CHAPTER X

FATS AND OILS, WAXES, AND LIPOIDS

Included in this group are several different kinds of compounds which have similar physical properties, and which, in general, belong to the type of organic compounds known as esters, i.e., alcoholic salts of organic acids. The terms "oil," "fat," and "wax," are generally applied more or less indiscriminately to any substance which has a greasy feeling to the touch and which does not mix with, but floats on, water. There are many oils which are of mineral origin which are entirely different in composition from natural fats. These have no relation to plant life and will not be considered here.

The natural fats, vegetable oils, and plant waxes are all esters. There is no essential difference between a fat and an oil, the latter term being usually applied to a fat which is liquid at ordinary temperatures. The waxes, however, are different in chemical composition from the fats and oils, being esters of monohydric alcohols of high molecular weight, such as cetyl alcohol, C_{16}H_{33}OH, myristic alcohol, C_{30}H_{61}OH, and cholesterol, C_{27}H_{45}OH; whereas the fats and oils are all esters of the trihydric alcohol glycerol, C_{3}H_{5}(OH)_{3}. Lipoids are much more complex esters, having some nitrogenous, or phosphorus-containing, group and sometimes a sugar in combination with the fatty acids and glycerol which make up the characteristic part of their structure.

In general, waxes and lipoids have a harder consistency than fats: but this is not always the case, since "wool-fat" and spermaceti, both of which are true waxes in composition, are so nearly liquid in form as to be commonly called fats; while certain true fats, like "Japan wax," are so hard as to be commonly designated as waxes. It is plain that physical properties alone cannot be relied upon in the classification of these bodies. In fact, there is no single definite property by which members of this group can be accurately identified. There are many other types of substances belonging to entirely different chemical groups, which have oily, or fat-like, properties.

A. FATS AND OILS

OCCURRENCE

Fats and oils are widely distributed in plants. They occur very commonly in the reproductive organs, both spores and seeds, as reserve food material. In fungi, oils are often found in the spores, but sometimes also in sclerotia, mycelia, or filaments. For example, the sclerotia of ergot have been found to contain as much as 60 per cent of oil. In higher plants, many seeds contain high percentages of oil, so as to make them commercial sources for edible or lubricating oils, such as olive oil, rape-seed oil, cottonseed oil, castor oil, corn oil, sunflower-seed oil, etc., etc. Nuts often contain large proportions of oil, the kernel of the Brazil nut, for example, sometimes contains as high as 70 per cent of oil, while an oil content of 50 per cent, or more, is common in almonds, walnuts, etc.

Oils also occur as reserve food material in other storage organs of plants, such as the tubers of certain flowering plants, and the roots of many species of orchids. Sometimes the appearance of oils in the stems of trees, or the winter leaves of evergreens, seems to be only temporary and to occur only during periods of very low temperatures.

Much less frequently, fats or oils are found in the vegetative organs of plants, as in the leaves of evergreens. Their appearance and functions in these organs seem to be much less certain than in the other cases cited above; although in rare cases a considerable proportion of oily material has been found to exist in definite association with the chloroplasts.

The vegetable fats and oils have many important industrial uses. Some of them, such as olive oil, cottonseed oil, cocoanut oil, etc., are largely used as human food. Others, as castor oil, are used as lubricants. The so-called "drying oils" (see page 132), such as linseed oil, etc., are used in the manufacture of paints and varnishes. Some cheap vegetable oils are used as the basis for the manufacture of soaps, etc. Hence, industrial plants and processes for the extraction of oils from plant tissues are of very great economic importance.

CHEMICAL CONSTITUTION

The fats (of either plant or animal origin) are glycerides, i.e., glycerol esters of organic acids. As has been pointed out, esters are derived from organic acids and alcohols in exactly the same way that mineral salts are derived from inorganic acids and metallic bases.

-------- Thus, Na|OH + H|ONO_2(HNO_{3}) = NaNO_{3} + H_{2}O -------- Base Acid Salt

----------- and, C_{2}H_{5}|OH + H|OOC·H = C_{2}H_{5}OOC·H + H_{2}O ----------- Alcohol Acid Ester

--------------- or, R·|OH + H|OOC·R = R·OOC·R + H_{2}O --------------- Any alcohol Any acid Any ester

Glycerol is, however, a trihydric alcohol, i.e., it contains three replaceable (OH) groups. Its formula is C_{3}H_{5}(OH)_{3}, or CH_{2}OH·CHOH·CH_{2}OH. Hence, three molecules of a monobasic acid are required to replace all of its (OH) groups.

For example,

C_{2}H_{5}OH + HOOC·C_{17}H_{35} = CH_{2}OOC·C_{17}H_{35} | | CHOH + HOOC·C_{17}H_{35} = CHOOC·C_{17}H_{35} + 3H_{2}O | | C_{2}H_{5}OH + HOOC·C_{17}H_{35} = CH_{2}OOC·C_{17}H_{35}

It is theoretically possible, of course, to replace either one, two, or three of the (OH) groups in the glycerol with acid radicals, thus producing either mono-, di-, or triglycerides. If the primary alcohol groups in the glycerine molecule are designated by (1) and the secondary one by (2), thus, CH_{2}OH(1)·CHOH(2)·CH_{2}OH(1), it is conceivable that there may be either (1) or (2) monoglycerides, either (1, 1) or (1, 2) diglycerides, or a triglyceride, depending upon which of the (OH) groups are replaced. Compounds of all of these types have been produced by combinations of glycerol with varying proportions of organic acids under carefully controlled conditions; and all of them found to possess fat-like properties. All natural fats are triglycerides, however. Most natural fats are mixtures of several different triglycerides in each of which the three (OH) groups of the glycerol has been replaced by the same organic acid radical, as in the example of stearin shown above. But recent investigations have shown that some of the common animal fats, and perhaps some plant oils, may be made up of mixed glycerides, i.e., those in which the different (OH) groups have been replaced by different acid groups, as oleo-stearin, oleo-stearo-palmitin, etc.

THE ACIDS WHICH OCCUR IN NATURAL FATS

The acids which, when combined with glycerol, produce fats are of two general types. The first of these are the so-called "fatty acids" having the general formula C_{_n_}H_{2_n_+1}·COOH. These are the "saturated" acids, i.e., they contain only single-bond linkages in the radical which is united to the ·COOH group; hence, they cannot take up hydrogen, oxygen, etc., by direct addition. The second type are the "unsaturated" acids belonging to several different groups, as discussed below, but all having one or more double-linkages between the carbon atoms of the alkyl radical which they contain. Because of these double linkages, they are all able to take on oxygen, hydrogen, or the halogen elements, by direct addition. When exposed to the air, for example, these "unsaturated" acids, or the oils derived from them, take up oxygen, increasing in weight, and becoming solid or hard and stiff. Hence, natural oils which contain considerable proportions of glycerides of these "unsaturated" acids are known as "drying oils" and are largely used in the manufacture of paints, varnishes, linoleums, etc.; while oils which contain little of these glycerides are known as "non-drying," and are used for food, for lubrication, or for other technical purposes in which it is essential that they remain in unchanged fluid condition when exposed to the air.

The following are some of the more important of the acids which occur as glycerides in natural fats: Saturated Acids:

(_a_) Acetic, or stearic, acid series--general formula, C_{_n_}H_{2_n_+1}·COOH.

(1) Formic acid, H·COOH, occurs free in nettles, ants, etc.

(2) Acetic acid, CH_{3}·COOH, occurs free in vinegar.

(3) Butyric acid, C_{3}H_{7}·COOH, in butter fat.

(4) Capric acid, C_{9}H_{19}·COOH, in butter fat and cocoanut oil.

(5) Myristic acid, C_{13}H_{27}·COOH, in cocoanut oil and spermaceti.

(6) Palmitic acid, C_{15}H_{31}·COOH, in palm oil and many fats.

(7) Stearic acid, C_{17}H_{35}·COOH, in most fats and oils.

Intervening members of this series, such as caprylic acid, C_{7}H_{15}·COOH, and lauric acid, C_{11}H_{23}·COOH, are also found in smaller quantities in cocoanut and palm nut oils, in butter fat, and in spermaceti; while higher members of the series, as arachidic acid, C_{19}H_{39}·COOH, and lignoceric acid, C_{23}H_{47}·COOH, are found in peanut oil; and cerotic acid, C_{25}H_{51}·COOH, and melissic acid, C_{29}H_{59}·COOH, in beeswax and carnauba wax.

Unsaturated Acids:

(_b_) Oleic acid series--general formula, C_{_n_}H_{2_n_-1}·COOH.

(1) Crotonic acid, C_{3}H_{5}·COOH, occurs in croton oil.

(2) Oleic acid, C_{17}H_{33}·COOH, occurs in many fats and oils.

(3) Brassic acid, C_{21}H_{41}·COOH, occurs in rape-seed oil.

(4) Ricinoleic acid, C_{17}H_{32}OH·COOH, occurs in castor oil.

(_c_) Linoleic acid series--general formula, C_{_n_}H_{2_n_-3}·COOH.

(1) Linoleic acid, C_{17}H_{31}·COOH, occurs in linseed and other drying oils.

(_d_) Linolenic acid series--general formula, C_{_n_}H_{2_n_-5}·COOH.

(1) Linolenic acid, C_{17}H_{29}·COOH, occurs in many drying oils.

It will be observed that all of these acids contain a multiple of two total carbon atoms. No acid containing an uneven number of carbon atoms has been found in a natural fat. Furthermore, the acids which occur most commonly in natural fats are those which contain eighteen carbon atoms; in fact, more than 80 per cent of the glycerides which compose all animal and vegetable fats are those of the C_{18} acids. This fact, in addition to the one that the sugars and starches all contain multiples of six carbon atoms in their molecules, indicates a very great biological significance of the chain of six carbon atoms. This has been alluded to in connection with the discussion of the biological significance of molecular configuration (see page 57) and will be mentioned again in other connections.

THE ALCOHOLS WHICH OCCUR IN NATURAL FATS

=Glycerol=, as has been pointed out, is by far the most common alcoholic constituent of natural fats and oils. This substance, which is familiar to everyone under its common name "glycerine," is a colorless, viscid liquid having a sweetish taste. It is a very heavy liquid (specific gravity 1.27) which mixes with water in all proportions and when in concentrated form is very hygroscopic.

Glycerine is made from fats and oils by commercial processes which clearly prove that the constitution of fats is as described above. The fat is boiled with a solution of caustic soda and is decomposed, the sodium of the alkali taking the place of the glyceryl (C_{3}H_{5}) group, the latter combining with three (OH) groups from the three molecules of alkali necessary to decompose the fat. A sodium salt of the organic acid, or soap, and glycerol are thus produced, and are separated by saturating the hot solution with common salt, which causes the soap to separate out as a layer on the surface of the liquid, which, on cooling, solidifies into a solid cake, which is then cut and pressed into the familiar bars of commercial soap. From the remaining solution, the glycerine is recovered by evaporation and distillation under reduced pressure. Taking stearin, a common fat, as the example, the reaction which takes place in the above process may be expressed by the following equation:

C_{3}H_{5}(C_{17}H_{35}·COO)_{3} + 3NaOH = 3C_{17}H_{35}COONa + Stearin Sodium stearate--a soap C_{3}H_{5}(OH)_{3}. Glycerol

This process, since it yields soap as one of its products, is called "saponification." All fats, when saponified, yield soaps and either glycerol or (more rarely) some of the other alcohols which are described below.

Glycerine is also prepared from fats by hydrolysis with superheated steam. Using olein, a glyceride which is present in olive oil and many common fats, as the example in this case, the equation for the reaction is:

C_{3}H_{5}(C_{17}H_{33}·COO)_{3} + 3H_{2}O = 3C_{17}H_{33}·COOH + Olein Steam Oleic acid C_{3}H_{5}(OH)_{3} Glycerol

In this case the free fatty acid, instead of a soap, is the product which is obtained in addition to glycerol.

In the equations presented above, a single glyceride has been used as the example in each case. In the saponification, or hydrolysis, of natural fats and oils which, as has been shown, are mixtures of many glycerides, the resultant soaps, or fatty acids, are mixtures of as many compounds as there were individual glycerides of the original fat, but the glycerol is identical in every case.

When glycerol is heated with dehydrating agents, it is easily converted into _acrolein_, an unsaturated aldehyde having a peculiar characteristic pungent odor. Hence, the presence of glycerol, or glycerides, in any substance may usually be detected by mixing the material with anhydrous acid potassium sulfate and heating the mixture in a test tube, when the characteristic odor of acrolein will appear.

Glycerol possesses all the characteristic properties of an alcohol, forming alcoholates with alkalies, esters with acids, etc. It is an active reducing agent, being itself easily oxidized to a variety of different products depending upon the strength of the oxidizing agent used and the conditions of the experiment. Microorganisms affect it in a variety of ways, either converting it into simple fatty acids, or condensing it into longer-chain compounds.

=Open-chain monohydric alcohols=, higher members of the ethyl alcohol series, such as cetyl, C_{16}H_{33}OH, carnaubyl, C_{24}H_{49}OH, ceryl, C_{26}H_{53}OH, and melissyl, C_{30}H_{61}OH, are found in the esters which constitute the major proportion of the common waxes.

=Cholesterol and phytosterol= are empirical names for certain closed-ring, monohydric alcohols which are found in relatively small amounts in all fats, the former term designating those found in animal fats and the latter those of plant origin. Their composition has not yet been definitely established. They are known to contain two, or three, closed rings, probably of the phenanthrene type; to form dichlor- and dibrom- addition products, showing that they contain one side-chain double linkage; and to yield ketones when oxidized, indicating that they are secondary alcohols. They form acetyl esters, or acetates, which can be separated from each other and identified by their crystal forms and melting points. Because of this fact and of the further fact that they are present in detectable quantities in practically all fats and oils, they afford a qualitative means of distinguishing between fats of animal and of plant origin. This possibility is the most interesting fact known concerning these complex alcohols; although their presence as esters in all plant and animal fats indicates that they must have some biological function.

_Phytosterol_ is not a single alcohol, but a mixture of at least two, which have been separated and studied as _sitosterol_, C_{27}H_{43}OH, and _stigmasterol_, C_{30}H_{49}OH. As has been said, these are found in small proportions in all vegetable fats, being present in largest amounts in oily seeds, especially those of the legumes.

The saponification of esters of cholesterol and phytosterol is a difficult and unsatisfactory process; but since this affords the only known means to distinguish between fats of plants and of animal origin, its technique has been fairly well worked out, and the process used in the study of the changes which take place in plant fats when they are used by animals as food.

HYDROLYSIS AND SYNTHESIS OF FATS

The reaction for the hydrolysis of fats has been discussed in connection with the process for the manufacture of glycerine. This reaction takes place very slowly with cold water alone, can be easily brought about by the action of superheated steam, and much more easily and rapidly in the presence of some catalyst (sulfuric acid is an especially effective catalyst for this purpose).

Fats can be artificially synthetized by heating mixtures of glycerol and fatty acids, under considerable pressure, for some time at temperatures of 200° to 240° C.; or by heating a mixture of the disulfuric ester of glycerol with a fatty acid dissolved in sulfuric acid. Recently, fatty acids have been prepared from carbohydrates, by first breaking the hexoses down into three-carbon compounds, then carefully oxidizing these to pyruvic acid, CH_{3}·CO·COOH, which can then be condensed into acids having longer chains. The violent reagents and long-continued processes which must be employed for the artificial hydrolysis or synthesis of the fats are in sharp contrast with the easy and rapid transition of carbohydrates to fats, and _vice versa_, which take place in both plant and animal nutrition.

THE EXTRACTION OF OILS FROM PLANT TISSUES

There are three types of methods which are employed for the extraction of oil from oil-bearing seeds, etc., either as a commercial industry or for the purposes of scientific study. These are (1) by pressure; (2) extraction with volatile solvents; and (3) boiling the crushed seeds or fruits with water.

By the first method, the seeds are first cleaned, then "decorticated" (hulls removed), crushed or ground, then subjected to intense pressure in an hydraulic press. In the commercial process, the ground seeds are first pressed at ordinary temperature, which yields "cold-drawn" oil, then the press cake is heated and pressed again, whereby "hot-drawn" oil is obtained. The crude oil is refined by heating it to coagulate any albumin which it may contain, and is sometimes bleached by different processes before it is marketed. The press cake from many seeds, such as flaxseed (linseed), cottonseed, etc., is ground up and sold for use as stock feed.

In the second method, the finely crushed seeds are treated with solvents such as gasoline or carbon bisulfide, in an apparatus which is so arranged that the fresh material is treated first with solvent which has already passed through various successive lots of material and has become highly charged with the oil, followed by other portions which contain less oil, and finally by fresh solvent, whereby the last traces of oil are removed from the material. The saturated solvent is transferred to suitable boilers and the solvent distilled off and condensed for repeated use, leaving the oil in the boiler in very pure form.

Extraction by boiling with water is sometimes used in the preparation of castor oil and olive oil. In such cases, the crushed seeds are boiled with water and the oil skimmed off as fast as it rises to the surface.

IDENTIFICATION OF FATS AND OILS

Fats and oils are identified by determinations of their physical properties, such as specific gravity, melting point, refractive index, etc., and by certain special color reactions for particular oils; or by measurements of certain chemical constants, such as the percentage of free fatty acids which they contain, the saponification value (i.e., the number of milligrams of KOH required to completely saponify one gram of the fat), the iodine number (percentage by weight of iodine which is absorbed by the unsaturated fatty acids present in the fat), percentage of water-insoluble fatty acids obtained after saponification and acidifying the resultant soap, etc., etc. Most of these tests must be carried out under carefully controlled conditions in order to insure reliable identifications, and need not be discussed in detail here. Full directions for making such tests, together with tables of standard values for all common fats and oils, may be found in any reference book on oil analysis.

PHYSIOLOGICAL USE OF FATS AND OILS

In animal organisms, fats are the one important form of energy storage. They also form one of the most important supplies of energy reserve material in plants. Carbohydrates commonly serve this purpose in those plants whose storage reservoirs are in the stems, tubers, etc.; but in most small seeds the reserve supply of energy is largely in the form of oil, and even in those seeds which have large endosperm storage of starch, the embryo is always supplied with oil which seems to furnish the energy necessary for the first germinative processes.

Fats are the most concentrated form of potential energy of all the different types of organic compounds which are elaborated by plants. This is because they contain more carbon and hydrogen and less oxygen in the molecule than any other group of substances of vegetable (or animal) origin. It has been pointed out that a quantity of fat capable of yielding 100 large calories of heat will occupy only about 12 cc. of space, whereas from 125 to 225 cc. of space in the same tissue would be required for the amount of starch of glycogen necessary to yield the same amount of heat, or energy, when oxidized.

The fats undoubtedly catabolize first by hydrolysis into glycerol and fatty acids, and then by oxidation possibly first into carbohydrates and then finally into the end-products of oxidation, namely, carbon dioxide and water. The following hypothetical equation to represent the oxidation of oleic acid into starch, suggested by Detmer, is interesting as a suggestion of how much oxygen is required and how much heat would be liberated by such a transformation:

C_{18}H_{34}O_{2} + 27O = 2(C_{6}H_{10}O_{5}) + 6CO_{2} + 7H_{2}O

Complete oxidation of oleic acid to the final end-products, carbon dioxide and water, would require much more oxygen, thus:

C_{18}H_{34}O_{2} + 51O = 18CO_{2} + 17H_{2}O.

Hence, Detmer's reaction would yield only approximately one-half the total energy available in the acid; but it does indicate the possibility of redevelopment of fatty acids or fats from the unoxidized carbohydrate material which remains in the equation. Moreover, there is abundant evidence to show that, in both animal and plant tissues, energy changes are brought about chiefly by the transformation of fats into carbohydrates and _vice versa_.

Many different hypotheses have been put forward concerning the mode of transformation of fats into carbohydrates, and the changes which take place in oily seeds during their germination have been carefully studied by many investigators. The following seem to be fairly well established facts. First, that fats as such may be translocated from cell to cell, since cell-walls and cell protoplasm seem to be permeable to oil if it is a sufficiently fine emulsion; or they may be hydrolyzed into glycerol and fatty acids and translocated from cell to cell in these forms and recombined into fats in the new location. Second, that fats are formed from glucose in some plants, from sucrose and from starch in others, and from mannite and similar compounds in still other species. Third, that in germination the fatty acids are used up in the order of their degree of unsaturation, those which contain the largest number of double-bond linkages being used first, and the saturated acids last of all. Fourth, that the sugar produced by the oxidation of fats is derived either from the glycerol or from the fatty acids of the fat, depending upon the nature of the latter. If the fat is saturated, the glycerine is converted into sugar while the fatty acids are oxidized; but if the fat contains large proportions of unsaturated acids, these contribute to the formation of sugar.

Recent studies seem to show that in the animal body fats serve an important function in connection with the production of antibodies to disease germs. But there is as yet no evidence to show that fats and oils have any similar function in plant tissues. The fact that they are found almost wholly in the storage organs of plants seems to indicate that their use as food reserve material is their principal, if not their sole, function in the plant economy.

B. THE WAXES

Waxes are most commonly found in or on the skin of leaves or fruits. They are similar to fats in chemical composition, except that, instead of being glycerides, they are esters of monohydric alcohols of high atomic weight. The term wax, when used in the chemical sense, has reference to this particular type of esters rather than to any special physical properties which the compound possesses, and both solid and liquid waxes are known.

Carnauba wax, found on the leaves of the wax-palm (_Copernicia cerifera_) contains ceryl alcohol (C_{23}H_{53}OH) and myricyl alcohol (C_{30}H_{61}OH) esters of cerotic acid (C_{25}H_{51}·COOH) and carnaubic acid (C_{23}H_{47}·COOH). It is the best known vegetable wax. Poppy wax is composed chiefly of the ceryl ester of palmitic acid (C_{17}H_{35}·COOH).

Since waxes contain no glycerol, they give no odor of acrolein when heated with dehydrating agents, do not become rancid, and are less easily hydrolyzed than the fats. They are soluble in the same solvents as the fats, but generally to a less degree.

The facts that waxes are impervious to water and usually occur on the surfaces of plant tissues have led to the conclusion that their chief function is to provide against the too-rapid loss of water by evaporation from these tissues. This seems to be borne out by the common experience that many fresh fruits and vegetables will keep longer without shriveling if their waxy coating is undisturbed. No other function than that of regulation of water losses has been suggested for the plant waxes.

C. THE LIPOIDS

The lipoids, or "lipins," as some authors prefer to call them, are substances of a fat-like nature which are found in small quantities in nearly all plant and animal tissues and in considerable proportions in nerve and brain substance, in egg yolk, etc., and in the seeds of plants. When hydrolyzed, they yield fatty acids or derivatives of fatty acids and some other group containing either nitrogen only or both nitrogen and phosphorus. The facts that they are extracted from tissues by the same solvents which extract fats and that they yield fatty acids when hydrolyzed account for the name "lipoid," which comes from the Greek word meaning fat. Some writers, who object to the word "lipoid" as a group name, prefer to call these substances the "fat-like bodies."

The first group of lipoids to be studied were those which occur in the brain; and the name _cerebroside_ was given to those lipoids which, when hydrolyzed, yield fatty acids, a carbohydrate and a nitrogen-containing compound but no phosphoric acid; while those lipoids which contain both nitrogen and phosphorus were called _phosphatides_. Substances which correspond in composition to both these types are found in plant tissues and the same class names are applied in a general way to lipoids of either plant or animal origin.

Plant lipoids have not been studied to nearly the same extent as have those which occur in the animal body; and certain observers believe that there are significant differences between the lipoids of plants and those of animal origin. However, most investigators use the same methods of study and the same systems of nomenclature for these fat-like substances, regardless of their origin.

LECITHIN

This phosphatide is by far the best-known lipoid. It occurs in the brain, the heart, the liver, and in the yolk of the eggs of many animals; and either lecithin or a substance so nearly like it in character as to be regarded by most investigators as identical with it, is present in small, but constant, quantities in nearly all seeds, especially those of leguminous plants. In many legume seeds, it constitutes from 50 to 60 per cent of the "ether extract," or "crude fat," which can be extracted from the crushed seeds, using ether as the solvent.

Lecithin is a glyceride. Only two of the (OH) groups of the glycerol are replaced by fatty acids, however; the third being replaced by phosphoric acid, H_{3}PO_{4}, or PO(OH)_{3}, which, in turn, has one of its hydrogen atoms replaced by the base _choline_. Choline is a nitrogenous base, or amine, which may be regarded as ammonium hydroxide with three of its hydrogen atoms replaced by methyl groups and the fourth by the ethoxyl group, the latter being the ethyl group with an OH in place of one of its hydrogens. Thus,

Ammonium hydride Choline

H H CH_{3} C_{2}H_{4}OH \ / \ / H-N CH_{3}-N / \ / \ H OH CH_{3} OH

Without the choline, lecithin would be a di-fatty acid derivative of glycero-phosphoric acid. These relations may be seen in the following formulas:

Glycerol Glycero-phosphoric acid Fatty Acid

CH_{2}OH CH_{2}OH OH HOOC·R | | / | | / CHOH CH--O--P=O | | \ | | \ CH_{2}OH CH_{2}OH OH

Choline Lecithin

HOC_{2}H_{4} CH_{2}OOC·R OH \ | / \ | / (CH_{3})_{3}[trb]N CH---O---P=O C_{2}H_{4} / | \ / \ / | \ / N[trb](CH_{3})_{3} HO CH_{2}OOC·R O / HO

fatty acid / Or, glycerol-fatty acid + Choline = Lecithin + H_{2}O \ phosphoric acid

There are many different possible linkages of the constituent groups which make up the lecithin molecule. In the first place, if the (OH) groups of the glycerol molecule be numbered (1) and (2), thus,

CH_{2}OH (1) | CHOH (2) | CH_{2}OH (1)

the fatty acid radicals may be attached either in one (1) position and one (2) position, or in the two (1) positions; hence, two forms of glycero-phosphoric acid are possible, thus

fatty acid fatty acid / / (A) glycerol--fatty acid (B) glycerol--phosphoric acid \ \ phosphoric acid fatty acid

Again, the choline may be attached to the phosphoric acid either through its alcoholic (OH) group or through its basic (N) group, thus

OH OH / / -P=O C_{2}H_{4} -P=O \ / \ \ O N[trb](CH_{3})_{3} or, O-N[trb](CH_{3})_{3} / \ HO C_{2}H_{4}OH

The facts that in the arrangement (B) the central carbon atom of the glycerol would be asymmetric, and that both lecithin and the glycero-phosphoric acid derived from it by hydrolysis are optically active, prove that formula (B) correctly represents the arrangement of that part of the lecithin molecule; and there is ample theoretical and experimental evidence to prove that the choline linkage is through the alcoholic (OH) group. Hence the formula for lecithin indicating the linkage as shown above is the correct one.

The fatty acids in the lecithin molecule may be different in lecithins from different sources, just as they are different in fats from different sources. Both oleic acid and a solid fatty acid have been found in the hydrolysis products of lecithin from leguminous seeds. In certain lupine seeds, the fatty acids present in the lecithin appear to be palmitic and stearic.

OTHER PLANT PHOSPHATIDES

Phosphatides other than lecithin are common in plants. In these, various sugars replace part or all of the glycerol as the alcoholic part of the ester. Percentages of sugar varying from mere traces up to 17 per cent of the weight of material taken, have been found in the products of hydrolysis of phosphatides prepared from vetch seeds, potato tubers, plant pollens, and whole wheat meal.

Furthermore, betaine

CH_{2} / \ (tri-methyl glycocoll, OC N[trb](CH_{3})_{3}) \ / O

and perhaps other vegetable amines (see Chapter XII) sometimes replace choline as the basic group in the phosphatides.

PLANT CEREBROSIDES

Bodies similar to the animal cerebrosides seem to occur in many plant tissues, since plant lipoids which yield no phosphorus when hydrolyzed have often been isolated. The sugar which constitutes the alcoholic portion of their structure appears to be galactose in every case which has been reported. Beyond this, little is known of the structure of these plant cerebrosides, as they are very difficult to prepare in pure form and not easily hydrolyzed.

PHYSIOLOGICAL USES OF LIPOIDS

Lipoids are so universally present in plant and animal tissues and so commonly found in those parts of the organism in which vital phenomena are most pronounced (brain, heart, embryo of egg, embryo of seeds, etc.), that it is evident that they must play some important rôle in the activity of living protoplasm. There is, as yet, however, no definite and certain knowledge of what this rôle is. Various theories concerning the matter have been put forward in recent years. For example, Overton, in 1901, presented the idea that every living cell is surrounded by a semi-permeable membrane consisting of lipoid material, which regulates the passage into and out of the cell of substances necessary to its metabolism and growth. Recent investigations by Osterhout and others indicate, however, that Overton's hypothetical lipoid membrane is not essential to a proper explanation of the migration into and out of the cell protoplasm of nutritive materials, etc. Other investigators have cited results which appear to indicate that lipoids play an important, but as yet unknown, part in the process of fat metabolism. Others go even further than this, and argue that since the extraordinary rapidity of the chemical changes which take place in plant protoplasm indicates the necessity of the presence there of exceedingly labile substances, and since both fats and proteins are relatively stable compounds, it is possible that the lipoids, which contain both nitrogenous and fatty acid groups, play an exceedingly important part in the metabolism processes. Bang, in particular, has pointed out (in 1911) that the lipoids are probably the most labile of all the components which constitute the colloidal system known as plant protoplasm. The importance of such considerations will be more apparent after the relation of colloidal phenomena to the activities of plant cell contents has been more fully discussed (see Chapter XVI).

Experimental studies of the physiological uses of lipoids have thus far been devoted almost exclusively to those of animal tissues. They have been seriously hampered by the difficulty of securing properly purified extracts of lecithin and similar lipoids. The same labile character which apparently makes them so important in the chemical changes in the cell makes them equally unstable compounds to work with in attempting to secure pure preparations for the purposes of experimental study. On this account, there is, as yet, no certain knowledge concerning their actual physiological uses. It is evident, however, that they have some really important rôle to play, which opens up a promising field for further study.

REFERENCES

ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 3, Fette, Wachse, Phosphatide, Cerebroside, ..." 340 pages, Berlin, 1911.

HOPKINS, E.--"The Oil-Chemist's Handbook," 72 pages, New York, 1902.

LEATHES, J. B.--"The Fats," 138 pages, _Monographs_ on Biochemistry, London, 1913.

LEWKOWITSCH, J.--"Chemical Technology and Analysis of Oils, Fats, and Waxes," Vol. I, 542 pages, 54 figs.; Vol. II, 816 pages, 20 figs.; and Vol. III, 406 pages, 28 figs., London, 1909.

MACLEAN, H.--"Lecithin and Allied Substances," 206 pages, _Monographs_ on Biochemistry, London, 1913.

SOUTHCOMBE, J. E.--"Chemistry of the Oil Industries," 204 pages, 13 figs., London, 1918.