saponi6cation products f waxes are water-soluble soaps (sodium Its of higher fatty acids); while the water insoluble long-chain alcohols appear in the "unsaponifiable matter" fraction. Waxes contain about 31 -55% of the unsaponifiable matter, while fats and oils contain only 1 - 2% unsaponifiable matter.

Common wax. Beeswax. It contains esters derived from alcohols having 24 - 30 carbon atoms, viz. palmitate of mrl alcohol (30H61) and n-hexacosanol (2653).

3(C2)14COOC30H61 3 (C2)14COOC26H53

myricyl patmitate n- hexacosanyl patmitate

Spermaceti. 1 is obtained from the head of the sperm whale. It is rich in ester of cetyl alcohol (1633) and palmitic acid: 3 (C 2 ) 14COOC16H33 - cetyl palmitate

Spermaceti is used in making candles.

Sperm Oil. 1t is liquid wax and occurs with spermaceti in the sperm whale. It is valuable lubricant used for delicate instruments, such as watches. It does not become gummy, as many oils do.

Carnauba wax. It is found in the leaves of the carnauba palm of Brazil. It is used as an ingredient in the manufacture of various wax polishes. Because waxes are very inert chemically, they make an excellent protective coating.

Lanolin or wool wax. It is obtained from wool and is used in making ointments and salves. It readily forms and emulsion with water, and for this reason makes it possible for drugs which are soluble in water to be incorporated into salves Chinese wax is the secretion ! an insect.

Some of the waxes found in skin are esters of hydroxylated fatty acids and open chain alcohols.

wax found in blood plasma is found to have cholesteryl palmitate, i.e. ester of cholesterol and paimitic acid.

Physiological importance of waxes.

-     The most important physiological function of waxes is as protective agent on surfaces of animals and plants.

-     Waxes are found on the surface of feathers and hair with the result they remain soft and piiable.

-     Waxes prevent aquatic animals from becoming wet. On coming out of water duck will shake herself once and becomes apparently dry

-     Waxy coating on the surface of plants protects them from excessive loss of moisture. Hence desert plants like palm and cactus can live for long periods without rain.

-     Waxy coating protects the plant from becoming infected with fungi and bacteria which cause disease.

-     Waxy coating on several fruits like apples and citrus fruits prevent them fruits during out and thus these fruits can be stored for long periods of time. The waxy covering also protects such fruit~ from organisms which cause rot.

 

Compound Lipids.

As already mentioned, compound lipids are those which contain some chemical group in addition to fatty acids and an alcohol. On the nature of the additional chemical group, compound lipids are sub-divided into two main groups.

(a) Phospholipids: which contain phosphate group.

(b) Glycolipids: which contain carbohydrate.

Other classification divides the complex lipids into three main groups, viz.

(1) Glycerophosphatides (glycerol phospholipids) which are glycerol containing phospholipids.

(2) Phosphoinositides: which contain inositol ( hexahydric alcohol) as the base.

(3) Phosphosphingosides or sphingolipids (sphingosine lipids)- which contain sphingosine or dihydrosphingosine as the base.

Phospholipids (phosphatides). This group is the most abundant among the complex lipids It is found in every living cell and makes up as much as 70% of the complex lipid contents of the tissues. These substances are also known as phosphatides and are sometimes named as derivatives of the parent compound, phosphatidic acid.

phosphatidic acid

Phospholipids may be defined as those lipids which yield on hysrolysis an alcohol, fatty acid, phosphoric acid, and nitrogen base.

Functions of phospholipids. Phospholipids are involved in many functions. Sme of these possible functions are listed below.

() As structural component. Phospholipids are said to be components of cellular membranes including membranes of mitochondria. The operation of the oxidative chain and oxidative chain phosphorylation in mitochondria is inactivated by removal of the phospholipids, which may be controlling or participating in the transport of metabolites from one side of the membrane to the other. Despite their structural differnces, all phospholipids have hydrophobic and hydrophilic domains. The hydrophobic domain is composed largely of the hydrocarbon chains of fatty acids; the hydrophilic domain, called a polar head group, contains phosphate and other charged or polar groups.

(b) In blood coagulation. Phospholipids having ethanolamine or serine as base are believed to function in the process of blood coagulation.

(c) In absorption and transport of lipids. Phospholipids may act as emulsifying agents during digestion and absorption of lipids and are believed to be important components of the coating of chylomicrons in the form of lipoproteins. Phospholipids are also involved in the transport of lipids in the blood. (A surface active agents is a subsstance that lowers the surface tension of a liquid, usually water, so that it spreads out over a surface.)

(d) Transport of ions. Some phospholipids help in the transport of inorganic ions mainly cations across the membrane.

Classification of phospholipids:

Name of X -OH

Formula of X

Name of phospholipid

Water

-H

Phosphatidic acid

Choline

Phosphatidylcholine (Lecithin)

Ethanolamine

Phosphatidylethanolamine (cephalin)

Serine

Phosphatidylserine

Glycerol

Phosphatidylglycerol

Phosphatidyl

glycerol

Diphosphatidylglycerol (cardiolipin)

Inositol

 

Phosphatidylinositol

Phosphoacylglycerols are triesters of 1 in which two- groups are esterified with fatty acids asnd one the third is esterified with phosphofic acid, which in turn is ecterified to an alcohol. The block diagram for phosphoacylglycerol has the following general structure:

The most abundant phosphoacylglycerols have an amino alcohol (choline, ethanolamine, or serine) attached to the phosphate group. The structures of these three aminoalcohols, given in terms of the charged forms that they adopt in neutral solution, are

Choline Ethanolamine Serine

Phosphoacylglycerols containing these three amino alcohols are respectively known as phosphatidylcholines, phosphatidylethanolamines, and phosphatidylserines. The fatty acid, glycerol, and phosphoric acid portions of phosphoacylglycerol structure constitute phosphatidyl group.

Although the general structural features of phosphoacylglycerols are similar in many respects to those of triacylglycerols, these two types of lipids have quite different biological functions. Triacylglycerols serve as storage molecules for metabolic fuel. Phosphoacylglycerols function almost exclusively as components of cell membranes and are not stored. major structural difference between the two types of lipids, that of polarity, is related to their differing biological functions. Triacylglycerols are nonpolar class of lipids, whereas phosphoacylglycerols are polar. In general, membrane lipids have polarity associated with their structures.

Further consideration of general phosphoacylglycerol structure reveals an additional structural characteristic of most membrane lipids. Let us consider phosphatidylcholine containing stearic and oleic acids as our example. There are two important things to notice about this model:

(1) There is "head" part, the choline and phosphate, and (2) there are two "tails," the two fatty acid carbon chains. The head part is polar. The two tails, the carbon chains, are nonpolar.

All phosphoacylglycerols have "head" "and two "tails. simplified representation for this structure uses circle to represent the polar head and two wavy lines to represent the nonpolar tails. The polar head group of phosphoacylglycerol is soluble in water. The nonpolar tail chains are insoluble in water but soluble in nonpolar substances. This dual-polarity feature, previously encountered when soaps were discussed, is structural characteristic of most membrane lipids.

Phosphatidylcholines are also known as lecithins. There are number of different phosphatidylcholines because different fatty acids may be bonded to the glycerol portion of the phosphatidylcholine structure. In general, phosphatidylcholines are waxy solids that form colloidal suspensions in water. Egg yolks and soybeans are good dietary sources of these lipids. Within the body, phosphatidylcholines are prevalent in cell membranes.

Periodically, claims arise that phosphatidylcholine should be taken as nutritive supplement; some claims indicate it will improve memory. There is no evidence that these supplements are useful. The enzyme lecithinase in the intestine hydrolyzes most of the phosphatidylcholine taken orally before it passes into body fluids, so it does not reach body tissues. The phosphatidylcholine present in cell membranes is made by the liver; thus phosphatidylcholines are not essential nutrients.

The food industry uses phosphatidylcholines as emulsifiers to promote the mixing of otherwise immiscible materials. Mayonnaise, ice cream, and custards are some of the products they are found in. It is the polar - nonpolar (head - tail) structure of phosphatidylcholines that enables them to function as emulsifiers.

(a) Lecithins. Lecithins occur in nearly all animal and plant organisms. In animals, lecithins are mainly found in egg yolk, brain, nerve tissues and sperm; while in plants they are particularly abundant in seeds and sprouts. The most important source of lecithins is egg yolk, from which its name is derived (Greek, lekithos meaning yolk). Man lecithins have been prepared synthetically. Lecithins are poorly crystalline substances They usually swell in water and form colloidal solutions. They are dextro-rotatory, and become inactive when heated in sealed tube at 100 0C.

Lecithins are esters of glycerol in whose molecule two hydroxyl groups have been estert6ed by fatty acids, while the third has been esterified by phosphoric acid which in turn has formed an ester with choline. The nature of fatty acids depends upon the source of the lecithins, thus the general structure of lecithins will be as below.

Lecithins in whose molecules the b-hydroxyt group of glycerol has been esterified by phosphoric acid are rare. Such lecithins are known as b-lecithins and produce optically inactive glycero b-phosphoric acid on hydrolysis.

glycero-b-phosphoric acid

As fr as the nature of the fatty acids is concerned stearic, paimitic, oleic, linoleic, arachidonic and clupanodonic acids have been isolated from lecithins obtained trom animal tissues. Recent evidence indicates that in the above general formula of lecithins R is usually saturated fatty acid, and R' is usually an unsaturated fatty acid.

This is the most common form of phoipholipid, at least in animals. The lecithins are required for normal transport and utilization of other lipids, especially is the liver Anything which interferes with the synthesis of choline ( constituent of lecithins) will also restrict the synthesis of lecithins, and thus disturb the normal transportation of lipids to and from the liver. This will result in the accumulation of lipid material and thus give rise to the condition called "fatty liver".

Certain enzymes in the snake venom can cause the hydrolysis of the unsaturated fatty acids on 2 of phospholipids, resulting in the production of compounds known as lysolecithins t1ysocephalins, see (b) below). The latter compounds have strong hemolytic (red-blood cell destroying) action. If the hemolysis is extensive enough, it may result in the dearth of the victim of snake bite.

The affinity of lecithins towards water form emulsions makes lecithins an important constituent of protoplasm. Lecithin aids in the organization of the cell structure.

Commercially lecithin is prepared from soyabeans and has several important applications. Added to the chocolate it is used in making candy, it prevents the formation of white spots on the surface of chocolate creams. When added to oleomargarine it gives the product consistency similar to that of butter.

(b) Cephalins (kephalins) are nearly similar to that of lecithins with the difference that they, unlike lecithins, contain colamine (ethanolamine) residue (sometimes serine or myoinositol) instead of the choline residue. They are present in brain matter from which the nme kephalin is derived (Greek kephale meaning heads). The separation of cephalins and lecithins is based on the fact that the cadmium chloride compound of' the former is soluble while the corresponding salt of the latter is insoluble.

As in the lecithias, two fatty acids are present in the molecule, usually one saturated and the other unsaturated. Moreover, there are also two cephalins, viz. a- and b-. Following is the structure for an a-cephalin.

a-cephalin

Where, X= - an ethanolamine cephalin; X= - serine cephalin

Stearic, oleic, linoleic, and arachidonic acids have been found as fatty acid constituents of the cephalins. Enzymes in snake venom can cause the formation of lysocepbalins. The cephaliris have been implicated in the process of blood coagulation (clotting). Cepbalins also play fundamental role in the structure of living organism.

(c) Plasmalogeas. These phospbolipids are characterized by the fact that on treatment with acid they form long chain fatty aldehydes The general structure of plasmalogens is the general structure of lecithin or cephalin in which one of the fatty acids is replaced by aldehydogenic group.

Plasmalogens or plasmologens are widely distributed in animal tissues, especially in the myelin of brain heart nerve and skeletal muscle.

Cardiolipids are polymers of phosphatidic acids of any of the above type of phospholipids minus the base. These are isolated from beef heart extract and are said to be the active substances responsible for the serological test for syphilis.

Shingomyelins. These are composed of a complex basic dihydric amino alcohol, sphingosirse (sphingosinol) with a fatty acid in amide linkage on the amino group and the hosphorylcholine group attached by way of the terminal alcohol group. Thus they differ chemically from other phospholipids in the following two important respects.

All lipids derived from sphingosine have (1) fatty acid connected to the - NH, group via an amide linkage, and (2) group attached to the - group on the terminal carbon atom via an ester linkage.

Note again, as in phosphoacylglycerols and waxes, the structural features of head and two tails. For sphingolipids, the fatty acid is one of the tails, and the long carbon chain of sphingosine itself s the other tail. The "additional component" is the heal, and it is phosphoric acid choline group.

Sphingolipids are the second major class of nonglycerol-based saponifiable lipids. Like phosphoacylglycerols, they are polar lipids and are major constituents of cell membranes.

Sphingolipids have structures based on the long-chain amino dialcohol sphingosine. sphingolipid is saponijiable lipid derived from the amino dialcohol sphingosine. The structure of sphingosine is:

Phytospingosine is found in plant spingolipids. The core structure of each type of spingolipid is ceramide, a fatty acidamide derivative of shingosine. In shingomyelin, ceramide is esterified to phosphatidylcholine or phosphatidylethanolamine. Sphingomyelin is found in most animal cell membranes. However, as its name suggests, spingomyelin is found in greatest abundance in the myelin sheath of nerve cells. (The myelin sheath is found by successive wrappings of the cell membrane of a specialized myelinating cell around a nerve cell axon. It facilitates the rapid transmission of nerve impulses).The ceramides are also precursors for the glycolipids, sometimes referred to as the glycosphingolipids. Clicolipids differ from spingomyelin in that they contain no phosphate. In glycolipids a monosaccharide, disaccharideor oligosaccharide is attached to aceramede through an O-glycosidic linkage. The most important glycolipid classes are cerebrosides, the sulfatides, and the gangliosides. Cerebrosides are shingolipids in which the head group is a monosaccharide.Galactocerebrosides, the most common axample of this class, are almost entifely found in thecell membranes of the brain. if a cerebroside is sulfated, it is referred to as a sulfatide. Sulfatidesare negatively charged at physiological pH. Spingolipids that possess oligosaccharide groups with one or more sialic acid residues are called gangliosides. Althhough gangliosides were first isolated form nerve tissue, they also occur in most other animal tissues. The names of individual gangliosides include subscript letter and numbers. The letters M, D and T indicate whether the molecule contains one, two, or three sialic residues, respectively. The number notation designates the sequence of sugars that are attached to ceramide. The structure of the Tay-Sachs ganglioside. The rol of glycolipids is still unclear. Certain glycolipid molecules have been implicated in the binding of bacterial toxins, as well as bacterial cells, to animal cell membranes. For example, the toxins that are responsible for cholera, tetanus, and botulism bind to glycolipid cell membrane receptors. Bacteria that have been shown to bing to glycolipid receptors include Eschefichia coli, Streptococcus pneumoniae, and Neisseria gonorrhoea, the causative agents of urinary tract infections, pneumonia, and gonorrhea respectively.

Following structure has been assigned to shinomyelin on the basis of the usual hydrolysis.

Three fatty acids namely stearic, lignoceric or nervonic were obtained from pure sphingomyelin fraction.

Like other phospholipids, it 1s occurs practically in all animal tissues particularly in brain and spinal cord and in plant seeds. It is very sparingly soluble in ether, and thus can be easily separated from the lecithins and cephalins.

Cerebrosides and Gangliosides. Some sphingosine-based membrane lipids have small carbohydrate as the head group. Cerebrosides, the simplest of such carbohydrate-containing lipids, usually have glucose or galactose as the carbohydrate unit. The cerebrosides, as the name suggests, occur primarily in the brain (7 % of dry mass) and in the myelin sheath of nerves. Gangliosides contain more complex carbohydrate heads; up to seven monosaccharide units are present. These substances occur in the gray matter of the brain as well as in the myelin sheath.

The specific structure of the cerebroside in which stearic acid (18:0) is the fatty acid and galactose is the monosaccharide is

The accompanying Chemistry at Glance briefly outlines the classification of saponifiable lipids.

Galactolipias occur in considerable amounts in the white matter of the brain and of all nervous tissue. They usually occur in the amorphous state. but are also known as liquid crystals. They are insoluble in ether but soluble in hot alcohol. On hydrolysis, give fatty acid, sphingosinol, and sugar, usually galactose. As mentioned earlier, there are four individual members of this group which differ only in the nature of their fatty acids. The general structural formula for galactolipids is as below.

The sphingosine-fatty acid moieiy is called ceramde. In addition to the above compound lipids there are globosides, hematosides and gangliosides. They are structurally similar to glycosides with the only difference that in the former the sugar residues are acetylated amino sugars, .g. D-galactosamine, in the middle one they are sialic acid while in the latter acetylated amino sugars and sialic acid both are present.

Nonsaponifiable Lipids do not undergo hydrolysis in alkaline solution. Their structures are much different from those of the saponifiable lipids; neither ester nor amide linkages are present.

Nonsaponifiable Lipids: steroids, eicosanoids, terpenes, pheromones, fat-soluble vitamins.

Steroids are lipids with structures that are based on fused-ring system involving three 6-membered rings and one 5-membered ring. The fused-ring system of steroids, called steroid nucleus, has the following structure:

Steroid nucleus

Note that the rings are customarily labeled with letters and each carbon atom is labeled with number.

Numerous steroids have been isolated from plants, animals, and human beings. Location of double bonds within the fused-ring system and the nature and location of substituents distinguish one steroid from another. Most steroids have an oxygen functional group (= or - ) at carbon 3 and some kind of side chain at carbon 17. Many also have double bond from carbon 5 to either carbon 4 or carbon 6.

Cholesterol is the most abundant steroid in the human body. The name cholesterol has the -ol ending because it is an alcohol, with an - group on carbon 3 of the steroid nucleus. In addition, cholesterol has methyl groups bonded to carbon atoms 10 and 13 and small branched hydrocarbon chain on carbon 17.

Within the human body, cholesterol is found in cell membranes (up to 25 % by mass), nerve tissue, and brain tissue (about 10% by dry mass), and it is the main component of gallstones. Human blood plasma contains about 50 mg of free cholesterol per 100 mL and about 170 mg of cholesterol esterified with various fatty acids.

space-filling model of the cholesterol molecule shows the rather compact nature of this molecule. The "head and two tails" arrangement found in many lipids is not present. The lack of large polar head causes cholesterol to have limited water solubility. The - group on carbon 3 is considered the head of the molecule.

Cholesterol plays vital biological role in chemical synthesis within the human body. It is the starting material for the synthesis of numerous steroid hormones, vitamin D, and bile salts. Its presence in the body is essential to life.

It is important to note that cholesterol is usually stored within cells as a fatty acid ester. The esterification reaction is catalyzed by the enzyme acyl CoA: cholecterol acyltransferase (ACAT), located on the cytoplasmic face of the endoplasmic reticulum.

Coadioac glycosides are among the most interesting steroid derivatives recall that glycosides are carbohydrate-containing acetals or ketals. Although several cardiac glycosides are axtremely toxic (ouabain, obtained from the seeds of the plant Strophanthus groups), others have valuablemedical properties. For example, digitalis, an axtract of the dried leaves of Digitalis purpurea (the fixglove plant), is a time-honored stimulator of cardiac muscle contraction. Digitoxin, the major cardiotonic glycoside in digitalis, is used in the treatment of congestive heart failure. It is important to note that in higher than therapeutic doses, digitoxin is extremely toxic. Both ouabain and digitoxin are inhibitors of the Na+ -K+ATPase.

The human body, mainly within the liver, synthesizes about 1 gram of cholesterol each day, an amount suf5cient to meet the body's biosynthetic needs. Therefore, cholesterol it not necessary in the diet. When we ingest cholesterol, the amount synthesized by the body is reduced. However, the reduction is less than the amount ingested. Therefore, total body cholesterol level increases with dietary cholesterol level.

Medical science now considers high blood cholesterol, along with high blood pressure and smoking, as the major risk factors for cardiovascular disease (CVD). High blood cholesterol contributes to atherosclerosis, the main form of CVD, which is characterized by the buildup of plaque along the inner walls of the arteries. Plaque is mound of lipid material mixed with smooth muscle cells and calcium. Much of the lipid material in plaque is cholesterol. Extensive plaque formation leads to hardening of the arteries. Plaque deposits in the arteries that serve the heart reduce blood flow to the heart muscle and can lead to heart attack.

People who want to reduce their level of dietary cholesterol should reduce the amount of animal products they eat (meat, dairy products, etc.) and eat more fruit and vegetables. Plant foods contain no cholesterol; it is found only in foods of animal origin.

Derived Lipids. Derived lipids are those which although do not contain any ester linkage but may be considered to have been derived from naturally occurring esterified materials. In simple words, we can say that derived lipids are substances formed on the hydrolysis of simple or compound lipids which still retain the properties of this class of compounds. Derived lipids may be of following types.

1. Fatty acids. Saturated and unsaturated.

2. Alcohols. Alcohols of high molecular weight but not glycerol. These may again be of following types.

(a) Aliphatic alcohols such as cetyl, stearyl and myricyl alcohols

(b) Sterols. These contain phenanthrene nucleus important examples are cholesterol, ergosterol and stigmasterol.

(c) Alchols having b-ionone ring. These include vitamin A1 and carotenols (.g., lutein and zeaxanthin).

3. Hydrocarbons. These include aliphatic hydrocarbons, carotenes, and squalene.

4. Certain vitamins. These include vitamins D, E and .

5. Steroids hormones.

6. Bile acids.

The liver secretes clear, golden-yellow, viscous fluid known as bile. It is stored in the gall bladder and is mainly useful for digestive system. Bile consists of inorganic (chiefly 3-, 1-, Na+, + etc.) ions as well as organic compounds. Among organic compounds the main constituents are bile acids, bile pigments, lipids, fatty acids and cholesterol.

The bile acids are present as the sodium salt of amide with glycine or taurine, e.g. sodium glycocholate (glycine+cholic acid) and taurocholate (taurine + cholic acid). Bile acids are the hydoxy derivatives of either cholanic or allocholanic acid and dehydration followed by reduction of the bile acids give the latter (cholanic or allocholanic acid).

The bile acids form molecular compounds with various substances, .g. deoxycholic acid forms such complexes with fatty acids which are known as choleic acids.

Cholic acid

Bile salts are emulsifying agents that make dietary lipids soluble in the aqueous environment of the digestive tract. During digestion, bile salts are released into the intestine from the gallbladder, where they help digestion by emulsifying (solubilizing) fats and oils. Their mode of action is much like that of soap during washing.

Bile salts are cholesterol oxidation products. They are trihydroxy cholesterol derivatives in which the carbon-17 side chain has been oxidized to carboxylic acid. This acid side chain is then bonded to an amino acid through an amide linkage. The two principal bile salts are sodium glycocholate (glycine is the amino acid) and sodium taurocholate (taurine is the amino acid).

Sodium glycocholate

Sodium taurocholate

Cholesterol, an important molecule in animals, is a representative example of the steroids. In addition to its role as an essential component in animal cell membranes, cholecterol is a precursor in the biosynthesis of all steroid hormones, vitamin D. and bile salts:

Functions of bile acids. The important functions of the bile acids may be summarised as below.

(a) They facilitate digestion of fats by emulsifying them and thereby increasing the surface area of the material for pancreatic enzymes.

(b) They also activate the enzyme choiesterol esterase and pancreatic lipase.

(c) They help in the absorption of cholesterol, fat soluble vitamins (, D, , F, ), etc. by forming water soluble complexes.

(d) They also keep cholesterol in solution, if the ratio between bile acids and cholesterol falls than the normal, cholesterol is precipitated and forms gallstones in liver, and gallbladder.

The bile acid in the bile entering the intestine are rapidly absorbed into the blood, taken back by the liver and reutilized. This is called enierohepatic circularion of bile salts. Unabsorbed bile acids are attacked by bacteria and decomposed into various products which are excreted in faeces.

Steroid Hormones. Hormones are chemical messengers produced by ductless glands. They serve as means of communication between various tissues. Many, but not all, hormones in the human body are steroids. Cholesterol is the ultimate starting material for the production of all steroid hormones, so they contain its characteristic system of four fused rings. Steroid hormone synthesis is always multistep process.

There are two major classes of steroid hormones: the sex hormones, which control reproduction and secondary sex characteristics, and the adrenocorttcal hormones, which regulate numerous biochemical processes in the body.

The sex hormones can be classified into three major groups:

1. Estrogens the female sex hormones

2. Androgens the male sex hormones

3. Progestins the pregnancy hormones

Estrogens are synthesized in the ovaries and adrenal cortex and are responsible for the development of female secondary sex characteristics at the onset of puberty and for regulation of the menstrual cycle. They also stimulate the development of the mammary glands during pregnancy and induce estrus (heat) in animals.

Androgens are synthesized in the testes and adrenal cortex and promote the development of secondary male characteristics. They also promote muscle growth.

Progestins are synthesized in the ovaries and the placenta and prepare the lining of the uterus for implantation of the fertilized ovum. They also suppress ovulation.

The fact that seemingly minor changes structure effect great changes in biofunction points out, again, the extreme specificity of the enzymes that control biochemical reactions.

Increased knowledge of the structures and functions of sex hormones has led to the development of number of synthetic steroids whose actions often mimic those of the natural hormones. Among the best known of the synthetic steroids are oral contraceptives and anabolic agents.

Estradiol - the principal estrogen; responsible for secondary female characteristics.

Testosterone the principal androgen; responsible for secondary male characteristics.

Progesterone the principal progestin; prepares the uterus for pregnancy

Oral contraceptives are used to suppress ovulation as method of birth control. Generally, mixture of synthetic estrogen and synthetic progestin is used. The synthetic estrogen regulates the menstrual cycle, and the synthetic progestin prevents ovulation, thus creating false state of pregnancy. Compare its structure to that of progesterone (the real hormone); the structures are very similar.

The second major group of steroid hormones consists of the adrenocortical hormones,

Produced by the adrenal glands, small organs located on top of each kidney, at least 2g different hormones have been isolated from the There are two types of adrenocortical hormones.

1. Mineralocorticoidx control the balance of Na+ and + ions in cells.

2. Glucocorticoids control glucose metabolism and counteract inflammation.

The major mineralocorticoid is aldosterone, and the major glucocorticoid is cortisol (hydrocortisone). Cortisol is the hormone synthesized in the largest amount by the adrenal glands. Cortisol and its synthetic ketone derivative cortisone exert powerful anti-inflammatory effects in the body. Both cortisone and prednisolone, similar synthetic derivative, are used as prescription drugs to control inflammatory diseases such as rheumatoid arthritis. adrenal cortex (the outer part other glandi),

Aldosterone ( mineralocorticoid)

Cortisol ( glucocorticoid)

Cortisone (an anti-inflammatory drug)

Prednisolone (an anti-inflammatory drug)

Eicosanoids are oxygenated derivatives of polyunsaturated 20-carbon fatty acids. The metabolic precursor of most eicosanoids is arachidonic acid, the 20:4 fatty acid. The name eicosanoid is derived from the Greek word eikos, which means "twenty."

Almost all cells, except red blood cells, produce eicosanoids. These substances, like hormones, have profound physiological effects at extremely low concentrations. Eicosanoids are hormone-like molecules rather than true hormones, because they are not transported in the bloodstream to their site of action, as are true hormones; instead, they exert their effects in the tissues where they are synthesized. The physiological effects of eicosanoids include mediation of:

1. The inflammatory response, normal response to tissue damage

2. The production of in and fever

3. The regulation of blood pressure

4. The induction of blood clotting

5. The control of reproduction functions, such as induction of labor

6. The regulation of the sleep/wake cycle

There are three principal types of eicosanoids: prostaglandins, thromboxanes, and leukotrienes.

Prostaglandins are 20-carbon ftty acid derivatives that contain cyclopentane ring and oxygen-containing functional groups. Twenty-carbon fatty acids are converted into prostaglandin structure when the eighth and twelfth carbon atoms of the fatty acid become connected to form five-membered ring.

Prostaglandins are named after the prostate gland, which was first thought to be their only source. Today, more than 20 prostaglandins have been discovered in variety of tissues in both males and females.

Within the human body, prostaglandins are involved in many regulatory functions, including raising body temperature, inhibiting the secretion of gastric juices, relaxing and contracting smooth inl, directing water and electrolyte balance, intensifying pain, and enhancing inflammation responses. Aspirin reduces inflammation and fever because it inactivates the enzyme needed for prostaglandin synthesis.

Thromboxanes are 20-carbon fatty acid derivatives that contain cyclic ether ring and oxygen-containing functional groups. As with prostaglandins, the cyclic structure involves bond between carbons 8 and 12. An important function of thromboxanes is to promote the formation of blood clots. Thromboxanes are produced by blood platelets and promote platelet aggregation.

Leukotrienes are 20-carbon fatty acid derivatives that contain three conjugated double bonds and hydroxy groups. Fatty acids and their derivatives do not normally contain conjugated double bonds, as is the case in leukotrienes. Leukotrienes are found in leukocytes (white blood cells). Their source and the presence of the three conjugated double bonds account for their name. Various inflammatory and hypersensitivity (allergy) responses are associated with elevated levels of leukotrienes. The development of drugs that inhibit leukotriene synthesis has been an active area of research.

Isoprenoids are not synthesized form isoprene (methylbutadiene). Instead, their biosynthetic pathways all begin with the formation of isopentenyl pyrophosphate from acetyl-CoA.

The isoprenoids consist of terpenes and steroids. Terpenes are an enormous group of molecules that are found largely in the essential oils of plants. Steroids are derivatives of complex hydrocarbon ring system.

The terpenes are classiffied according to the number of isoprene residues they contain. Monoterprnes are composed of two isoprene units.Geraniol is a monoterpene found in oil of geranium. Terpenes that contain three isoprenes are referred to as sesquiterpenes. Fernesene, an important constituent of oil of citronella (a substance used in soap and perfumes), is a sesquterpene. Phytol, a plant alcohol, is an example of the diterpenes, molecules composed of four isoprene units. Squalene, which is found in large quantities in shark liver oil as well as olive oil and yeast, is a prominent example of the triterpenes. Squalene is an intermediate in the synthesis f the steroids). Carotenoids, the orange-colored pigment found in most plants, are the only example of the tetraterpenes (molecules composed of eight isoprene units). The cerotenes are hydrocarbon members of this group. The xanthophylls are oxygenated derivatives of the carotenes. Polyterpenes are high-molecular-weight molecules composed of hudreds or thousands of isoprene units. natural rubber is a polyterpene composed of between 3000 and 6000isoprene units.

Severol important biomolecules are composed of nonterpene components attached to isoprenoid groups (aften referred to as prenyl or isoprenyl groups). Examples of these biomolecules, referred to as mixed terpenoids, include vitamin E (a-tocopherol, ubiquione, vitamin K, and some cytokinins(plant hormones).

It has recently become apparent that a variety of proteins in eukaryotic cells are covalently attached to prenyl groups that are most often involved in his process, referred to as prenylation, are farnesyl and geranylgeranyl groups. The function of protein prenylation is not clear. There is some avidence that it plays a role in the control of cell growth. For example, Res proteins, a group of cell growth regulators, are activated by prenylation reactions.

 

 

 

Role of Lipids

The main purpose of lipids is to store energy.

Other very important function include:

-      structural elements (cell membrane)

-      dissolving fat soluble vitamins

-      transportation of some molecules through the blood

-      emulsifying agent (bile salts)

For many years, lipids were considered to be intractable and uninteresting oily materials with two main functions to serve as a source of energy and as the building blocks of membranes. They were certainly not considered to be appropriate candidates for such important molecular tasks as intracellular signalling or local hormonal regulation. In 1929, George and Mildred Burr demonstrated that linoleic acid was an essential dietary constituent, but it was many years before the importance of this finding was recognized by biochemists in general. With the discovery by Bergström, Samuelsson and others in 1964 that the essential fatty acid arachidonate was the biosynthetic precursor of the prostaglandins with their effects on inflammation and other disease states, the scientific world in general began to realize that lipids were much more interesting than they had previously thought.

A major milestone was achieved in 1979 with the discovery of the first biologically active phospholipid, platelet-activating factor. At about the same time, there arose an awareness of the distinctive functions of phosphatidylinositol and its metabolites. Since then, virtually every individual lipid class has been found to have some unique biological role that is distinct from its function as a source of energy or as a simple construction unit of a membrane. Indeed it is now recognised that lipids in membranes function also in the trafficking of cellular constituents, the regulation of the activities of membrane proteins and signalling.

All multi-cellular organisms, use chemical messengers to send information between organelles and to other cells and as relatively small hydrophobic molecules, lipids are excellent candidates for signalling purposes. The fatty acid constituents have well-defined structural features, such as cis-double bonds in particular positions, which can carry information by binding selectively to specific receptors. In esterified form, they can infiltrate membranes or be translocated across them to carry signals to other cells. During transport, they are usually bound to proteins so their effective solution concentrations are very low, and they are can be considered to be inactive until they reach the site of action and encounter the appropriate receptor.

Storage lipids, such as triacylglycerols, in their cellular context are inert, and indeed esterification with fatty acids may be a method of de-activating steroidal hormones, for example, until they are actually required. In contrast, polar phospholipids have both hydrophobic and hydrophilic sites that can bind via various mechanisms to membrane proteins and influence their activities. Glycosphingolipids carry complex carbohydrate moieties that have a part to play in the immune system, for example. Lipids have been implicated in a number of human disease states, including cancer and cardiovascular disease, sometimes in a detrimental and sometimes in a beneficial manner. In short, every scientist should now be aware that lipids are just as fascinating as all the other groups of organic compound that make up living systems.

 

Saturated fatty acids have

     Single CC bonds.

     Molecules that fit closely together in a regular pattern.

     High melting points that make them solids at room temperature.

 

Unsaturated fatty acids

Unsaturated fatty acids are long-chain carboxylic acid containing one or more carboncarbon double bonds.

 

Unsaturated fatty acids

     Have one or more double C=C bond

     Typically contain cis double bonds.

     Have low melting points.

     Are liquids at room temperature.

 

 

The two carbon atoms in the chain that are bound next to either side of the double bond can occur in a cis or trans configuration. In most naturally occurring unsaturated fatty acids all are cis bonds. Most fatty acids in the trans configuration (trans fats) are not found in nature and are the result of human processing (e.g., hydrogenation).

 

 

 

Essential fatty acids

Fatty acids that are required by the body but cannot be made in sufficient quantity from other substrates, therefore must be obtained from food and are called essential fatty acids. Two fatty acids are essential in humans, linoleic acid and linolenic acid. They are widely distributed in plant oils.

 

Omega-3 and Omega-6 Fatty Acids

Common names have been also developed; certain fatty acid names have been popularized by the media. The acids can be named for how far the double bond lays away from the final tail carbon. Linolenic acid has a double bond, three carbons from the fatty acids end. It is classified as an omega-3 fatty acid, the omega carbon being the terminal carbon and the bond being found on the third carbon from the end. Linoleic acid is an Ω-6 fatty acid.

Unsaturated fats such as those in vegetable oils and fish are recognized as more beneficial to health than saturated fats.

Vegetables contain omega-6 acids, meaning the first double bond occur at carbon 6. Examples of omega-6 acids are linoleic and arachidonic acids.

Fish have high levels of omega-3 acids, meaning the first double bond occur at carbon 3. Examples of omega-3 acids include linolenic, eicosapentaenoic, and docosahexaenoic acids.

Cold-water fish are a source of omega-3 fatty acids.

: : : : http://t3.gstatic.com/images?q=tbn:ANd9GcTQ8nysrPpS5wHwMS1Xue1GEsw38ze7mK40nR3o820_c7SnmH4e3g : : : : http://20-20-24-7.com/wp-content/uploads/2010/11/omega-3-s15-photo-of-assortment-of-fish-300x203.jpg

 

 

Glycerides

 

Triglycerides (triglycerols) consist of

a glycerol esterifies with three fatty acids.

They a formed by dehydration reactions in which the three OH groups of a glycerol molecule react with the carboxylic acid groups of three different or same fatty acids to form three ester bonds and also three molecules of water.

 

 

These triglycerides (or triacylglycerols) are found in both plants and animals, and compose one of the major food groups of our diet. Tryglicerides that are solid or semisolid at room temperature are classified as fats, and occur predominantly in animals. Those triglycerides that are liquid are called oils and originate chiefly in plants, although triglycerides from fish are also largely oils.

As might be expected from the properties of the fatty acids, fats have a predominance of saturated fatty acids and oils are composed largely of unsaturated acids.

Triglycerides having three identical acyl chains, such as tristearin and triolein are called simple, while those composed of different acyl chains are called mixed (such as oleopalmitostearin).

 

 

Reactions

Hydrolysis Reaction

Just as the fats can be assembled (or synthesized) by means of a dehydration reaction, they can also be dissembled by hydrolysis reactions to reform the glycerol and fatty acids which were present when it was put together. In our bodies, this reaction is generally carried out in the presence of enzymes that catalyze the hydrolysis reaction.

Saponification Reaction

This reaction can take place outside our bodies. When a fat is heated with a strong base such as sodium hydroxide, saponification of the fat gives glycerol and the sodium salts of the fatty acids, which are soaps. KOH produces a softer, liquid soap. Oils that are polyunsaturated produce softer soaps. Names like coconut or avocado shampoo tell you the sources of the oil used in the reaction.

Soap is mainly used for washing, bathing and cleaning.

When used for cleaning, soap serves as a surfactant in conjunction with water. The cleaning action of this mixture is attributed to the action of micelles, tiny sphres coated on the outside with polar carboxylate groups, encasing a hydrophobic (lipophilic) pocket that can surround the grease particles, allowing them to dissolve in water.

 

 

The hydrophobic portion is made up of the long hydrocarbon chain from the fatty acid. In other words, whereas normally oil and water do not mix, the addition of soap allows oils to dissolve in water, allowing them to be rinsed away. Synthetic detergents operate by similar mechanisms to soap.

 

When a dirty cloth is put in water containing soap than the hydrocarbon ends of the soap molecule in the micelle attach to the oil or grease particles present on the surface of dirty cloth. In this way the soap micelles entraps the oily particles by using the hydrocarbon ends. The ionic ends of the soap molecules remain attached to the water when the dirty cloth is agitated in soap solution. The oily particles presents on its surface gets dispersed in the water due to which the cloth gets clean

 

Soaps and Detergents

Carboxylic acids and salts having alkyl chains longer than eight carbons exhibit unusual behavior in water due to the presence of both hydrophilic (CO2) and hydrophobic (alkyl) regions in the same molecule. Such molecules are termed amphiphilic (Gk. amphi = both) or amphipathic. Fatty acids made up of ten or more carbon atoms are nearly insoluble in water, and because of their lower density, float on the surface when mixed with water. Unlike paraffin or other alkanes, which tend to puddle on the waters surface, these fatty acids spread evenly over an extended water surface, eventually forming a monomolecular layer in which the polar carboxyl groups are hydrogen bonded at the water interface, and the hydrocarbon chains are aligned together away from the water. This behavior is illustrated in the diagram on the right. Substances that accumulate at water surfaces and change the surface properties are called surfactants.

Alkali metal salts of fatty acids are more soluble in water than the acids themselves, and the amphiphilic character of these substances also make them strong surfactants. The most common examples of such compounds are soaps and detergents, four of which are shown below. Note that each of these molecules has a nonpolar hydrocarbon chain, the "tail", and a polar (often ionic) "head group". The use of such compounds as cleaning agents is facilitated by their surfactant character, which lowers the surface tension of water, allowing it to penetrate and wet a variety of materials.

: : : : http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Images3/surfact.gif


Very small amounts of these surfactants dissolve in water to give a random dispersion of solute molecules. However, when the concentration is increased an interesting change occurs. The surfactant molecules reversibly assemble into polymolecular aggregates called micelles. By gathering the hydrophobic chains together in the center of the micelle, disruption of the hydrogen bonded structure of liquid water is minimized, and the polar head groups extend into the surrounding water where they participate in hydrogen bonding. These micelles are often spherical in shape, but may also assume cylindrical and branched forms, as illustrated on the right. Here the polar head group is designated by a blue circle, and the nonpolar tail is a zig-zag black line.

An animated display of micelle formation is presented below. Notice the brownish material in the center of the three-dimensional drawing on the left. This illustrates a second important factor contributing to the use of these amphiphiles as cleaning agents. Micelles are able to encapsulate nonpolar substances such as grease within their hydrophobic center, and thus solubilize it so it is removed with the wash water. Since the micelles of anionic amphiphiles have a negatively charged surface, they repel one another and the nonpolar dirt is effectively emulsified. To summarize, the presence of a soap or a detergent in water facilitates the wetting of all parts of the object to be cleaned, and removes water-insoluble dirt by incorporation in micelles.

: : : : http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Images3/micelles.gif

The oldest amphiphilic cleaning agent known to humans is soap. Soap is manufactured by the base-catalyzed hydrolysis (saponification) of animal fat (see below). Before sodium hydroxide was commercially available, a boiling solution of potassium carbonate leached from wood ashes was used. Soft potassium soaps were then converted to the harder sodium soaps by washing with salt solution. The importance of soap to human civilization is documented by history, but some problems associated with its use have been recognized. One of these is caused by the weak acidity (pKa ca. 4.9) of the fatty acids. Solutions of alkali metal soaps are slightly alkaline (pH 8 to 9) due to hydrolysis. If the pH of a soap solution is lowered by acidic contaminants, insoluble fatty acids precipitate and form a scum. A second problem is caused by the presence of calcium and magnesium salts in the water supply (hard water). These divalent cations cause aggregation of the micelles, which then deposit as a dirty scum.
These problems have been alleviated by the development of synthetic amphiphiles called detergents (or syndets). By using a much stronger acid for the polar head group, water solutions of the amphiphile are less sensitive to pH changes. Also the sulfonate functions used for virtually all anionic detergents confer greater solubility on micelles incorporating the alkaline earth cations found in hard water. Variations on the amphiphile theme have led to the development of other classes, such as the cationic and nonionic detergents shown above. Cationic detergents often exhibit germicidal properties, and their ability to change surface pH has made them useful as fabric softeners and hair conditioners. These versatile chemical "tools" have dramatically transformed the household and personal care cleaning product markets over the past fifty years.

 

Hydrogenation Reaction

Hydrogenation of unsaturated fats converts carbon-carbon double bonds to single bonds. The hydrogen gas bubbled through the heated oil the presence of a nickel catalyst (or another transition metal).

 

Margarine and shortening originate from vegetable oils (i.e. soybean oil and sunflower oil) that have been hydrogenated. This process is called hardening of oils.

 

Halogenation

Neutral fats containing unsaturated fatty acids have the ability of adding halogens (e.g., hydrogen or hydrogenation and iodine or iodination) at the double bonds. It is a very important property to determine the degree of unsaturation of the fat or oil that determines its biological value

Oxidation (Rancidty)

This toxic reaction of triglycerides leads to unpleasant odour or taste of oils and fats developing after oxidation by oxygen of air, bacteria, or moisture. Also this is the base of the drying oils after exposure to atmospheric oxygen.

Rancidity it is a physico-chemical change in the natural properties of the fat leading to the development of unpleasant odor or taste or abnormal color particularly on aging after exposure to atmospheric oxygen, light, moisture, bacterial or fungal contamination and/or heat. Saturated fats resist rancidity more than unsaturated fats that have unsaturated double bonds.

 

 

Prevention of rancidity is achieved by:

Avoidance of the causes (exposure to light, oxygen, moisture, high temperature and bacteria or fungal contamination). By keeping fats or oils in well-closed containers in cold, dark and dry place (i.e., good storage conditions).

Addition of anti-oxidants to prevent peroxidation in fat (i.e., rancidity). They include phenols, naphthols, tannins and hydroquinones. The most common natural antioxidant is vitamin E that is important in vitro and in vivo.

 

For describe the fat composition using the following number:

Iodine number (or iodine value or iodine index) is the mass of iodine in grams that is consumed by 100 grams of fat. Iodine number uses for determination of the amount of unsaturation contained in fatty acids. This unsaturation is in the form of double bonds which react with iodine compounds. The higher the iodine number, the more unsaturated fatty acid bonds are present in a fat.

Saponification number (or saponification value) represents the number of milligrams of potassium hydroxide (KOH) or sodium hydroxide (NaOH) required to saponify 1 g of fat under the conditions specified. The long chain fatty acids found in fats have low saponification value because they have a relatively fewer number of carboxylic functional groups per unit mass of the fat as compared to short chain fatty acids. It more moles of base are required to saponify N grams of fat then there are more moles of the fat the chain lengths are relatively small.

Acid number (or acid value, or neutralization number) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize free fatty acids which present in one gram of fat. As oil-fats rancidity, triglycerides are converted into fatty acids and glycerol, causing an increase in acid number.

 

Saturated fat

Everyone should eat less saturated fat. Doctors believe that people who eat saturated fat diets increase their risk of developing life-threatening cholesterol deposits in the lining of their arteries.

Sources:

                     High-fat dairy products such as full-fat cheese, cream, ice cream, whole milk, 2% milk and sour cream.

                     High-fat meats like regular ground beef, bologna, hot dogs, sausage, bacon and spareribs

                     Lard

                     Butter

                     Fatback and salt pork

                     Cream sauces

                     Gravy made with meat drippings

                     Chocolate

                     Palm oil and palm kernel oil

                     Coconut and coconut oil

                     Poultry (chicken and turkey) skin

                     Cream

Try to eat less than 10% of your total calories as saturated fat. For most people this means less than 20 grams of saturated fat per day. For people with high cholesterol, this is less than 15 grams of saturated fat per day. Many adults, especially women or sedentary men, may need less.

Trans fat

Trans fats are produced when liquid oil is made into a solid fat. This process is called hydrogenation. Trans fats act like saturated fats and can raise your cholesterol level. Sources of trans fat include:

                     Processed foods like snacks (crackers and chips) and baked goods (muffins, cookies and cakes) with hydrogenated oil or partially hydrogenated oil

                     Stick margarines

                     Shortening

                     Some fast food items such as french fries.

Unsaturated fat

Unsaturated fat subdivided into two groups: monounsaturated fat and polyunsaturated fat. These fats, basically, are liquid in the room temperature, but some of them are solid (e.g. margarine).

Monosaturated fat

Monounsaturated fats are called "good or healthy" fats because they can lower your bad (LDL) cholesterol.

Sources:

                     Avocado

                     Canola oil

                     Nuts like almonds, cashews, pecans, and peanuts

                     Olive oil and olives

                     Peanut butter and peanut oil

                     Sesame seeds

ADA recommends eating more monounsaturated fats than saturated or trans fats in your diet. To include more monounsaturated fats, try to substitute olive or canola oil instead of butter, margarine or shortening when cooking. Sprinkling a few nuts or sesame seeds on a salad is an easy way to eat more monounsaturated fats. But be careful! Nuts and oils are high in calories, like all fats. If you are trying to lose or maintain your weight, you want to eat small portions of these foods. For example, 6 almonds or 4 pecan halves have the same number of calories as 1 teaspoon of oil or butter. Work with your dietitian to include healthy fats into your meal plan without increasing your total calories.

Polyunsaturated fat

Polyunsaturated fats are also "healthy" fats. ADA recommends that you include these in your diet as well as monounsaturated fats.

 

 

Sources:

                     Corn oil

                     Cottonseed oil

                     Safflower oil

                     Soybean oil

                     Sunflower oil

                     Walnuts

                     Pumpkin or sunflower seeds

                     Soft (tub) margarine

                     Mayonnaise

                     Salad dressings

                     Flax oil.

 

Waxes

Waxes are esters of fatty acids with long chain monohydric alcohols. Waxes are biosynthesized by many plants or animals. They typically consist of several components, including wax esters, wax acids, wax alcohols and hydrocarbons. Wax esters are typically derived from a variety of carboxylic acids and a variety of fatty alcohols.

Animal waxes.

Beeswax. Glands under the abdomen of bees secrete a wax, which they use to construct the honeycomb. The wax is recovered as a by-product when the honey is harvested and refined.

A major component of beeswax is the aster myricyl palmitate.

Lanolin - The grease obtained from the wool of sheep during the cleaning or refining process is rich in wax

 

Bird waxes -Special glands of birds secrete waxes that consist largely of wax esters. The main purpose of the waxes is to give a water-proof layer to the feathers

 

Spermaceti, a wax, liquid at body temperature, obtained from the head of a sperm whale. Spermaceti was used chiefly in ointments, cosmetic creams, fine wax candles, pomades, and textile finishing; later it was used for industrial lubricants. The substance was named in the mistaken belief that it was the coagulated semen of the whale.

37C liquid (body temperature)

31C crystallizes (increase density)

One of its main constituents is cetyl palmitate:

 

 

Plant waxes

Especially in warm climates, plants secrete waxes as a way to control evaporation and hydration. Plant waxes provides protection from disease and insects, and helps the plants resist drought.

Waxes are mainly consumed industrially as components of complex formulations, often for coatings.

Waxes and hard fats such as tallow have long been use to make candles.

 

Phospholipids

 

Phospholipids are made from glycerol, two fatty acids, and a phosphate group. Phospholipids are made up of 1 glycerol molecule and only 2 fatty acid molecules (not 3!). This is a big difference because in place of the third fatty acid, phospholipids have a polar group attached to the glycerol molecule.

Remember, triglycerides were entirely hydrophobic because all three positions on the glycerol molecule were taken up by nonpolar fatty acids. But, phospholipids have a polar group in one place on the glycerol molecule, making phospholipids partly hydrophilic; the polar group helps to form the polar head.  The two fatty acids, maintain a hydrophobic portion of the phospholipid, the tails. This means that phospholipids are soluble in both water and oil.

The general form of a phospholipid depicts a polar head with two long tails. What is really happening is that the phosphate and its functional group are atop the two long fatty-acid chains.

The phosphate group in phospholipids is attached to the glycerol but has another molecule attached to its other end. The phosphate moiety of the resulting phosphatidic acid is further esterified with ethanolamine, choline or serine in the phospholipid itself.

 

Examles of phospholipids:

Lecithin and cephalin are abundant in brain and nerve tissues, found in egg yolk, wheat germ, and yeast.

Phospholipids are the most abundant lipids in cell membranes and play an important role in cellular permeability

The membrane of a cell separates the contents of a cell from the external fluids. It is semipermeable so that nutrients can enter the cell and waste products can leave. There are two rows of phospholipids in a cell membrane, that they are arranged like a sandwich. Their nonpolar tails, which are hydrophobic (water-fearing), move to the center, while their polar heads, which are hydrophilic (water-loving) align on the outer edge of the membrane. This double row arrangement of phospholipids is called a lipid bilayer.

 

 

Most of the phospholipids in the lipid bilayer contain unsaturated fatty acids. Due to the kinks in the carbon chains at the cis double bonds, the phospholipid s do not fit closely together. As a result, the lipid bilayer is not a rigid, fixed structure, but one that is dynamic and fluid -like. In this liquid-like bilayer, there are also proteins, carbohydrates, and cholesterol molecules. For this reason, the model of biological membranes is referred to as the fluid mosaic model of membranes.

 

Sphingolipids

Sphingolipids are similar to phospholipids. Contain sphingosine (a long-chain amino alcohol), a fatty acid, phosphate, and a small amino alcohol. Have polar and nonpolar regions.

Sphingosine is a long-chain unsaturated amino alcohol.

Sphingomyelins are found in large amounts in brain and nerves and in smaller amounts in lung, spleen, kidney, liver and blood

Glycosphingolipids

Glycosphingolipids contain monosaccharides bonded to the OH of sphingosine by a glycosidic bond.

They are present in cerebral tissue, therefore are called cerebrosides

According to the number and nature of the carbohydrate residue(s) present in the glycosphingolipids the following are

Cerebrosides. They have one galactose molecule (galactosides).

Gangliosides. They have several sugar and sugaramine residues.

 

Cerebrosides occur in myelin sheath of nerves and white matter of the brain tissues and cellular membranes. They are important for nerve conductance.

 

Gangliosides

Gangliosides are similar to cerebrosides, but contain two or more monosaccharides. They are more complex glycolipids that occur in the gray matter of the brain, ganglion cells. They transfer biogenic amines across the cell membrane and act as a cell membrane receptor for hormones and viruses.

Gangliosides contain sialic acid (N-acetylneuraminic acid), ceramide (sphingosine + fatty acid of 18-24 carbon atom length), 3 molecules of hexoses (1 glucose + 2 galactose) and hexosamine. The most simple type of it the monosialoganglioside. It works as a receptor for cholera toxin in the human intestine.

Steroids

Steroids constitute an important class of biological compounds. Steroids are nonhydrolyzable lipids.

Steroids are usually found in association with fat. They can be separated from fats after saponification since they occur in the unsaponifiable residue.

Steroids are compounds containing the steroid nucleus, which consists of three cyclohexane rings and one cyclopentane ring fused together (no fatty acids). The four rings in the steroid nucleus are designated A, B, C, and D. Numbered carbon atoms beginning in ring A

 

Cholesterol is the most abundant steroid in the body, has methyl groups (carbons 10, 13) an alkyl chain (carbon 17), and an OH group (carbon 3) attached to the steroid nucleus

 

Cholesterol in the body.

Cholesterol is a component of cellular membranes, myelin sheath, and brain and nerve tissue. It is also found in the liver, bile salts, and skin, where it forms vitamin D. In the adrenal gland, it is used to synthesize steroid hormones. Cholesterol in the body in obtained from eating meats, milk, and eggs, and it is also synthesized by the liver from fats, carbohydrates, and proteins. There is no cholesterol in vegetable and plant products.

If the diet is high in cholesterol, the liver produces less. A typical American daily diet includes 400-500 mg of cholesterol (one of the highest in the world). However, we should consume no more than 300 mg of cholesterol a day.

 

The number one killer of men and women over the age of 50 is heart disease. This is brought on by elevated levels of cholesterol which will block arteries. When the blockage occurs in the blood vessels supplying the heart with blood, a heart attack is very likely. Also, excess cholesterol levels can exceed the saturation level in bile, causing gallstones to form. Gallstones are almost all cholesterol with a small amount of minerals, like calcium.

Maximum cholesterol levels in the blood would be 220 mg/l of blood plasma.

 

 

When cholesterol exceeds its saturation level in the bile, gallstones may form. High levels of cholesterol are also associated with the accumulation of lipid deposits (plaque) that line and narrow the coronary arteries. Some research indicates that saturated fats in the diet may stimulate the production of cholesterol by the liver.

 

Bile Salts

Bile acids are produced from oxidation of cholesterol in the liver producing cholic and chenodeoxycholic acids that are conjugated with glycine or taurine to produce glycocholic, glycochenodeoxycholic, taurocholic and taurochenodeoxycholic acids. They react with sodium or potassium to produce sodium or potassium bile salts.

 

Bile salts are synthesized in the liver and stored in the gallbladder. Have polar and nonpolar regions that act like soaps to make fat soluble in water. Bile salts assist in the digestion of lipids and other non-soluble molecules. Help in absorption of cholesterol. Activation of pancreatic lipase. Help digestion and absorption of fat-soluble vitamins. Intestinal antiseptic that prevent putrefaction

 

When large amounts of cholesterol accumulate in the gallbladder, gallstones are formed. If a gallstone is large enough it may block the channel used to release bile and the person will suffer from jaundice and turn yellow. One other problem is these stones can cause extreme pain.

 

Steroid hormones

Hormones are chemical messengers that serve as a kind of communication system from one part of the body to another. The steroid hormones, which include the sex hormones are closely related in structure to cholesterol and depend on cholesterol for their synthesis. Two important male sex hormones, testosterone and androsterone, promote the growth of muscle and of facial hair and the maturation of the male sex organs and of sperm. The estrogens, a group of female sex hormones, direct the development of female characteristics: the uterus increases in size, fat is deposited in the breasts, and the pelvis broadens. Progesterone prepares the uterus for the implantation of a fertilized egg.

Adrenal corticosteroids are produced by the adrenal glands located on the top of each kidney. Include aldosterone, which regulates electrolytes and water balance by the kidneys. Include cortisone, a glucocorticoid, which increases blood glucose level and stimulates the synthesis of glycogen in the liver.

 

Lipoproteins

Lipids are nonpolar and made more soluble by combining them with glycerophospholipids and proteins to form water-soluble complexes called lipoproteins.

Lipoproteins surround nonpolar lipids with polar lipids and protein for transport to cells. They are soluble in water because the surface consists of polar lipids

Lipoproteins in the body.

Lipids must be transported through the bloodstream to tissues where they are stored, used for energy, or to make hormones. However, most lipids are nonpolar and insoluble in the aqueous environment of blood. They are made more soluble by combining them with phospholipids and proteins to form water-soluble complexes called lipoproteins.

 

 

In general, lipoproteins are spherical particles with an outer surface of polar proteins and phospholipids that surround hundreds of nonpolar molecules of triacylglycerols and cholesteryl esters (formed by the esterification of the hydroxyl group in cholesterol with a fatty acid).

There are two main categories of lipoproteins distinguished by how compact/dense they are. LDL or low density lipoprotein is the bad guy, being associated with deposition of cholesterol on the walls of someones arteries. HDL or high density lipoprotein is the good guy, being associated with carrying cholesterol out of the blood system, and is more dense/more compact than LDL.

HIGH DENSITY LIPOPROTEINS (HDL)

More protein; less cholesterol

Transports cholesterol from cells back to the liver

 

LOW DENSITY LIPOPROTEINS (LDL)

Less protein, more cholesterol

Transports cholesterol from the liver to cells

 

 

Prostaglandins Thromboxanes & Leukotrienes

The members of this group of structurally related natural hormones have an extraordinary range of biological effects. They can lower gastric secretions, stimulate uterine contractions, lower blood pressure, influence blood clotting and induce asthma-like allergic responses. Because their genesis in body tissues is tied to the metabolism of the essential fatty acid arachadonic acid (5,8,11,14-eicosatetraenoic acid) they are classified as eicosanoids. Many properties of the common drug aspirin result from its effect on the cascade of reactions associated with these hormones.

The metabolic pathways by which arachidonic acid is converted to the various eicosanoids are complex and will not be discussed here. A rough outline of some of the transformations that take place is provided below. It is helpful to view arachadonic acid in the coiled conformation shown in the shaded box.

: : : : http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Images3/eicosoid.gif

Leukotriene A is a precursor to other leukotriene derivatives by epoxide opening reactions. The prostaglandins are given systematic names that reflect their structure. The initially formed peroxide PGH2 is a common intermediate to other prostaglandins, as well as thromboxanes such as TXA2.

Lipid Soluble Vitamins

The essential dietary substances called vitamins are commonly classified as "water soluble" or "fat soluble". Water soluble vitamins, such as vitamin C, are rapidly eliminated from the body and their dietary levels need to be relatively high. The recommended daily allotment (RDA) of vitamin C is 100 mg, and amounts as large as 2 to 3 g are taken by many people without adverse effects. The lipid soluble vitamins, shown in the diagram below, are not as easily eliminated and may accumulate to toxic levels if consumed in large quantity. The RDA for these vitamins are:

Vitamin A   800 μg ( upper limit ca. 3000 μg)
Vitamin D   5 to 10
μg ( upper limit ca. 2000 μg)
Vitamin E   15 mg ( upper limit ca. 1 g)
Vitamin K   110
μg ( upper limit not specified)

From this data it is clear that vitamins A and D, while essential to good health in proper amounts, can be very toxic. Vitamin D, for example, is used as a rat poison, and in equal weight is more than 100 times as poisonous as sodium cyanide.

: : : : http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Images3/vitamin1.gif

From the structures shown here, it should be clear that these compounds have more than a solubility connection with lipids. Vitamins A is a terpene, and vitamins E and K have long terpene chains attached to an aromatic moiety. The structure of vitamin D can be described as a steroid in which ring B is cut open and the remaining three rings remain unchanged. The precursors of vitamins A and D have been identified as the tetraterpene beta-carotene and the steroid ergosterol, respectively.

 

Polyfunctional compounds

 

Molecules with more than one functional groups, called polyfunctional.

Hydroxy acids, are a class of chemical compounds that consist of carboxylic and hydroxyl groups.

Oxo acid include aldehydo- or keto- group besides carboxyl group.

Example of hydroxyl acids:

 

Glycolic acid

Lactic acid

Tartaric acid

Malic acid

Citric acid

γ (gamma)-hydroxybutyric acid

Example of oxo carboxylic acids

 

oxaloacetic acid

α-ketoglutaric acid

acetoacetic acid

 

 

Nomenclature of Hydroxy Carboxylic Acid

 

Specific Properties of Hydroxy Carboxylic Acid

Alpha-hydroxy acids contain both hydroxyl and carboxyl functional groups, they can undergo self-esterification when heated to form cyclic six-membered diesters. These diesters are sometimes referred to generically as lactides. However, the term "lactide" is used herein to refer only to the cyclic diester of lactic acid.

Alpha hydroxy acids (AHAs) are naturally occurring carboxylic acids with a hydroxyl group (-OH) on the carbon adjacent to the carboxyl group.

 

When heated, α-hydroxy-acid can dehydrate to form lactide, β- hydroxy-acid can form α,β-unsaturated carboxylic acid, while γ-hydroxy-acid can form γ-lactone, depending on the positions of the hydroxyl group of hydroxyl acid.

 

 

 

Glycolic acid

Glycolic acid (or hydroxyacetic acid) is the smallest α-hydroxy acid. This colorless, odorless and hygroscopic crystalline solid is highly soluble in water.

Due to its excellent capability to penetrate skin, glycolic acid finds applications in skin care products, most often as a chemical peel performed by a dermatologist or plastic surgeon in concentrations of 20 to 70 % or at-home kits in lower concentrations between 10 and 20 %.

Glycolic acid is used to improve the skin's appearance and texture. It may reduce fine lines wrinkles, acne scarring, hyperpigmentation and improve many other skin conditions, including actinic keratosis, hyperkeratosis, and seborrheic keratosis. Once applied, glycolic acid reacts with the upper layer of the epidermis, weakening the binding properties of the lipids that hold the dead skin cells together. This allows the stratum corneum to be exfoliated, exposing live skin cells.

Glycolic acid is also a useful intermediate for organic synthesis, in a range of reactions including: oxidation-reduction, esterification and long chain polymerization. It is used as a monomer in the preparation of polyglycolic acid and other biocompatible copolymers (e.g. PLGA). Among other uses this compound finds employment in the textile industry as a dyeing and tanning agent, in food processing as a flavoring agent and as a preservative. Glycolic acid is often included into emulsion polymers, solvents and additives for ink and paint in order to improve flow properties and impart gloss.

 

Lactic acid

Lactic acid, also known as milk acid, is chemical compound that plays a role in several biochemical processes.

Lactic acid in the form of its salt (lactate) is prodused in muscle tissue as a result of the anaerobic breakdown of glucose. Excess lactate is the cause of muscle soreness produced after strenuous exercise when the bodys supply of oxygen is reduced.

Lactic acid is found primarily in sour milk products, such as koumiss, leban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid. Lactic acid is also responsible for the sour flavor or sourdough breads. This acid is used in beer brewing to lower the pH and increase the body of the beer.

 

Tartaric acid

Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds, and is one of the main acids found in wine. It is added to other foods to give a sour taste and is used as an antioxidant. Salts of tartaric acid are known as tartrates.

Fellings reagent an aqueous solution of copper sulfate, sodium hydroxide, and potassium sodium tartrate used to test for the presence of sugars and aldehydes in a substance, such as urine.

 

Citric acid

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Citric acid is a natural component and common metabolite of plants and animals. It is the most versatile and widely used organic acid in foods, beverages, and pharmaceuticals. Because of its functionality and environmental acceptability, citric acid and its salts (primarily sodium and potassium) are used in many industrial applications for chelation, buffering, pH adjustment, and derivatization. These uses include laundry detergents, shampoos, cosmetics, enhanced oil recovery, and chemical cleaning. Citric acid occurs in the terminal oxidative metabolic system of virtually all organisms. This oxidative metabolic system, variously called the Krebs cycle, the tricarboxylic acid cycle, or the citric acid cycle, is a metabolic cycle involving the conversion of acetate derived from carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organisms growth, movement, luminescence, chemosynthesis, and reproduction.

Citric acid decomposed in presence of sulphuric acid.

 

 

Pyruvic acid

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Pyruvate is an important chemical compound in biochemistry. It is the output of the anaerobic metabolism of glucose known as glycolysis. One molecule of glucose breaks down into two molecules of pyruvate, which are then used to provide further energy, in one of two ways. Pyruvate is converted into acetyl-coenzyme A, which is the main input for a series of reactions known as the Krebs cycle. Pyruvate is also converted to oxaloacetate by an anaplerotic reaction which replenishes Krebs cycle intermediates; alternatively, the oxaloacetate is used for gluconeogenesis.

These reactions are named after Hans Adolf Krebs, the biochemist awarded the 1953 Nobel Prize for physiology, jointly with Fritz Lipmann, for research into metabolic processes. The cycle is also called the citric acid cycle, because citric acid is one of the intermediate compounds formed during the reactions.

If insufficient oxygen is available, the acid is broken down anaerobically, creating lactate in animals and ethanol in plants and microorganisms. Pyruvate from glycolysis is converted by anaerobic respiration to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation, or to acetaldehyde and then to ethanol in alcoholic fermentation.

Pyruvate is a key intersection in the network of metabolic pathways. Pyruvate can be converted into carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine and to ethanol. Therefore it unites several key metabolic processes.

Aromatic acids

Aromatic acids include compounds that contain a COOH group bonded to an aromatic ring. The simplest aromatic acid is benzoic acid.

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Aromatic carboxylic acids show not only the acidity and other reactions expected of carboxylic acids (as an acid, benzoic acid is slightly stronger than acetic acid) but, similar to other aromatic compounds, also undergo electrophilic substitution reactions. The COOH group is deactivating, meaning electrophilic substitutions take place less readily than with benzene itself (Friedel-Crafts reactions do not occur), and meta-directing, meaning that the incoming entity will enter at a position meta to the COOH group, rather than at an ortho or para position, as in, for example, the nitration of benzoic acid.

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Benzoic acid, a solid at room temperature (melting point 122 C [252 F]), was first described in 1560, having been prepared by distilling gum benzoin, a resin obtained from certain Asian trees. It occurs in various plants, both in free acid form and in ester form. It is also a constituent of the urine of certain animals, especially horses, as an amide of glycine called hippuric acid, C6H5CONHCH2COOH. The sodium salt, sodium benzoate, is used as a preservative in many foods.

Some other important aromatic acids include the following:

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Salicylic acid is both a carboxylic acid and a phenol, so it can be esterified in two ways, with both giving rise to familiar products. In methyl salicylate (oil of wintergreen), the COOH group of salicylic acid is esterified with methanol (CH3OH), whereas in acetylsalicylic acid (aspirin) the acid component of the ester is acetic acid, and salicylic acid contributes the phenolic −OH group.

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Gallic acid is found in tea, as well as in other plants, and it also occurs as part of a larger molecule, called tannin, which is present in galls (such as the swellings of the tissue of oak trees caused by the attack of wasps). Tannins are used in making leather, and gallic acid is employed in the production of inks. Three of the most important aromatic dicarboxylic acids are called phthalic, isophthalic, and terephthalic acid, for the ortho, meta, and para isomers, respectively. Phthalic acid is converted to its anhydride simply by heating. Phthalic anhydride is used to make polymeric resins called alkyd resins, which are used as coatings, especially for appliances and automobiles. The paraisomer, terephthalic acid, is also used to make polymersnamely, polyesters.

Several important acids contain an aromatic ring but, because the carboxyl group is not bonded directly to it, they are not considered to be aromatic acids.

Phenylacetic acid is used to synthesize many other organic compounds.

Mandelic acid is toxic tobacteria in acidic solution and is used to treat urinary infections.

Cinnamic acid, an unsaturated carboxylic acid, is the chief constituent of the fragrant balsamic resin storax.

Ibuprofen and naproxen are important painkilling and anti-inflammatory drugs. Ibuprofen is sold over-the-counter under proprietary names such as Advil and Nuprin. Naproxen is sold under names such as Aleve. Both ibuprofen and naproxen have a stereocentre and are chiral.