very high degree of unsaturation. On exposure to dry air, it absorbs 34.3
per cent of its own weight of oxygen, which corresponds to 11½ atoms of
oxygen, computed on the basis of the molecular formula C_{40}H_{56}, and
would indicate a formula of (C_{40}H_{56})_{2}O_{23} for the oxygenated
compound; this being three oxygen atoms less than would be required to
bring the compound to the theoretical stage of saturation represented by
the unimolecular formula C_{_n_}H_{_2n+2_}. In moist air, two more oxygen
atoms are absorbed, probably forming two OH groups in the molecule.
Moreover, carotin absorbs iodine. When the calculated amount of iodine is
used, a definite compound having the formula C_{40}H_{56}I_{2} is produced;
but in the presence of an excess of iodine another compound having the
apparent formula C_{40}H_{56}I_{3} (or 2C_{40}H_{56}I_{2}+I_{2}) is
obtained. (Note that 2 atoms of iodine plus 12 atoms of oxygen, or 3 of
iodine plus 11 of oxygen, produce the degree of saturation required by
the formula C_{_n_}H_{_2n+2_}.) It is evident from these experimental data,
that a part of the unsaturated linkage in the carotin molecule is of a type
which can easily be saturated by direct addition of oxygen, while the
remainder may be saturated by iodine.
The reaction of carotin toward bromine is peculiar. With this element, it
forms a compound having the formula C_{40}H_{36}Br_{22}, indicating the
direct addition of two atoms of bromine and the substitution of twenty
atoms of this element for the same number of hydrogen atoms.
The oxygenated carotins are colorless substances, while the iodide
crystallizes in beautiful dark-violet prisms, having a coppery red
fluorescence.
=Xanthophyll= is closely related to carotin. It has the molecular formula
C_{40}H_{56}O_{2}. It absorbs 36.55 per cent of oxygen (corresponding to 13
atoms, which would indicate the formation of two OH groups in addition to
the saturation required by the C_{_n_}H_{_2n+2_} formula); and an iodine
addition product having the formula C_{40}H_{56}O_{2}I_{2}, which
crystallizes in dark-violet needles.
Xanthophyll differs markedly from carotin in its solubilities, being
insoluble in petroleum ether and only sparingly soluble in carbon
disulfide. It may be fairly easily reduced to carotin. This transformation
is reversible, and suggests a similarity to the change from hæmoglobin to
oxyhæmoglobin, and the reverse, in the blood of animals, as a part of their
respiration process.
=Separation of the Chlorophylls, Carotin, and Xanthophyll.=--These
pigments, which exist together in most plant tissues, may easily be
separated from each other by taking advantage of the differences in their
solubilities, according to the following procedure. Grind up a small
quantity of the fresh tissue (leaves of the stinging nettle furnish a
conveniently large supply of each of these pigments) with fine sand in a
mortar. Cover with acetone, let stand a few moments and then filter on a
Büchner funnel. Pour the filtrate into a separatory funnel, add an equal
volume of ether and two volumes of water. Shake up once and then allow the
ether layer to separate; the pigments will be in this layer. Drain off the
water-acetone layer. Now to the etherial solution, add about half its
volume of a concentrated solution of potassium hydroxide in methyl alcohol.
Shake well and allow to stand until the mixture becomes permanently green.
Now add an equal volume of water and a little more ether, until the mixture
separates sharply into two layers. The chlorophylls will now be in the
lower dilute alcohol layer, and the carotinoids in the upper ether, and may
be separated by draining of each layer separately. To separate the carotin
from xanthophyll place the ether solution in a small open dish and
evaporate to a small volume. Now add about ten volumes of petroleum spirit
and an equal volume of methyl alcohol, stir up well, transfer to a
separatory funnel and allow the two layers to separate. The carotin will
now be in the upper layer of petroleum ether, and the xanthophyll in the
lower alcohol layer; these layers may be drained off separately and the
solvents evaporated in order to recover the pigments in dry form.
=Lycopersicin= (or lycopin) is a hydrocarbon pigment having the same
formula as carotin. It is, however, brilliantly red in color, and
crystallizes in a different form and has a different adsorption spectrum
from carotin. It is the characteristic pigment of red tomatoes, and is
found also in red peppers. Yellow tomatoes have only carotin as their
skin-pigment, while lycopersicin is usually present in the flesh of the
ripe fruits of all varieties and in the skin of red ones. It has been
shown, however, that if varieties of tomatoes which are normally red when
ripe, are ripened at high temperatures, 90° F. or above, their skins will
be yellow instead of red when fully ripe. Hence, the occurrence of carotin,
or of lycopersicin, as the skin pigment is determined in part by the
varietal character (being different in different varieties when ripened at
normal temperatures) and in part by the temperature at which the fruit
ripens. The two pigments are, of course, isomers; but the difference in
their structural arrangement is not known.
=Fucoxanthin=, C_{40}H_{54}O_{6}, is a brownish-red pigment, found in fresh
brown algæ, and in some brown sea-weeds. Its formula indicates that it is
an oxidized carotin. With iodine, it forms a compound having the formula
C_{40}H_{54}O_{6}I_{4}. It is unlike carotin and xanthophyll in that it has
basic properties, forming salts with acids, which are blue in color.
PHYCOERYTHRIN AND PHYCOPHÆIN
These are the principal pigments of red and brown seaweeds, respectively.
Their most characteristic difference from the pigments of non-aquatic
plants is that they are easily soluble in water, and insoluble in most
organic solvents, such as alcohol, ether, etc. At first thought, this would
appear to be impossible, since the plants grow in water and it would seem
that their water-soluble pigments would be continuously dissolved out of
the tissues. The reason why this does not occur lies in the fact that these
pigments exist in the cells of the seaweeds in colloidal form (see Chapter
XV), and, hence, cannot diffuse out through the cell-walls. The only way in
which they can be extracted from the tissues is by rupturing the cells, by
grinding with sharp sand, etc., after which the pigments can readily be
dissolved out by water.
=Phycoerythrin= is the red pigment. It is a colloidal, nitrogenous
substance, allied to the proteins (see Chapter XIII) but not a true protein
compound. Hydrolysis by acids indicates that it contains leucine and
tyrosine, two amino-acids which are constituents of proteins, along with
other bodies of unknown composition.
The colloidal solution of phycoerythrin in water has a brilliant rose-red
color, with an orange fluorescence. It readily sets to a gel (see Chapter
XV), so that the solution is almost impossible to filter. On this account,
purified solutions of this pigment are very difficult to secure, and no
satisfactory analysis to indicate its composition has yet been obtained.
Actinically, it is a complementary pigment to chlorophyll, that is, it
absorbs the blue and green rays and permits the passage of light which is
of the wave length that is absorbed by chlorophyll.
=Phycophæin.=--Still less is known of the composition of this pigment than
of that of phycoerythrin. It is the characteristic pigment of brown
seaweeds. It is supposed to exist in the cells of algæ, chiefly as a
colorless chromogen, which becomes first yellow and then brown on exposure
to air. Associated with it are other pigments, which have been variously
reported as carotin, phycoxanthin, etc.
THE ANTHOCYANS
These are a group of pigments of red, blue, or violet color, which occur in
the flowers, fruits, or leaves of many species of plants. They are
essentially ornamental pigments, and constitute a large proportion of the
brilliant colors of flowers, etc. They occur not only dissolved in the
cell-sap, but also as deposits of definite crystals or amorphous compounds
in the cell protoplasm.
They are all glucosides. When the anthocyans are hydrolyzed, the sugar
molecules are split off and the characteristic hydroxy-derivatives of the
three-ring anthocyan nucleus (figured on page 83), known as
"anthocyanidins," remain. These anthocyanidins are themselves pigments.
They have been shown to be all derivatives of the anthocyan nucleus. The
oxygen atom in this nucleus is very strongly basic and exhibits its
quadrivalent property by forming stable salts by direct addition of acid
radicles. The variation of color of the anthocyanins has been explained by
Willstätter, as follows; the red is the acid salt, the blue is a neutral
metallic salt, and the violet is the anhydride of the anthocyanidin in
question, thus
Cl Cl
| |
O _____ O _____
HO__ / \ / \ __/ \ KO__ / \ / \ __/ \
| | | \_____/ | | | \_____/
| | | | | |
\ / \ / \ / \ /
C O C
| \
Red | \ Blue
| O _____
|/ \ / \ __/ \
| | | \_____/
| | |
\ / \ /
C
Violet
All of the natural anthocyanin pigments appear to contain a chlorine atom
attached directly to the ring oxygen, as shown in the above partial
formulas. In addition, they have four, five, or six hydroxyl (OH), or
methoxy (OCH_{3}), groups attached at various points around the three
rings. The following formula for _[oe]nidin_, one of the most complex of
these anthocyanidins, will illustrate their structural arrangement.
Cl
| _____OCH_{3}
O / \
HO__ / \ / \ _____/ \OH
| | | \ /
| | | \_____/
\ / \ / OCH_{3} OH
OH C
_Delphinidin_ is the corresponding compound without the two CH_{3} groups;
while _cyanidin_ contains only five OH groups; and _pelargonidin_, only
four OH groups.
The anthocyanin pigments are soluble in water, alcohol, and ether, the
solutions being red or blue in color according to the acidity or alkalinity
of the medium. Their presence in many species of plants is hereditable, as
these plants come true to color from seed, as in the case of red beets, red
cabbage, several species of blue berries, etc. In other cases, the
anthocyanin development depends largely upon the conditions of growth,
particularly those which prevail during the later stages of development: as
in the case of apples, where the amount of red color in the skin depends to
a large extent upon the conditions under which the fruit ripens.
Anthocyanin pigments often make their appearance late in the season; in
fruits, etc., as the result of the normal ripening process but in leaves as
the result of shorter daylight illumination accentuated also by sharp
frosts.
THE ANTHOXANTHINS
The yellow plant pigments, other than the carotinoids, are almost without
exception glucosides having a xanthone or flavone nucleus. These typical
nuclei are illustrated on page 83. In these nuclei, as in the anthocyan
one, the oxygen atom is strongly basic and combines with mineral acids to
form salts (the oxygen becoming quadrivalent) and the color of the pigment
depending upon the nature of the combination formed in this way.
The anthoxanthin pigments are yellow, crystalline solids, which are only
slightly soluble in water. They dissolve readily in dilute acids and
alkalies, giving yellow or red solutions which are of the same color in
either acid or alkaline media. They are extensively used as yellow dyes.
Many of the common members of this group have been mentioned in the chapter
dealing with the glucosides. The characteristic pigment nucleus of several
of these is as follows:
_Chrysin_, found in various species of poplar and mallows,
O _____
HO / \ / \ __/ \
| | | \_____/
| | |
\ / \ /
HO C
¦
O
_Apigenin_, found in parsley and celery, as the glucoside apiin,
O _____
HO / \ / \ __/ \OH
| | | \_____/
| | |
\ / \ /
HO C
¦
O
_Campferol_, found in Java indigo, as the glucoside campferitrin,
O _____
HO / \ / \ __/ \OH
| | | \_____/
| | |OH
\ / \ /
HO C
¦
O
_Fisetin_, found in quebracho wood and fiset wood,
O ____OH
HO / \ / \ __/ \OH
| | | \_____/
| | |
\ / \ /
C
¦
O
_Quercitrin_, found in oak bark, horse-chestnut flowers, and in the skin of
onions,
O ____OH
HO / \ / \ __/ \OH
| | | \_____/
| | |OH
\ / \ /
HO C
¦
O
_Morin_, found in yellow wood (_Morus tinctoria_).
O HO____
HO / \ / \ __/ \OH
| | | \_____/
| | |OH
\ / \ /
HO C
¦
O
_Gentisin_, found in yellow gentian (_Gentiana lutea_),
O
CH_{3}O / \ / \ / \
| | | |
| | | |
\ / \ / \ / OH
HO C
¦
O
As a rule, the most brilliant of these yellow pigments are found in the
largest quantities in the bark and wood of various species of tropical
plants; although they are also present, in smaller amounts, in the blossoms
of species growing in temperate zones.
The anthoxanthins are easily converted into anthocyanins, and _vice versa_,
by the action of oxidizing and reducing enzymes which are commonly present
in the tissues of the plants which develop the pigments.
THE PRODUCTION OF ORNAMENTAL PIGMENTS IN FLOWERS, ETC.
The breeding of flowering plants having blossoms of almost any desired
color has become a commercial enterprise of large importance. The results
which have been obtained, in many cases, have been made the object of
scientific study of the genetics of color inheritance. These studies have
developed certain interesting facts with reference to the chemistry of the
development of these ornamental pigments, which may be briefly mentioned
here.
In many of the plants which have been studied, the color of the flowers
depends upon several different factors, as follows:
_C_, a chromogen (or color-producing substance) which is generally a
flavone or xanthone glucoside, and which may be either yellow or
colorless.
_E_, an enzyme which acts upon _C_, to produce a red pigment.
_e_, another enzyme which acts upon the red pigment, changing it to some
other anthocyanin color.
_A_, an antioxidase, or antienzyme, which prevents the action of _E_.
_R_, an enzyme which changes reds to yellows.
Thus, if a plant whose flower contains only the factor _C_ be crossed with
one which contains the factor _E_, a red blossom will result, or if it
contains the factor _e_ more intense pigments are developed. But if either
_A_ or _R_ are present, no change in the color of the original parents will
result from the crossing.
THE PHYSIOLOGICAL USES OF PIGMENTS
The vegetative pigments undoubtedly serve as agencies for regulating the
rate of metabolic processes. At the same time, it is extremely difficult to
determine whether the presence of a pigment in any given case is the cause
or the effect of the changes in the plant's activities which result from
changes in its external environment.
The chlorophylls are, of course, the regulator of photosynthesis, absorbing
solar energy with which the photosynthetic process may be brought about.
The simultaneous presence of carotinoids in varying amounts undoubtedly
serves to modify the amount and character of the radiant energy absorbed,
as these pigments absorb a different part of the spectrum of light and
hence undoubtedly produce a different chemical activity or "actinic effect"
of the absorbed energy. The variations in depth of color of foliage during
different growing conditions, from a pale yellow when conditions are
unfavorable and growth is slow to the rich dark green of more favorable
conditions, is a familiar phenomenon. Whether this change in pigmentation
is the result of an adjustment of the plant protoplasm, so that it can
absorb a more highly actinic portion of the light, or is a direct effect of
the lack of conditions favorable to chlorophyll-production and active
photosynthesis, has not yet been determined.
But there must be some influence other than response to environmental
conditions which controls the vegetative color in plants, since shrubs, or
trees, which have green, yellow, red, and purple leaves, respectively, will
grow normally, side by side, under identical external conditions of
sunlight, moisture supply, etc. The hereditary influence must completely
overshadow the apparent normal self-adjustment of pigment to
energy-absorbing needs, in all such cases.
Again, it appears that there is some definite connection between pigment
content and respiration. It is known, of course, that the gaseous exchanges
involved in animal respiration are accomplished through the reversible
change of hæmoglobin to oxyhæmoglobin, these being the characteristic blood
pigments. The easy change of carotin, C_{40}H_{56}, to xanthophyll,
C_{40}H_{56}O_{2}, and _vice versa_, and the reversible changes of the
yellow anthoxanthins to the red anthocyanins, under the influence of the
oxidizing and reducing enzymes which are universally present in plants,
would indicate the possibility of the service of these pigments as carriers
of oxygen for respiratory activities in plants in a way similar to that in
which the blood pigments serve this purpose in the animal body. The fact,
which has been observed in connection with the experimental studies of the
development of the lycopersicin, that tomatoes which normally would become
red remain yellow in the absence of oxygen, indicates that this
pigmentation, at least, is definitely connected with oxygen supply; and the
further fact that the development of lycopersicin in red tomatoes, red
peppers, etc., is dependent upon the temperature at which the fruit ripens,
may indicate a definite connection of this pigment with the need for more
oxygen (or for more heat, as suggested in the following paragraph) at these
lower temperatures.
Again, many investigators have concluded that at least one function of the
anthocyanin pigments is to absorb heat rays and so to increase
transpiration and other chemical changes. In support of this view, there
may be cited the general presence of such pigments in arctic plants, their
appearance in the leaves of many deciduous trees after a frost in the fall,
etc. Indeed, there is much to support the view that the autumnal changes in
foliage pigments have the physiological function of absorbing heat in order
to hasten the metabolic processes of ripening and preparation for winter
defoliation. The rapid and brilliant changes in foliage coloring after a
sharp frost which kills the tissues and makes rapid translocation of the
food material of the leaves to the storage organs immediately necessary,
have been explained as the response of the pigmentation of the leaves to
the need for increased heat-absorption. On the other hand, the red pigments
of the beet-root, etc., which seem to be identical in composition with the
other anthocyanin pigments, can have no such function as those which have
just been described. Furthermore, the fact that the pigment often varies in
color from red to yellow or brown, depending upon the temperature under
which the tissue is ripening, makes it an open question whether the pigment
is the regulating agency or whether its nature is the result of the
environmental conditions. Or, in other words, it is a question whether
these changes in color are a mechanism by which the plant cell adjusts its
absorptive powers, or whether they are only the inevitable result of the
changes in temperature upon a pigment material which is present in the cell
for an entirely different use.
A very interesting side-light upon the color changes which many species of
plants undergo when the external temperature falls has been shown by the
investigations of the relation of the sugar content of the plant tissues to
their pigmentation. It is a well-known fact that not only do many species
of deciduous plants show the characteristic reddening of their leaves after
frost in the autumn but also many evergreens (_Ligustrum_, _Hedera_,
_Mahonia_, etc.) exhibit a marked reddening, or purpling, of their foliage
during the winter months, with a return to the normal green color in the
spring. Earlier investigations, which have been confirmed by several
repetitions, showed that the red or purple leaves always contain higher
percentages of sugar than do green ones of similar types. More recent
studies have shown that artificial feeding of some species of plants with
abnormally large portions of soluble sugars produces a reddening of the
foliage tissues which is apparently identical with that which these tissues
undergo as the result of low temperatures. Thus, the connection between the
natural winter reddening of foliage and the development of sugar in the
tissues during periods of low temperatures (see page 64) seems to be
clearly demonstrated. It appears that at least a part of the seasonal
changes in color of plants is either the cause of, or the effect of,
variations in sugar content of the tissues of the plants, accompanying the
changes in external temperatures.
Oftentimes, the anthocyanin pigments seem to be associated with sugar
production, as contrasted with the chlorophylls, which seem to be more
favorable to the production of starch. But in this case also, it is
impossible to say whether the pigment is the direct causative agent in the
type of carbohydrate production or whether it is the effect of the same
external factors which determine, or modify, the character of the
carbohydrate condensation.
BIOLOGICAL SIGNIFICANCE OF ORNAMENTAL PIGMENTS
The ornamental pigments undoubtedly have definite biological significance.
When present in the storage roots, such as beet-roots, carrots, etc., or in
the above-ground parts of plants, they may have served to protect these
organs against herbivorous animals which were accustomed to consume green
foods.
In flowers, the brilliant ornamental pigments undoubtedly serve to attract
the insects which visit these blossoms in search of nectar, and in so doing
promote cross-fertilization. Recent experiments have demonstrated that
colors are much more efficient than odors in attracting insects.
Taken altogether, it is apparent that the pigments may have a variety of
important rôles in plants. At the same time, some of them may be waste
products, with no definite use in the plant economy.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 6, Farbstoffe der
Pflanzen- und der Tierwelt," 390 pages, Berlin, 1911.
PERKIN, A. G. AND EVEREST, A. E.--"The Natural Organic Colouring Matters,"
655 pages, London, 1918.
WAKEMEN, NELLIE A.--"Pigments of Flowering Plants," in _Transactions_ of
the Wisconsin Academy of Sciences, Arts, and Letters, Vol. XIX, Part II,
pages 767-906, Madison, Wisc., 1919.
WATSON, E. R.--"Colour in Relation to Chemical Constitution," 197 pages, 65
figs., 4 plates, London, 1918.
WHELDALE, M.--"The Anthocyan Pigments of Plants," 304 pages, Cambridge,
1916.
WILLSTÄTTER, R. AND STOLL, A.--"Untersuchung über Chlorophyllen, Methoden
und Ergebnisse," 432 pages, 16 figs., Berlin, 1913.
CHAPTER IX
ORGANIC ACIDS, ACID SALTS, AND ESTERS
Organic acids, either in free form, or partially neutralized with calcium,
potassium, or sodium, forming acid salts, or combined with various alcohols
in the form of esters, are widely distributed in plants. They occur in
largest proportions in the fleshy tissues of fruits and vegetables, where
they are largely responsible for the flavors which make these products
attractive as food for men and animals. But organic acids and their salts
are also found in the sap of all plants, and undoubtedly play an important
and definite part in the vital processes of metabolism and growth.
CHEMICAL CONSTITUTION
All organic acids contain one (or more) of the characteristic
O
//
acid group, --COOH, or --C, known as "carboxyl." This
\
OH
group is monovalent, and in the simplest organic acid, formic acid
(H_{2}CO_{2}), it is attached to a single hydrogen atom, thus, H·COOH. In
all other monobasic acids, it is attached to some other monovalent group,
usually an alkyl radical, i.e., a radical derived from an alcohol and
containing only carbon and hydrogen (as methyl, CH_{3}, ethyl, C_{2}H_{5},
butyl, C_{4}H_{9}, acryl, C_{2}H_{3}, etc.). Hence, the general formula for
all monobasic organic acids is R·COOH, the R representing any monovalent
radical. In the simplest dibasic acid, oxalic (H_{2}C_{2}O_{4}), two
carboxyl groups are united to each other, thus, HOOC·COOH; but in the
higher members of the series, the two characteristic acid groups are united
through one or more --CH_{2}-- groups, or their oxy-derivatives (as
HOOC·CH_{2}·COOH, malonic acid; HOOC·CH_{2}·CH_{2}·CH_{2}·COOH, glutaric
acid; HOOC·CHOH·CH_{2}·COOH, malic acid, etc.). Polybasic acids, containing
three or more carboxyl groups, linked together through one or more alkyl
carbon atoms, are also possible, and a few typical ones (as
COOH
|
HOOC·CH_{2}·COH·CH_{2}·COOH, citric acid)
are found in fruits and other plant tissues.
The H atom of the COOH group may be replaced by metals, in exactly the same
way as it is replaceable in inorganic acids, producing either neutral or
acid salts, depending upon whether all or only a part of the acid H atoms
are replaced by the basic element.
Thus, with sulfuric acid:
OH ONa
/ /
SO_{2} (H_{2}SO_{4}) + NaOH = SO_{2} (NaH_{2}SO_{4}) + H_{2}O
\ \
OH OH
Sulfuric Acid Acid sodium sulfate
OH ONa
/ /
or, SO_{2} (H_{2}SO_{4}) + 2NaOH = SO_{2} (Na_{2}SO_{4}) + 2H_{2}O
\ \
OH ONa
Sulfuric Acid Neutral sodium sulfate
Similarly, with oxalic acid:
COOH COOK
| (H_{2}C_{2}O_{4}) + KOH = | + H_{2}O
COOH COOH
Oxalic acid Acid potassium oxalate
or, COOH COOK
| (H_{2}C_{2}O_{4}) + 2KOH = | + 2H_{2}O
COOH COOK
Oxalic acid Neutral potassium oxalate
Similarly, the acid H atom of either an organic or an inorganic acid may be
replaced by the alkyl group of an alcohol, producing "ethereal salts," or
"esters."
Thus, with nitric acid;
NO_{2}OH(HNO_{3})+C_{2}H_{5}OH = NO_{2}OC_{2}H_5(C_{2}H_{5}NO_{3})+H_{2}O
Nitric acid Ethyl alcohol Ethyl nitrate
And, with acetic acid;
CH_{3}·COOH(H_{4}C_{2}O_{2})+C_{2}H_{5}OH = CH_{3}·COOC_{2}H_{5}+H_{2}O
Acetic acid Ethyl acetate
With dibasic or polybasic acids, either one or more of the carboxyl H atoms
may be replaced with an alcohol radical, so that both acid and neutral
esters of all such acids are possible. Examples of all of these different
types of derivatives of organic acids are frequently found in plant
tissues.
The occurrence, properties, and functions of a particular type of glycerol,
and other esters of organic acids, which are known as fats and waxes, are
not taken into consideration in the following discussions, but reserved for
a subsequent chapter dealing specially with them.
SOME COMMON ORGANIC ACIDS
Free organic acids, or their mineral salts or volatile esters, sometimes
occur as separate and characteristic individual compounds in particular
species of plants, or fruits; but much more commonly, two, three, or even
more acids or their derivatives, are associated together.
=Formic acid=, H·COOH (H_{2}CO_{2}), occurs in free form and in
considerable proportions in the leaves of several species of nettle, where
it is responsible for the unpleasant effects of the "sting." It may be
detected in small amounts in the vegetative parts of many, if not all,
plants, especially during periods of rapid growth, and is probably one of
the intermediate products in the photosynthesis of carbohydrates (see
Chapter III).
Higher members of the formic acid series (as acetic, CH_{3}·COOH;
propionic, C_{2}H_{5}·COOH; butyric, C_{3}H_{7}·COOH; etc.) are often found
in small quantities in the leaves of many plants and seem to be
characteristically present in certain species. They are easily produced
from carbohydrates by bacterial action and, hence, are always present in
fermenting tissues, such as silage, sauerkraut, etc. Furthermore, the
glycerol esters of higher members of this and other monobasic acid series
are constituents of all natural fats and oils (see Chapter X).
=Oxalic acid=, HOOC·COOH (H_{2}C_{2}O_{4}), is found in small amounts in
nearly all plants and in relatively large proportions in those of _Oxalis_,
rhubarb, etc. It occurs both as the free acid and as neutral, or acid,
oxalates of calcium, potassium, and, perhaps, of magnesium and sodium.
Solid crystals of insoluble calcium oxalate are often found in plant cells,
and it has been shown that when so deposited the calcium cannot become
again available for metabolic uses. It is stated, further, that such
crystals form only when calcium is in excess in the plant sap; hence, the
deposition of crystallized calcium oxalate seems to be a device for the
avoidance of excessive calcium rather than excessive oxalic acid, in the
plant juices.
=Succinic acid=, HOOC·CH_{2}·CH_{2}·COOH (H_{6}C_{4}O_{4}), occurs in many
fruits and vegetables, and is also found in some animal tissues. In fruits,
it is usually associated with its derivatives, malic and tartaric acids.
=Malic acid=, HOOC·CH_{2}·CHOH·COOH (H_{6}C_{4}O_{5}), occurs in apples and
in many small fruits, and in many vegetables. Acid calcium malate is now
produced commercially as a bye-product from the manufacture of syrups from
fruit juices, and is used as a substitute for "cream of tartar" in the
manufacture of baking powders.
=Tartaric acid=, HOOC·CHOH·CHOH·COOH (H_{6}C_{4}O_{6}), is found in many
fruits, but most characteristically in the grape, where it occurs as the
mono-potassium salt. During the fermentation of grape juice into wine, this
salt is deposited in considerable quantities in the bottom of the
wine-casks. This crude product is collected and sold under the name
"argols." From these argols, pure acid potassium tartrate is obtained by
decolorization and recrystallization, and constitutes the "cream of tartar"
of commerce.
COOH
|
=Citric acid=, HOOC·CH_{2}·COH·CH{2}·COOH (H_{8}C_{6}O_{7}), occurs
in large proportions in lemons, and associated with malic acid in
strawberries, cherries, currants, etc. It is also found in small quantities
in the seeds of the common leguminous vegetables, beans, peas, etc.
=Tannic acid= occurs widely distributed in the plant kingdom as a
constituent of the special type of glucosides known as _tannins_, whose
properties and functions have already been discussed (see Chapter VII).
PHYSIOLOGICAL USES OF ORGANIC ACIDS
No conclusive evidence concerning the rôle of organic acids in plant, or
animal, growth, has yet been produced. There can be no doubt that the
hypothetical _carbonic acid_ and its acid and normal salts have a
significant effect in regulating the acidity or alkalinity of plant juices,
or body fluids, and so determining the nature of the enzymic activities and
colloidal conditions of the biological systems (see Chapters XIV and XV).
It is probable that other organic acids, such as formic, acetic, oxalic,
and succinic acids, in plants and sarco-lactic acid, in animal tissues,
perform similar regulatory rôles; but there seems as yet to be no
indication as to why different acids should be used for this purpose by
different species, or organisms; or as to the methods by which they perform
their specific functions, whatever these may be.
In plants, the organic acids are usually in solution in the sap. When the
plant ripens, they generally disappear, either being neutralized by
calcium, or other bases, and deposited as crystals in the leaves or stems,
or else used up in the synthesis of other organic compounds. Small
proportions of these acids are usually present in mature seeds, and the
percentage increases materially during germination, indicating that they
play an important rôle in insuring the proper conditions for the conversion
of the reserve food of the seed into soluble materials available for the
nutrition of the young growing plant.
BIOLOGICAL SIGNIFICANCE OF FRUIT ACIDS, ETC.
The occurrence of organic acids, or their derivatives, which have
pronounced odors or flavors, in the flesh surrounding the seeds of fruits,
in the endosperm of vegetable seeds, or in the tubers, etc., of perennial
plants, thus making them attractive as food for animals and men,
undoubtedly serves to insure a wider distribution of the reproductive
organs of these plants; a fact which has unquestionably had a marked
influence upon the survival of species in the competitive struggle for
existence during past eras and in the development and cultivation of
different species by man. Indirect evidence that the proportion of these
attractive compounds present in certain species may have been considerably
increased by the processes of "natural selection" in the past is furnished
by the many successful attempts to increase the percentage of such
desirable constituents in fruits or vegetables by means of artificial
selection of parent stocks by skillful plant breeders.
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.
CHAPTER XI
ESSENTIAL OILS AND RESINS
Included in this group are all those substances to which the characteristic
odors of plants are due, along with others similar in structure and
possessing characteristic resinous properties. They have no such uniformity
in composition as is exhibited by the oils which are included among the
fats and waxes; but belong to several widely different chemical groups.
Furthermore, there is no sharp dividing line between the essential oils and
certain esters of organic acids on the one hand and the fats on the other.
For example, if an aromatic fluid essence is a light fluid, non-viscid, and
easily volatile, it is usually classed with the organic esters; denser
liquid substances, of oily or waxy consistency, and with comparatively
slight odor and taste are usually fats, while oils of similar physical
properties but possessing strong characteristic odors are classed as
essential oils, regardless of their chemical composition.
Included in this general class are compounds having a great variety of
chemical structures; e.g., hydrocarbons, alcohols, phenols, organic
sulfides and sulfocyanides, etc. Many of these compounds are crystalline
solids at ordinary temperatures, but melt to oily fluids at higher
temperatures. The characteristic property which assigns any given plant
extract to this group is that it has a strikingly characteristic odor or
taste, often accompanied by some definite physiological effect, or
medicinal property.
These compounds may be either secretions or excretions of plants, sometimes
normally present in the healthy tissue, and sometimes produced as the
result of injury or disease.
The essential oils and the resins often occur associated together in the
plant; or, the resins may develop from the oily juice of the plant after
exposure to the air.
THE ESSENTIAL OILS
These may be divided, according to their chemical composition, into two
major groups; (1) the hydrocarbon oils, or terpenes, and (2) the oxygenated
and sulfuretted oils.
The =terpenes= are of three different types, namely: (_a_) the
hemiterpenes, C_{5}H_{8}, unsaturated compounds of the valerylene series,
of which _isoprene_ (found in crude rubber) is the best-known example;
(_b_) the terpenes proper, C_{10}H_{16}, which constitute the major
proportion of the whole group; and (_c_) the polyterpenes
(C_{5}H_{8})_{_n_}, of which _colophene_ and _caoutchouc_ are the most
common examples.
Eleven different terpenes having the formula C_{10}H_{16} have been
isolated from various plant juices, and their molecular arrangement
carefully worked out. The following three examples will serve as typical of
the general structural arrangement of these hydrocarbons:
Limonene Camphene Pinene
H
|
C
/|\
/ | \
/ | \
/ | \
/ | \
CH_{3} H / | \
| | / | \
C C /CH_{3} | CH_{3}\
/ \\ CH_{3} / | \ H_{3}C \|/ CH_{2}
/ \\ \ / | \ | C |
H_{2}C CH C | CH_{2} | / |
| | /| | | | / |
| | CH_{3} | HCH | | / |
| | | | | | / |
H_{2}C CH_{2} CH_{2}=C | CH_{2} | / |
\ / \ | / | / |
\ / \ | / | / |
CH C |/ |
| | HC CH
C H \ //
// \ \ //
H_{2}C CH_{3} \ //
\ //
\ //
\ //
\ //
\ //
C
|
CH_{3}
A discussion of the evidence which supports these formulas as properly
represented the molecular arrangements of the various isomeric forms would
be out of place here, as its only particular interest is in connection with
the medicinal effects of the different compounds. It is clear, however,
that they are six-membered hydrocarbon rings, with additional hydrocarbon
groups attached to one or more of the carbon atoms in the ring.
Different modifications, or varieties, of the terpenes constitute the main
proportions of the oils of turpentine, bergamot, lemon, fir needles,
eucalyptus, fennel, pennyroyal, etc.
The =oxygenated essential oils= may be either alcohols, aldehydes, ketones,
acids, esters, or phenols, derived from either five-membered or
six-membered closed-ring hydrocarbons. They are usually present in the
plant oil in mixtures with each other or with a terpene. Since most of them
have pronounced physiological or medicinal properties, their structure has
been well worked out, in most cases; but it seems to be hardly worth while
to present these matters in detail here, as they are of interest chiefly on
account of their medicinal properties rather than their botanical
functions.
_Borneol_, C_{10}H_{17}OH, and _menthol_, C_{10}H_{19}OH, are typical
_alcohols_. The latter is a crystalline substance, which melts at 42°,
which is present in peppermint oil, both as the free alcohol and as an
ester of acetic acid.
Amyl acetate, CH_{3}·COOC_{5}H_{11}, and linalyl acetate,
CH_{3}·COOC_{10}H_{17}, the latter occurring in the oils of lavender and
bergamot, are typical esters classed as essential oils.
As examples of the _aldehyde_ oils, benzoic aldehyde, C_{6}H_{5}CHO, "oil
of bitter almonds," and cinnamic aldehyde, C_{6}H_{5}CH=CHCHO, found in the
oils of cinnamon and cassia, may be cited.
Camphor, C_{10}H_{16}O, is a _ketone_, having the following structural
formula:
H
|
C
/ | \
/ | \
/ | \
/ | \
/ | \
/ | \
CH_{2} | CH_{2}
| | |
|CH_{3}-C-CH_{3}|
| | |
CH_{2} | C=O
\ | /
\ | /
\ | /
\ | /
\ | /
\ | /
C
|
CH_{3}
There are a considerable number of essential oils which are _phenols_.
_Thymol_, C_{6}H_{3}·(CH_{3})·(C_{3}H_{7})·OH, in oil of thyme, and
carvacrol, its isomer, in oil of hops, are familiar examples.
O-C=O
/ |
Coumarin, the anhydride of cinnamic acid C_{6}H_{4} | ,
\ |
HC=CH
is an example of an acid substance which is classed as an essential oil,
even though it is a solid at ordinary temperatures. It has an odor and
flavor similar to that of _vanillin_, the essential flavoring material of
the vanilla bean, and is often used as a substitute for the latter in the
preparation of artificial flavoring extracts.
Of the =essential oils containing sulfur=, there are two common examples;
oil of mustard, allyl isosulfocyanide, C_{3}H_{5}NCS, and oil of garlic,
allyl sulfide (C_{3}H_{5})_{2}S. The latter is present in onions, garlic,
water cress, radishes, etc., the difference in flavor of these vegetables
being due to the fact that the allyl sulfide is united with other different
groups in the glucoside arrangement, in the different plants. Similarly,
mustard oil is not present in mustard seeds as such, but as a glucoside
which, when hydrolyzed by the enzyme _myrosin_ which is always present in
other cells of the same seeds, yields C_{3}H_{5}NCS, KHSO_{4}, and
C_{6}H_{12}O_{6}.
THE RESINS
The resins were formerly supposed to be the mother substances from which
the terpenes are derived. It is now known, however, that they are the
oxidation products of the terpenes. Their exact structure is still a matter
of some uncertainty, as their peculiar "resinous" character makes them very
difficult to study by the usual methods of chemical investigations.
Resins are divided into two classes: (_a_) the balsams, and (_b_) the solid
or hard resins. Canada balsam and crude turpentine are familiar examples of
the first class. They consist of resinous substances, dissolved in or mixed
with fluid terpenes. Ordinary resin, or _colophony_, consists chiefly of a
monobasic acid having the empirical formula C_{20}H_{30}O_{2}, known as
sylvinic acid, whose exact structure is not known. Its sodium salt is used
as the basis for cheap soaps.
The hard resins are amorphous substances of vitreous character, which
consist of very complex aromatic acids, alcohols, or esters, combined with
other complicated structures, known as _resenes_, whose definite chemical
nature is not yet known. Among the hard resins are many substances which
are extensively used in the manufacture of varnishes, such as copal, amber,
dammar, sandarach, etc.
There are also resinous substances, such as asaf[oe]tida, myrrh, gamboge,
etc., which are mixtures of gums (see Chapter VI) and true resins. Some of
these have considerable commercial value for medicinal or technical uses.
PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE OF ESSENTIAL OILS
No theory has yet been advanced concerning the possibility of the use of
essential oils and resins by plants in their normal metabolic processes.
The very great diversity in their chemical nature makes it impossible that
they should all be considered as having the same physiological function,
if indeed any of them actually have any such function.
It is evident that those aromatic compounds which occur as normal
secretions of plants and which give to the plants their characteristic
odors may act either as an attraction to animals which might utilize the
plants as food and so serve to distribute the seed forms, or as a repellent
to prevent the too rapid destruction of the leaves, stems, or seeds of
certain species of plants whose slow-growing habits require the
long-continued growth of these portions of the plant for the perpetuation
of the species. The presence of these compounds in larger proportions in
those species of conifers, etc., which grow in tropical regions, in
competition with other rapid-growing vegetation, suggests the latter
possibility. It must be admitted, however, that their presence in such
cases may be the result of climatic conditions, as indicated by the fact
that most spice plants are tropical in habit, rather than the result of
their protective influence in the struggle for survival during past ages.
Many of the oils and resins which are secreted as the result of injury by
disease or wounds have marked antiseptic properties and undoubtedly serve
to prevent the entrance into the injured tissue of destructive organisms.
But apart from these possible protective influences which may have had an
important effect upon the preservation and perpetuation of the species of
plants which secrete them, there is no known biological necessity for the
presence of these aromatic substances in plants.
REFERENCES
ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 7, Gerbstoffe,
Flechtenstoffe, Saponine, Bitterstoffe, Terpene, Aetherische Oele, Harze,
Kautschuk," 822 pages, Berlin, 1912.
ALLEN'S Commercial Organic Analysis, Vol. IV, "Resins, Rubber, Guttapercha,
and Essential Oils," 461 pages, 7 figs., Philadelphia, 1911 (4th ed.).
HEUSLER, F., trans. by Pond, F. J.--"The Chemistry of the Terpenes," 457
pages, Philadelphia, 1902.
PARRY, E. J.--"The Chemistry of Essential Oils and Perfumes," 401 pages, 20
figs., London, 1899.
CHAPTER XII
THE VEGETABLE BASES
We come, now, to the consideration of the characteristically nitrogenous
compounds of plants. None of the groups of compounds which have been
considered thus far have, as a group, contained the element nitrogen. This
element is present in the chlorophylls and in certain other pigments, but
not as the characteristic constituent of the molecular structure of the
group of compounds, nor do these compounds serve as the source of supply of
nitrogen for the plant's needs.
The characteristic nitrogen-containing compounds may all be regarded as
derived from ammonia, or ammonium hydroxide, by the replacement of one or
more hydrogen atoms with organic radicals of varying type and complexity.
If the group, or groups, which be considered as having replaced a hydrogen
atom in ammonia, in such compounds, is an alkyl group, the compound is
strongly basic in character and is known as an _amine_; whereas if the
replacing group is an acid radical, the resulting compound may be neutral
(known as _acid amides_), or weakly acid (known as _amino-acids_) in type.
Compounds of the first type constitute the _vegetable bases_; while those
of the second type are the _proteins_.
The vegetable bases may be divided into three groups. These are (_a_) the
_plant amines_, which are simple open-chain amines; (_b_) the _alkaloids_,
which are comparatively simple closed-ring amines, containing only one
nitrogen atom in any single ring; and (_c_) the _purine bases_, which are
complex compounds containing a nucleus with four carbon atoms and four
nitrogen atoms arranged alternately to form a double-ring group.
THE PLANT AMINES
The simple amines bear the relation to ammonia, or ammonium hydroxide,
represented by the following formulas, in which the R indicates any simple
alkyl radical:
H R R R R R H H
/ / / / \ / \ /
N-H N-H N-H N-R N-R N-H
\ \ \ \ / \ / \
H H R R R OH H OH
Ammonia Primary Secondary Tertiary Quaternary Ammonium
amine amine amine amine hydroxide
The simple amines which occur in animal tissues are known as "ptomaines"
and "leucomaines." The ptomaines are all decomposition products resulting
from the putrefactive decay of proteins caused by moulds or bacteria. Some
of these are highly toxic, producing the so-called "ptomaine-poisoning";
while others are wholly innocuous. They are all simple amines. Putrescine,
di-amino butane, NH_{2}·CH_{2}·CH_{2}·CH_{2}·CH_{2}·NH_{2}, and cadaverine,
di-amino pentane, HN_{2}·(CH_{2})_{5}·NH_{2}, are common non-toxic
ptomaines, resulting from the decay of meat. Neurine, trimethyl-ethylene
ammonium hydroxide, (CH_{3})_{3}(C_{2}H_{3})·NOH, is a violently poisonous
ptomaine produced in the decay of fish. Amines of similar structure to
these are occasionally found in living animal tissues. Such compounds are
known as _leucomaines_, to distinguish them from the _ptomaines_, which are
found only in dead material.
Corresponding in structure and properties to these amines of animal origin,
there is a series of basic substances, found in many plants, known as the
_plant amines_. The following are common examples:
=Trimethyl amine=, (CH_{3})_{3}N, is a very volatile compound, found in the
flowers of several species of the Rose family, the leaves of certain weeds,
etc. When crushed, these tissues give off a very fetid odor, which is due
to this amine.
=Choline=, =muscarine=, and =betaine= are plant amines which are closely
related to each other and to neurine (the toxic ptomaine) in composition
and structure, as shown in the following formulas:
CH_{2}CH_{2}OH CH_{2}CHO
/ /
(CH_{3})_{3}N (CH_{3})_{3}N
\ \
OH OH
Choline Muscarine
CH_{2}CO CH=CH_{2}
/ / /
(CH_{3})_{3}N / (CH_{3})_{3}N
\ / \
O OH
Betaine Neurine
Choline and betaine are non-toxic; while muscarine and neurine are violent
poisons.
Choline and muscarine occur in certain toadstools. Betaine and choline
often occur together in the germs of many plants. Betaine is found in the
beet root and the tubers of Jerusalem artichoke. Choline occurs alone in
the seeds and fruits of many plants, sometimes as the free amine, but more
often as a constituent of lecithin (see page 141).
Phenyl derivatives of simple amines are sometimes found in
---
/ \
plants. _Hydroxyphenylethyl amine_, HO-C CH_{2}·CH_{2}·NH_{2},
\ /
---
---
/ \
found in _ergot_, and _hordeine_, HO-C CH_{2}·CH_{2}·N·(CH_{3})_{2},
\ /
---
found in barley, are examples. The former has marked medicinal properties.
There is no known physiological use for these simple amines in plants. By
some investigators, they are regarded as intermediate products in the
synthesis or decomposition of proteins; but it would seem that if this were
a normal procedure, these amines would occur in varying proportions in all
plants, under different conditions of metabolism, instead of in practically
constant proportions in only a few species, as they do.
ALKALOIDS
These are a group of strong vegetable bases whose nitrogen atom is a part
of a closed-ring arrangement.
As a rule, alkaloids are colorless, crystalline solids, although a few are
liquids at ordinary temperatures. They are generally insoluble in water,
but easily soluble in organic solvents. Being strong bases, they readily
form salts with acids, and these salts are usually readily soluble in
water.
Alkaloids are usually odorless; although nicotine, coniine, and a few
others, have strong, characteristic odors. Most of them have a bitter
taste, and many of them have marked physiological effects upon animal
organisms, so that they are extensively used as narcotics, stimulants, or
for other medicinal purposes.
Most of the alkaloids contain asymmetric carbon atoms and are, therefore,
optically active, usually levorotatory, although a few are dextrorotatory.
The alkaloids are precipitated out of their solutions by various solutions
of chemical compounds, known as the "alkaloidal reagents": iodine dissolved
in potassium iodide solution gives a chocolate-brown precipitate; tannic
acid, phosphotungstic acid, phosphomolybdic acid, and mercuric iodide
solutions give colorless, amorphous precipitates; while gold chloride and
platinic chloride solutions give crystalline precipitates, many of which
have sharp melting points and can be used for the identification of
individual alkaloids. There are a great many specific color reactions for
individual alkaloids, which are important to toxicologists and pharmacists,
but which it would not be desirable to consider in detail here.
The alkaloids are conveniently divided into groups, according to the
characteristic closed-ring arrangements which they contain. The several
closed-ring arrangements which are found in common alkaloids, and upon
which their grouping is based, may be illustrated by the following
formulas:
H H H
H{2}_C---CH_{2} | \ /
| | C C
| | // \ / \
H{2}_C CH_{2} HC CH H_{2}C CH_{2}
\ / | | | |
N HC CH H_{2}C CH_{2}
| \\ / \ /
H N N
|
Pyrrolidine, C_{4}H_{9}N Pyrridine, C_{5}H_{5}N H
Piperidine, C_{5}H_{11}N
H H
\ / H
C |
/ \ H_{2}C-----C-----CH_{2}
H_{2}C CH_{2} | | |
| | | NH CH_{2}
HC CH | | |
|\ /| or H_{2}C-----C-----CH_{2}
| N | |
| /|\ | H
|/ H \|
H_{2}C-----CH_{2}
Tropane, C_{7}H_{13}N
H H H H
| | | |
C C C C
// \ / \\ // \ / \\
HC C CH HC C CH
| ¦ | | ¦ |
HC C CH HC C N
\\ / \ // \\ / \ //
C N C C
| | |
H H H
Quinoline, C_{9}H_{8}N Isoquinoline, C_{9}H_{8}N
The common alkaloids are distributed in the several groups as follows:
Pyrridine--piperidine group; piperine, coniine, nicotine.
Pyrrolidine group; hygrine and stachydrine.
Tropane group; atropine, hyoscine, cocaine, lupinine.
Quinoline group; quinine, cinchonine, strychnine, brucine.
Isoquinoline group; papaverine, hydrastine, morphine, codeine, berberine.
The composition and properties of the individual alkaloids have been
extensively studied, because of their medicinal uses. As they have no known
metabolic use to the plants which elaborate them, it will not be worth
while to consider all of these investigations in detail here. The following
facts with reference to certain typical members of each group will serve to
illustrate the general constitution and properties of the alkaloids.
=Piperine=, C_{17}H_{19}O_{3}, is found in black peppers. Its constitution
is represented by the following formula, the group which is united to the
piperidine ring, in this case, being piperic acid:
H_{2}
¦
C
/ \
H_{2}C CH_{2}
| | /\ --O
H_{2}C CH_{2} | | \
\ / | | CH_{2}
N----OC·CH=CH·CH=CH--| | /
\/ --O
=Coniine=, C_{8}H_{17}N, is found in the umbelliferous plant, _Conium
maculatum_. Structurally, it is a propyl-piperidine, represented by the
following formula:
H_{2}
¦
C
/ \
H_{2}C CH_{2}
| |
H_{2}C CH-C_{3}H_{7}
\ /
N
|
H
=Nicotine=, C_{10}H_{14}N_{2}, is the alkaloid of tobacco leaves. It is an
extremely poisonous, oily liquid, with a strong odor and a burning taste.
Its structural formula shows it to contain both a pyrridine ring and a
pyrrolidine ring, linked together thus
H
|
C H_{2}C---CH_{2}
/ \\ | |
HC C------HC CH_{2}
¦ | \ /
HC CH N
\ // |
N CH_{3}
=Hygrine=, C_{7}H_{13}NO, from coca leaves, is an acetic acid salt of
pyrrolidine, represented by the following formula:
H_{2}C---CH--OC·CH_{3}
| |
H_{2}C CH_{2}
\ /
N
|
CH_{3}
=Atropine= and =hyoscyamine=, C_{17}H_{23}NO_{3}, are optical isomers.
Atropine is an extremely poisonous, white crystalline compound, which is
obtained from deadly nightshade and henbane, and used in medicine, in
minute doses, as an agent for reducing temperature in acute cases of
fevers. Structurally, it is a tropic acid ester of tropane, represented
by the following formula:
H_{2}C---CH--------CH_{2} C_{6}H_{5}
| | | |
| N-CH_{3} CHOOC---CH
| | | |
H_{2}C---CH--------CH_{2} CH_{2}OH
=Cocaine=, C_{17}H_{21}NO_{4}, is found in coca leaves. It is a white
crystalline solid, which is largely used as a local anæsthetic for minor
surgical operations. Its structural formula is
H_{2}C---CH--------HC-OOC·CH_{3}
| | |
| N-CH_{3} HC-OOC·C_{6}H_{5}
| | |
H_{2}C---CH---------CH_{2}
It is, therefore, a di-ester of acetic and benzoic acids with tropane.
=Cinchonine=, C_{19}H_{22}N_{2}O, and =quinine=, C_{20}H_{24}N_{2}O_{2},
are alkaloids found in cinchona bark. They are white crystalline solids,
which are extensively used in medicine. They have been shown to contain a
quinoline group combined with modified piperidine groups, as represented in
the following formulas:
H
|
N C
/ \ / \ /|\
| | | / | \
| | | H_{2}C HCH CH-CH=CH_{2}
\ / \ / | | |
CH-CHOH-HC HCH CH_{2}
\ | /
N
Cinchonine
H
|
N C
/ \ / \ / \
| | | / \
| | | H_{2}C CH-CH=CH_{2}
\ / \ / | |
CH-CHOH-HC CH_{2}
\ /
N
Quinine
=Strychnine=, C_{21}H_{22}N_{2}O_{2}, =brucine=,
C_{21}H_{20}(OCH_{3})N_{2}O_{2}, and =curarine= are three alkaloids which
are present in the seeds of several species of _Strychnos_. They are all
highly poisonous. Beyond the fact that when they are hydrolyzed they yield
quinoline and indole, their composition is unknown.
=Morphine=, C_{17}H_{19}NO_{3}, is the chief alkaloid of opium, which is
the dried juice of young pods of the poppy. Both the alcoholic solution of
opium (known as "laudanum") and morphine itself are extensively used in
medicine as narcotics to deaden pain. Morphine has an exceedingly complex
structure, being a combination of an isoquinoline and a phenanthrene
nucleus, which is probably correctly represented by the following formula:
H H_{2}
| ¦
C C
// \ / \
HOC C CH_{2}
| ¦ |
C C N-CH_{3}
/ \\/ \ /
/ C CH
O | |
\ HC CH_{2}
\ / \ /
HC C
| ¦
H_{2}C CH
\ /
C
¦
H_{2}
=Codeine=, C_{17}H_{18}(OCH_{3})NO_{2}, which is also found in opium, is a
methyl derivative of morphine. =Papaverine=, =laudanosine=, =narcotine=,
and =narceine= are four other alkaloids found in opium. They each contain
an isoquinoline nucleus, combined by one bond to a benzene ring, with one
or more methyl groups and three or more methoxy (OCH_{3}) groups attached
at various points around the three characteristic rings. The following
formula for laudanosine will illustrate their structure:
/ \ / \
CH_{3}O | |
| | | OCH_{3}
CH_{3}O | N-CH_{3} / \
\ / \ / | |OCH_{3}
| | |
| \ /
H_{2}C----------------CH
The above discussions of the composition of typical alkaloids clearly
indicate the extreme complexity of their molecular structure. It is
generally supposed that they are formed by the decomposition of proteins.
But they are developed in only a few particular species of plants and are
always present in these plants in fairly constant quantities. Hence, it
appears that, in these species, the production of alkaloids is in some way
definitely connected with protein metabolism; but it is certain that this
is not a common relationship, as it is manifested by such a limited number
of species of plants, and there is absolutely no knowledge as to its
character and functions. Some authorities prefer to regard the alkaloids as
waste-products of protein metabolism; but here, again, it is difficult to
understand why such products should result in certain species of plants and
not in others.
THE PURINE BASES
This is a group of compounds, widely distributed in both plant and
animal tissues, all of which are derivatives of the compound known as
_purine_, C_{5}H_{4}N_{4}. All of the naturally occurring compounds of
this group may be regarded as derived from purine, either by the
addition of oxygen atoms, or by the replacing of one or more of its
hydrogen atoms with a methyl (CH_{3}) group or an amino (NH_{2}) group.
The following structural formula represents the arrangement of the
purine nucleus, the numbers being used to designate the nitrogen or
carbon atoms to which the additional atoms, or groups, are attached in
the more complex compounds of the group. In purine itself, the four
hydrogen atoms are attached in the 2, 6, 7, and 8 positions.
6
1N==C--
| | 7
--2C 5C---N--
¦ ¦ \
¦ ¦ 8C--
¦ ¦ //
3N--4C--9N
The double bonds, in each case except those between the 4 and 5 carbon
atoms, are easily broken apart and readjusted, so that other atoms or
groups can be attached to any atom in the nucleus except the 4 and 5 carbon
atoms. In all of the statements with reference to the structure of the
purine bases, the term "oxy" is used to mean an oxygen atom attached by
both its bonds to one of the carbons in the nucleus, instead of its
customary use to mean the monovalent OH group replacing a hydrogen, as in
the case of all other nomenclature of organic compounds. With this
understanding, reference to the numbered nucleus formula above will make
plain the structure of all of the purine bases which are included in the
following list:
Hypoxanthine, C_{5}H_{4}N_{4}O, = 6-monoxypurine.
Xanthine, C_{5}H_{4}N_{4}O_{2}, = 2,6-dioxypurine.
Uric acid, C_{5}H_{4}N_{4}O_{3}, = 2,6,8-trioxypurine.
Adenine, C_{5}H_{3}N_{4}NH_{2}, = 6-aminopurine.
Guanine, C_{5}H_{3}N_{4}ONH_{2}, = 2-amino-6-oxypurine.
Theobromine, C_{5}H_{2}N_{4}O_{2}(CH_{3})_{2} =
3,7-dimethyl-2,6-dioxypurine, or dimethyl xanthine.
Theophylline, C_{5}H_{2}N_{4}O_{2}(CH_{3})_{2} =
1,3-dimethyl-2,6-dioxypurine.
Caffeine, C_{5}HN_{4}O_{2}(CH_{3})_{3} = 1,3,7-trimethyl-2,6-dioxypurine,
or trimethyl xanthine.
In order to make these structural relationships quite clear, the following
formulas for uric acid and for caffeine are presented as typical examples:
H N--C=O CH_{3}--N--C=O
| | | |
O=C C--N-H O=C C--N-CH_{3}
| ¦ \ | ¦ \
| ¦ C=O | ¦ CH
| ¦ / | ¦ //
HN--C--N-H CH_{3}-N--C--N
Uric acid Caffeine
=Uric acid= is found in the excrement of all animals; in the urine of
mammals, and in the solid excrement of birds and reptiles. It is not known
to occur in plants.
=Xanthine= and =hypoxanthine= occur in animal urine, and also in the
tissues of both plants and animals.
=Adenine= and =guanine= are constituents of all nucleic acids (see below)
and, hence, are found in all plant and animal tissues. Guanine is the chief
constituent of the excrement of spiders, and is found also in Peruvian
guano. It is also a constituent of the scales of fishes.
=Caffeine=, =theophylline=, and =theobromine= are not found in animal
tissues, but are fairly widely distributed in plants. Caffeine and
theobromine are the active constituents of tea leaves and coffee seeds and
are found also in cacao beans and kola nuts. The use of these three
compounds in the metabolism of the plants which elaborate them is wholly
unknown. They are not so directly related to protein metabolism as are the
other purine bases.
The purine bases, other than the three mentioned in the preceding
paragraph, are undoubtedly intermediate products in protein metabolism. In
animals, they constitute a large proportion of the waste-products from the
use of proteins in the body. It is not clear that there are similar
waste-products in plant metabolism, however. In both plants and animals,
the purine bases which are a part of the nucleic acids undoubtedly play an
important and essential part in growth, since they form the major
proportion of the nucleus, from which all cell-division proceeds.
THE PYRIMIDINE BASES
These compounds do not occur free in plants; but since they are constituent
groups in the plant nucleic acids (see below), a brief explanation of their
composition is desirable. They are nitrogenous bases, similar to, but
somewhat simpler than, the purine bases. Their general composition and
structural relationships are illustrated by the following typical formulas:
N==C-H H-N--C=O
| | | |
H-C C-H O=C C-H
¦ ¦ | ¦
N--C-H H-N--C-H
Pyrimidine Uracil
C_{4}H_{4}N_{2} C_{4}H_{4}N_{2}O_{2}
2,6-dioxypyrimidine
N==C-NH_{2} H-N--C=O
| | | |
O=C C-H O=C C-CH_{3}
| ¦ | ¦
H-N--C-H H-N--C-H
Cytosine Thymine
C_{4}H_{3}N_{2}ONH_{2} C_ {4}H_{3}N_{2}O_{2}CH_{3}
2,oxy-6-amino-pyrimidine 2,6-dioxy-5-methyl-pyrimidine
THE NUCLEIC ACIDS
The nuclei of cells are composed almost wholly of complex organic salts, in
which _proteins_ constitute the basic part and _nucleic acids_ the acid
part. These salts, or esters, are known under the general name
"nucleoproteins." The composition of the proteins is discussed in detail in
the following chapter, and it seems desirable to present a brief discussion
of the constitution of the nucleic acids here; although they are
essentially acids rather than vegetable bases.
The nucleic acids are complex compounds consisting of a carbohydrate,
phosphoric acid, two purine bases, and two pyrimidine bases. So far as is
known, all animal nucleic acids are identical and all plant nucleic acids
are identical; but those of plant origin differ from those found in animal
cells in the character of the carbohydrate and that of one of the
pyrimidine bases which are present in the molecule, as shown in the
following tabulation of their composition:
Animal nucleic acid Plant nucleic acid
Phosphoric acid Phosphoric acid
Hexose (levulose) Pentose (_d_-ribose)
Guanine Guanine
Adenine Adenine
Cytosine Cytosine
Thymine Uracil
The structure of the plant nucleic acid may be represented by the following
formula:
OH
|
O=P--O--carbohydrate-guanine group
|
O
|
O=P--O--carbohydrate-adenine group
|
O
|
O=P--O--carbohydrate-uracil group
|
O
|
O=P--O--carbohydrate-cytosine group
|
OH
That this is probably a correct representation of the general arrangement
in this compound, is indicated by the fact that by different methods of
hydrolysis it is possible to split off either the purine and pyrimidine
bases, leaving a carbohydrate ester of phosphoric acid; or the phosphoric
acid, leaving carbohydrate combinations with the nitrogenous bases.
Nucleic acid, prepared from animal glands which contain large proportions
of it, is a white powder, which is insoluble in water, but when moistened
forms a slimy mass. It is almost insoluble in alcohol, but dissolves
readily in alkaline solutions, forming a colloidal solution which readily
gelatinizes (see chapter on Colloids). Solutions of nucleic acids are
optically active, probably because of the carbohydrate constituents.
From their structure and properties, it is apparent that nucleic acids are
on the border line between carbohydrates, plant amines, and proteins. They
undoubtedly play an important part, both in cell-growth and in the
synthesis of proteins from carbohydrates and ammonium compounds.
References
BARGER, GEO.--"The Simpler Natural Bases," 215 pages, _Monographs_ on
Biochemistry, London, 1914.
FISCHER, E.--"Untersuchungen in der Puringruppe, 1882-1906," 608 pages,
Berlin, 1907.
HENRY, T. A.--"The Plant Alkaloids," 466 pages, Philadelphia, 1913.
JONES, W.--"The Nucleic Acids," 118 pages, _Monographs_ on Biochemistry,
London, 1914.
PICTET, A.--"La Constitution Chimique des Alcaloides Vegetaux," 421 pages,
Paris, 1897 (2d ed.).
VAUGHAN, V. C. and NOVY, F. G.--"Ptomaines, Leucomaines, Toxins and
Antitoxins," 604 pages, Philadelphia, 1896, (3d ed.).
WINTERSTEIN, E. and TRIER, G.--"Die Alkaloide," 340 pages, Berlin, 1910.
CHAPTER XIII
PROTEINS
The proteins are the most important group of organic components of plants.
They constitute the active material of protoplasm, in which all of the
chemical changes which go to make up the vital phenomena take place.
Combined with the nucleic acids, they comprise the nucleus of the cell,
which is the seat of the power of cell-division and, hence, of the growth
of the organism. Germ-cells are composed almost exclusively of protein
material. Hence, it is not an over-statement to say that proteins furnish
the material in which the vital powers of growth and repair and of
reproduction are located. A recognition of their importance is reflected in
the use of the name "protein," which comes from a Greek word meaning
"pre-eminence," or "of first importance."
In addition to the proteins which constitute the active protoplasm, plants
also contain large amounts of reserve, or stored, proteins, especially in
the seeds. In the early stages of growth, the proteins are present in
largest proportions in the vegetative portions of the plant; but as
maturity approaches, a considerable proportion of the protein material is
transferred to the seeds.
GENERAL COMPOSITION OF PROTEINS
The plant proteins are fairly uniform in their percentage composition. The
analyses of some sixteen different plant proteins show the following
maximum limits of percentages of the different chemical elements which they
contain: Carbon, 50.72-54.29; hydrogen, 6.80-7.03; nitrogen, 15.84-19.03;
oxygen, 20.86-24.29; sulfur, 0.17-1.09. Animal proteins vary more widely,
both in percentage composition and in properties, than do those of plant
origin.
Protein molecules are very large and, in the case of the so-called
"conjugated proteins" in particular, their structure is very complex.
The molecular weight of some of the proteins has been determined
directly, in the case of those particular ones which can be prepared in
proper form for the usual determination of molecular weight by the
osmotic pressure method; and has been computed for various others, from
the percentage of sulfur found on analysis, or (in the case of the
hæmoglobin of the blood) from the proportion by weight of oxygen
absorbed. From these determinations and computations, the following
formulas for certain typical proteins have been calculated: for zein
(from Indian corn), C_{736}H_{1161}N_{184}O_{208}S_{3}; for gliadin
(from wheat), C_{685}H_{1068}N_{196}O_{211}S_{5}; for casein (from
milk), C_{708}H_{1130}N_{180}O_{224}S_{4}P_{4}; for egg-albumin,
C_{696}H_{1125}N_{175}O_{220}S_{8}. These few examples will serve to
illustrate the enormous size and complexity of the protein molecule. The
conjugated proteins are still more complex than the simple proteins
whose formulas are here presented.
Fortunately for the purposes of the study of the chemistry of the proteins,
however, it has been found that most of the common plant proteins, known as
the "simple proteins," can easily be hydrolyzed into their constituent unit
groups, which are the comparatively simple amino-acids, whose composition
and properties are well understood. A study of the results of the
hydrolysis of some twenty common plant proteins has shown that it is rarely
possible to recover the amino-acids in sufficient quantities to account for
a full 100 per cent of the material used, the actual percentage of
amino-acids recovered usually totaling from 60 to 80 per cent. The
remaining material is supposed to be also composed of amino-acids which are
linked together in some arrangement which is not broken apart by any method
of hydrolysis which has yet been devised. This view is borne out by the
fact that substances which exhibit all the characteristic properties of
proteins have been artificially synthetized, by using only amino-acid
compounds. Animal proteins often show a much larger proportion of
unhydrolyzable material than do plant proteins.
AMINO-ACIDS AND PEPTID UNITS
The products of hydrolysis of the common simple proteins are all
amino-acids. These are ordinary organic acids with one (or more) of the
hydrogen atoms of the alkyl group replaced by a --NH_{2} (or sometimes by a
--NH--) group. They may be regarded as ammonia, NH_{3}, with one of its
hydrogen atoms replaced by an acid radical; or as the acid with one of its
hydrogens replaced by the NH_{2} group. For example, an amino-acid derived
from acetic acid, CH_{3}·COOH, is glycine, or amino-acetic acid,
CH_{2}NH_{2}·COOH; from propionic acid, CH_{3}·CH_{2}·COOH, there may be
obtained either [alpha]-amino-propionic acid, CH_{3}·CHNH_{2}·COOH, or
[beta]-amino-propionic acid, CH_{2}NH_{2}·CH_{2}·COOH, etc.
All of the amino-acids which result from the hydrolysis of proteins are
[alpha]-amino-acids, that is to say, the NH_{2} group is attached to the
[alpha]-carbon atom, i.e., the one nearest to the COOH group. Hence, the
general formula for all the amino-acids which are found in plants is
R·CHNH_{2}·COOH.