Structure and chemical properties of carboxylic acids. Heterofunctional compounds.Lipids: classification, structure and biological role.
Mono- and dicarboxylic acids.
Functional derivates of the carboxylic acids
CARBOXYLIC ACIDS
Carboxylic acids are а compound whose characteristic functional group is the carboxyl group, example:
Structure. Carboxylic acids are distinguished by the functional grouping CO2H. Four ways of writing this grouping, referred to as the carboxy group, are shown.
lewis structure kekule structure condensed structures
Either an organic group or а hydrogen may be attached to the carboxy group. The carbon atom in а carboxy group uses three hybrid orbitals to bond to the oxygen of the ОН group, the carboxy oxygen, and to hydrogen or an organic radical. These three orbitals are approximately spa hybrids that lie in one plane. The remaining p orbital on the carbon forms а m bond to а р orbital on the carboxy oxygen. There are two distinct С-O bond distances, corresponding to C=O and С-О. The bond angles and bond lengths formic acid determined by microwave spectroscopy. Note that the bond angles around the carboxy carbon are only approximately those expected for sp2-hybridization. The array НСОО is planar and the hydroxy hydrogen lies outside of this plane.
In the solid and liquid phases, as well as in the vapor phase at moderately high pressure, carboxylic acids exist largely in the dimeric form depicted:
Nomenclature: There are two systems of nomenclature currently in use for carboxylic acids, and the student should be acquainted with both. Since many of the simpler acids are naturally occurring and were discovered early in the history of organic chemistry, they have well-entrenched “common” names. At the 1892 IUPAC Congress, it was agreed to derive the name of а carboxylic acid:
1. Select as the parent carbon chain the longest carbon chain that includes the carbon atom of the carboxyl group.
2. Name the parent chain by changing the -е ending of the corresponding alkane to -oic acid.
3. Number the parent chain by assigning the number 1 to the carboxyl carbon atom.
4. Determine the identity and location of any substituents ш the usual manner and append this information to the front of the parent chain name.
2-hydroxy-6-methylheptanoic acid
When using commoames, the chain is labeled a, b ,g, d, and so on, beginning wit the carbon adjacent to the carboxy carbon.
d-phenylvaleric acid or 5- phenylpentanoic acid (not s-phenylpentanoic acid)
Physical properties
Solubility
Carboxylic acids are polar. Because they are both hydrogen-bond acceptors (the carbonyl) and hydrogen-bond donors (the hydroxyl), they also participate in hydrogen bonding. Together the hydroxyl and carbonyl group forms the functional group carboxyl. Carboxylic acids usually exist as dimeric pairs ionpolar media due to their tendency to “self-associate.” Smaller carboxylic acids (1 to 5 carbons) are soluble in water, whereas higher carboxylic acids are less soluble due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be rather soluble in less-polar solvents such as ethers and alcohols.
Boiling points
Carboxylic acids tend to have higher boiling points than water, not only because of their increased surface area, but because of their tendency to form stabilised dimers. Carboxylic acids tend to evaporate or boil as these dimers. For boiling to occur, either the dimer bonds must be broken, or the entire dimer arrangement must be vaporised, both of which increase enthalpy of vaporization requirements significantly.
Acidity
Carboxylic acids are typically weak acids, meaning that they only partially dissociate into H+ cations and RCOO– anions ieutral aqueous solution. For example, at room temperature, in a 1-molar solution of acetic acid, only 0.4% of the acid molecules are dissociated. Electronegative substituents give stronger acids.
Iontzation
Compounds containing the functional group-COOH are weakly acidic; in fact it is this property from which the class derives its name. When acetic acid is dissolved in the water, the equilibrium in equation:
The equilibrium constant for this reaction, is called as “acid dissociation constant”
Inductive effects. Electronegative groups whose bonds to carbon are highly polar have important effects on the acidity of acids, that this effect could be interpreted in terms of the electrostatic interaction of а bond dipole with the anionic negative charge. The вате behavior is manifest by substituent groups in carboxylic acids. Because of the higher acidity and ease of measurement of carboxylic acids, а wealth of quantitative acidity data is available. Atoms that have high electronegativity tend to withdraw electron density from carbon and have а marked acid-strengthening effect. Chloroacetic acid is 1.9 рKa, units more acidic than acetic acid. The С – Cl bond dipole is oriented in such а way that the positive end is closer to the negative charge on the carboxy group than is the negative end. Electrostatic attraction exceeds the repulsion and the negative charge of the anion в тоге stabilized.
Carbon-carbon double and triple bonds have а significant еlectron-attracting effect that is reflected in the enhanced acidity of vinylacetic and ethynylacetic acids. A sp2– hybridized carbon obital with its greater s character is effectively more electronegative than a sp3 orbital. The higher alkanoic acids are somewhat less acidic than acetic acid. Alkyl groups manifest a small but significant electron-donatin inductive effect in appropriate systems in solution. The indinductive effect of remote substituents falls off dramatically with increased distance from the charged center.
Classification:
1. By the nature of hydrocarbon radical:
a) Saturated acid is acid, which has only simple bonds in molecule. Example: acetic acid, formic acid, buthanic acid;
b) Ansaturated acid is an acid, which has both as simple bonds and duble bonds in molecule. Example: palmitoleic acid,, oleic acid linoleic acid, linolenic acid, arashdonic acid;
c) Aromacic acid is acid, which contain aromatic ring. Example benzoic acid.
2. By the number of carboxyl groups:
a) Monocarboxylic acid is acid, which has one carboxylic group in molecule. Example: acetic acid, formic acid, buthanic acid;
b) Dicarboxylic acid is acid, which has two carboxylic group in molecule. Example: oxalic acid, malonic acid.
Reactions: The chemistry of carboxylic acids may be divided mechanistically into four
categories: (а) reactions involving the acidic O-Н bond, (b) reactions occurring in the hydrocarbon side chain, (с) reactions occurring at the carboxy carbon atom, and (d) one-carbon degradations.
1) Reactions involving the ОН-bond
a) Important reaction of carboxylic acids involving the ОН bond – the reaction with bases to give salts.
b) Another important reaction involving this bond is the reaction of carboxylic acids with diazomethane. The products of this reaction are the methyl ester and nitrogen.
2) Reactions involving the hydrocarbon side chain
a) Carboxylic acids undergo the normal reactions of alkanes, as modified by the presence of the carboxy group, in the hydrocarbon chain of the molecule. For example, butyric acid undergoes combustion and free-radical chlorination.
Since these reactions are not selective for any particular position along the chain, they generally have по preparative utility.
b) One reaction of the aliphatic chain that does have utility is the reaction of carboxylic actds with phosphorus tribromide and bromine. This reaction is sometimes known as the Hell-Volhard-Zelinsky reaction, after its discoverers.
c) Note that the reaction is positionally selective – only the hydrogen on С-2 is replaced. This is not а free-radical halogenation reaction. The overall result, a-bromination, is accomplished by а sequence of steps. The key step involves the reaction of bromine with the enol form of the corresponding acyl bromide. Phosphorus tribromide facilitates the reaction by reacting with the carboxylic acid to yield the acyl bromide (bromoanhydride), which undergoes enollzation much more readily than the acid itself.
3) Reacttons Occurring at the Carbonyl Carbon
a) The formation amides. The most common reaction of this type is the reaction of carboxylic acids with ammonia or amines to give amides. When ammonia is bubbled through butyric acid at 1850, butyramide is obtained in 85% yield. The reaction involves two stages. At room temperature, or even below, butyric acid reacts with the weak base ammonia to give the salt ammonium butyrate. This salt is perfectly stable at normal temperatures. However, pyrolysis of the salt results in the elimination of water and formation of the amide.
b) esterification: Carboxylic acids react readily with alcohols in the presence of catalytic amounts of mineral acids to yield compounds called esters. The process is called esterification.
Unlike most of the reactions we have encountered, this one has an equilibrium constant of relatively low magnitude. The experimental equilibrium constant for the reaction of acetic acid with ethanol is:
c) reaction with halo-compounds
Carboxylic acids react with thionyl hloride, phosphorus pentachloride, and phosphorus tribromide in the same way that alcohols do. The products are acyl halides:
d) reaction formation anhydrides.
Dicarboxylic acid is a compound that contains two carboxylic groups of a carbon chain. Saturation acid of this type are named by appending the suffix –dioic acid to the corresponding alkane name.
Oxalic acid malonic acid
Decarboxylation reaction:
Dicarboxylic acid has same properties as monocarboxylic.
Urea. Reaction:
(a) hydrolysis by strong acids:
(b) dimerization:
biuret
Hydroxy acids are compounds, which molecules have one or more hydroxyl groups and one or more carboxyl groups. Many hydroxy acids are important iature and, correspondingly, have trivial names that are in common use.
Gligolic acid is a constituent of cane sugar juice. Lactic acid is responnsible for the characteristic odor and taste of sour milk. Other important hydroxy acid are dicarboxylic acids. Malic acid accurs in fruit juices. Tartaric acid has been known since antiguity as the mono potassium salt (cream of tartar) deposited in the lees of wine. The hydroxy acids with asyymmetric carbon are optically active iature.
Both the (-) and (+) forms of tartaric acid are found iature, although the (+) acid by the more common: two optically inactive forms are known. Racemic acid, m.p. 2060, is simply a mixture of (+)- and (-)-tartaric acid. meso-tartaric acid, m.p.1400, is the R,S diastereoisomer.
(+)-(R,R)- (-)-(S,S)-tartaric acid meso-tartaric acid
Syntesis hydroxy acid.
a– hydroxy acid can be prepared form a-halo acid by hydrolysis:
Recall that a-halo acid can be prepared from the carboxylic acids by the Hell-Vollhard-Zelinsky bromination:
Reactions: 1.Formation of lactones. Recall that carboxylic acids react with alcohols under acid catalysis to yield esters
А hydroxy acid contains both of these functional groups, and thus it can undergo intramolecular esterification to yield, а cyclic ester, called, а lactone.
g-hydroxybutyric acid g-butyrolactone
Lactonization, like normal esterification, is an equilibrium process. Only when the lactone has а five- or six-member ring is there а substantial amount of lactone present under equilibrium conditions.
2. Polimerization, formation of lactides. As discussed in the previous section, 4- and 5-hydroxy acids react rapidly in an intramolecular process to afford lactones. Other hydroxy acids, which cannot form 5- or 6-member rings, undergo polymerization unless the reaction is carried out under high dilution conditions
a-Hydroxy acids cannot form а stable lactone ring (three-membered), so they undergo intermolecular self-esterification under acid catalysis. However, the initial dimeric product is now а form of 5-hydroxy acid, во lactonization occurs. The product, which is а dilactone containing two molecules of the original a-hydroxy acid, is called а lactide.
Lactide
3. Dehydratation. Like b-hydroxy aldehydes and ketones, b-hydroxy acids and their derivatives undergo dehydration easily under acidic conditions. The mechanism is similar to that for dehydration of the other p-hydroxy carbonyl compounds discussed previously. В under the Benzoin Condensation). Since conjugation of а double bond with an acid or ester carbonyl group is less stabilizing than with an aldehyde or ketone carbonyl, mixtures of the a,b-unsaturated and b,g-unsaturated acid often result from dehydration of а b-hydroxy acid.
4. Like a-hydroxy aldehydes and ketones, a-hydroxy acids and their derivatives can hydration in water solution by strong mineral acids, such as sylfuric acid:
Lactic acid
Heterofunctional compounds are widespread in the nature. They are in fruits and vegetable leafs. Also they are formed in body. So, the lactic acid is product of transformation glucose (glycolysis) in human body. A malic and citric acid formed in a cycle of tricarboxylic acids, which is also known as citric acid cycle or Krebs’ cycle. Hydroxo acids such as: pyruvic acid, acetoacetic acid, oxaloacetic acid, -ketoglutaric acid is important in metabolism of carbohydrates.
LIPIDS
Lipids form а large class of relatively water-insoluble bioorganic compounds. In humans and many animals, excess carbohydrates and other energy-yielding foods are converted to, and stored in the body in the form of, lipids called fats. These fat reservoirs constitute а major way of storing chemical energy and carbon atoms in the body. Fats and other lipids also surround and insulate vital body organs, providing protection from mechanical shock and helping to maintain correct body temperature. Lipids function as coverings for nerve fibers and as the basic structural components of all cell membranes. Many chemical messengers in the human body, substances called hormones, are lipids.
Lipids, unlike carbohydrates and most other classes of compounds, cannot be defined from а structural viewpoint. А variety of functional groups and structural features are found in molecules classified as lipids. What lipids share are their solubility properties. Lipids are a structurally heterogeneous group of substances of biological origin that are only sparingly soluble, if at all, in water but are soluble ionpolar organic solvents. When biological material (animal or plant tissue) is homogenized in а blender and mixed with а nonpolar organic solvent, the substances that dissolve in the solvent are the lipids.
Functions. 1. The most important role of lipids is as а fuel. Much of the carbohydrates of the diet is converted to fat which is stored in various tissues and utilised at the time of requirement. Thus fat may be the major source of energy for many tissues. Actually, in some respects lipids (fats) are even superior to carbohydrates as source of energy.
2. Since fat is а bad conductor of heat, it provides excellent insulation. Thus in cold conditions, in which heat is lost to the environment, it provides both an insulating blanket and an extra energy source.
3. Fat may also provide padding to protect the internal organs. Brain and nervous tissue are rich in certain lipids, а fact which indicates the importance of these compounds to life.
4. Some compounds derived from lipids are important building blocks of biologically active materials; е.g. acetic acid can be used by the body to synthesize cholesterol and related compounds (hormones).
5. Lipoproteins are constitueats of cell walls. Тhe lipids present in lipoproteins constituting the cell walls are of the types of phospholipids. Since lipids are water msoluble they act as ideal barrier for preventiag water soluble materials from passing freely between the intra- and extra-cellular fluids.
6. One more important function of dietary lipids is that of supplying the sо-called essential fatty acids Although there are several functions о(essential fatty acids (EFA), none of them is well defined.
7. Dietary fat is also found to be necessary for the sufficient absorption of the essential fatty acids and fat-soluble vitamins from the gastro intestinal tract.
However, it is important to note that deficiency of an essential fatty acid has not been observed in man, because the amounts required are very small and an absolutely fat-free diet is practically unknown.
Classification: Lipids can be divided into two major classes on the basis of whether they undergo hydrolysis reactions in alkaline (basic) solution. Saponifiable lipids can be hydrolyzed under alkaline conditions to yield salts of fatty acids. Nonsaponifiable lipids do not undergo hydrolysis reactions in alkaline solution.
We begin our consideration of lipids with an in-depth discussion of saponifiable lipids. Information about nonsaponifiable lipids follows.
The basis of the nature of the products obtained on hydrolysis lipids are mainly divided into three type, viz. simple, compound and derived lipids.
1. Simple lipids. These are esters of fatty acids and alcohols and thus on hydrolysis give fatty acids and alcohols. They may be of two types.
а) Fats and oils. These are esters of fatty acids and glycerol (а trihydric alcohol). These are also known as glycerides.
b) Waxes. These are esters of long-chain fatty acids and long-chain monohydric alcohols or sterols.
2 Compound lipids. Compound lipids are esters of fatty acids and alcohols in combination with other compound and thus on hydrolysis give fatty acids, alcohol and other compounds. On the basis of the nature of the other group, compound lipids may again be of following types.
а) Phospholipids. These are fat like compounds containing phosphoric acid and а nitrogen base.
b) Glycolipids. These are compounds containing а fatty acid, а carbohydrate, а complex alcohol, and nitrogen. but nо phosphorus.
3. Derived lipids. These compounds although do not contain an ester linkage but are obtained by the hydrolysis of simple and compound lipids. They may be fatty acids, alcohols and sterols,
Fatty acids are saponifiable lipid building blocks. We begin our discussion of saponifiable lipids by considering fatty acids, compounds that are а building block in all saponifiable lipid structures.
Fatty acids are naturally occurring carboxylic acids with an unbranched carbon chain and an eveumber of carbon atoms. They are rarely found free iature but rather occur mostly in esterified form in the structures of saponifiable lipids. Because of the pathway by which fatty acids are biosynthesized they almost always contain an even number of carbon atoms. Long-chain fatty acids (12 to 26 carbon atoms) are found in meats and fish; medium-chain fatty acids (6 to 10 carbon atoms) and short-chain fatty acids (fewer than б carbon atoms) occur primarily in dairy products.
Almost all have an eveumber of carbon atoms, and although carboxylic acids withmore than 80 carbon atoms have been isolated from natural sources, those with 16-18 (or 16-22) carbon atoms are most common. Unsaturated acids preferentially exist as cis-isomers with the double bond in one of relatively few preferred locations.
Where there is more than one double bond in an unsaturated carboxylic acid, the double bonds usually occur in a methylene-interrupted pattern:
(-HC = CH- CH2– HC = CH- CH2-)n
Fats and oils.
The most abundant fatty-acid-containing (saponifiable) lipids are the fats and oils. Because these two types of compounds have the same general chemical structure, we will consider them at the same time.
Fats and oils are formally called triacylglycerols. А triacylglycerol is а compound formed by esterification of three fatty acids to glycerol. An acyl group is the group that remains after the – ОН group is removed from а fatty acid. Thus, as the name implies, triacylglycerols contain three fatty acid residues (acyl groups) esterified to glycerol. An older name that is still frequently used for а triacylglycerol is triglyceride.
The triacylglycerol produced from glycerol and three molecules of stearic acid is an example of а simple triacylglycerol. А simple triacylglycerol is а triester formed the reaction of glycerol with three identical fatty acid molecules.
If the reacting fatty acid molecules are not all identical, then the result is а mixed triacylglycerol. А mixed triacylglycerol is а triester formеd from the reaction of glycerol with more than one kind of fatty acid molecule.
Waxes.
Wax is а mixture of esters of high molecular weight alcohols and high molecular weight fatty acids.
Waxes are saроinfied with great difficulty than fats and are not attacked by lipase. Although waxes may be saponified by prolonged boiling with alcoholic KOH, they are more easily saponified by treating а solution of the wax in petroleum ether with absolute alcohol and metallic sodium, with sodium ethoxide. The saponi6cation products оf waxes are water-soluble soaps (sodium »Its of higher fatty acids); while the water insoluble long-chain alcohols appear in the “unsaponifiable matter” fraction. Waxes contain about 31 -55% of the unsaponifiable matter, while fats and oils contain only 1 – 2% unsaponifiable matter.
Compound Lipids.
As already mentioned, compound lipids are those which contain some chemical group in addition to fatty acids and an alcohol. On the nature of the additional chemical group, compound lipids are sub-divided into two main groups.
(a) Phospholipids: which contain а phosphate group.
(b) Glycolipids: which contain а carbohydrate.
Other classification divides the complex lipids into three main groups, viz.
(1) Glycerophosphatides (glycerol phospholipids) – which are glycerol containing phospholipids.
(2) Phosphoinositides: which contain inositol (а hexahydric alcohol) as the base.
(3) Phosphosphingosides or sphingolipids (sphingosine lipids)- which contain sphingosine or dihydrosphingosine as the base.
Phospholipids (phosphatides). This group is the most abundant among the complex lipids It is found in every living cell and makes up as much as 70% of the complex lipid contents of the tissues. These substances are also known as phosphatides and are sometimes named as derivatives of the parent compound, а phosphatidic acid.
Phospholipids may be defined as those lipids which yield on hysrolysis an alcohol, fatty acid, phosphoric acid, and а nitrogen base.
Functions of phospholipids. Phospholipids are involved in many functions. Sоme of these possible functions are listed below.
(а) As а structural component. Phospholipids are said to be components of cellular membranes including membranes of mitochondria. The operation of the oxidative chain and oxidative chain phosphorylation in mitochondria is inactivated by removal of the phospholipids, which may be controlling or participating in the transport of metabolites from one side of the membrane to the other. Despite their structural differnces, all phospholipids have hydrophobic and hydrophilic domains. The hydrophobic domain is composed largely of the hydrocarbon chains of fatty acids; the hydrophilic domain, called a polar head group, contains phosphate and other charged or polar groups.
(b) In blood coagulation. Phospholipids having ethanolamine or serine as base are believed to function in the process of blood coagulation.
(c) In absorption and transport of lipids. Phospholipids may act as emulsifying agents during digestion and absorption of lipids and are believed to be important components of the coating of chylomicrons in the form of lipoproteins. Phospholipids are also involved in the transport of lipids in the blood. (A surface active agents is a subsstance that lowers the surface tension of a liquid, usually water, so that it spreads out over a surface.)
(d) Transport of ions. Some phospholipids help in the transport of inorganic ions mainly cations across the membrane.
Phosphoacylglycerols are triesters of фусего1 in which two- ОН groups are esterified with fatty acids asnd one the third is esterified with phosphofic acid, which in turn is ecterified to an alcohol. The block diagram for а phosphoacylglycerol has the following general structure:
Cardiolipids are polymers of phosphatidic acids of any of the above type of phospholipids minus the base. These are isolated from beef heart extract and are said to be the active substances responsible for the serological test for syphilis.
Shingomyelins. These are composed of a complex basic dihydric amino alcohol, sphingosirse (sphingosinol) with a fatty acid in amide linkage on the amino group and the hosphorylcholine group attached by way of the terminal alcohol group. Thus they differ chemically from other phospholipids in the following two important respects.
All lipids derived from sphingosine have (1) а fatty acid connected to the – NH, group via an amide linkage, and (2) а group attached to the – ОН group on the terminal carbon atom via an ester linkage.
Note again, as in phosphoacylglycerols and waxes, the structural features of а head and two tails. For sphingolipids, the fatty acid is one of the tails, and the long carbon chain of sphingosine itself s the other tail. The “additional component” is the heal, and it is а phosphoric acid – choline group.
Sphingolipids are the second major class of nonglycerol-based saponifiable lipids. Like phosphoacylglycerols, they are polar lipids and are major constituents of cell membranes.
Sphingolipids have structures based on the long-chain amino dialcohol sphingosine. А sphingolipid is а saponijiable lipid derived from the amino dialcohol sphingosine.
Phytospingosine is found in plant spingolipids. The core structure of each type of spingolipid is ceramide, a fatty acidamide derivative of shingosine. In shingomyelin, ceramide is esterified to phosphatidylcholine or phosphatidylethanolamine. Sphingomyelin is found in most animal cell membranes. However, as its name suggests, spingomyelin is found in greatest abundance in the myelin sheath of nerve cells. (The myelin sheath is found by successive wrappings of the cell membrane of a specialized myelinating cell around a nerve cell axon. It facilitates the rapid transmission of nerve impulses).The ceramides are also precursors for the glycolipids, sometimes referred to as the glycosphingolipids. Clicolipids differ from spingomyelin in that they contain no phosphate. In glycolipids a monosaccharide, disaccharideor oligosaccharide is attached to aceramede through an O-glycosidic linkage. The most important glycolipid classes are cerebrosides, the sulfatides, and the gangliosides. Cerebrosides are shingolipids in which the head group is a monosaccharide.Galactocerebrosides, the most common axample of this class, are almost entifely found in thecell membranes of the brain. if a cerebroside is sulfated, it is referred to as a sulfatide. Sulfatidesare negatively charged at physiological pH. Spingolipids that possess oligosaccharide groups with one or more sialic acid residues are called gangliosides.
Cerebrosides and Gangliosides. Some sphingosine-based membrane lipids have а small carbohydrate as the head group. Cerebrosides, the simplest of such carbohydrate-containing lipids, usually have а glucose or galactose as the carbohydrate unit. The cerebrosides, as the name suggests, occur primarily in the brain (7 % of dry mass) and in the myelin sheath of nerves. Gangliosides contain more complex carbohydrate heads; up to seven monosaccharide units are present. These substances occur in the gray matter of the brain as well as in the myelin sheath.
Nonsaponifiable Lipids do not undergo hydrolysis in alkaline solution. Their structures are much different from those of the saponifiable lipids; neither ester nor amide linkages are present.
Nonsaponifiable Lipids: steroids, eicosanoids, terpenes, pheromones, fat-soluble vitamins.
Steroids are lipids with structures that are based on а fused-ring system involving three 6-membered rings and one 5-membered ring.
Derived Lipids. Derived lipids are those which although do not contain any ester linkage but may be considered to have been derived from naturally occurring esterified materials. In simple words, we can say that derived lipids are substances formed on the hydrolysis of simple or compound lipids which still retain the properties of this class of compounds. Derived lipids may be of following types.
1. Fatty acids. Saturated and unsaturated.
2. Alcohols. Alcohols of high molecular weight but not glycerol. These may again be of following types.
(a) Aliphatic alcohols such as cetyl, stearyl and myricyl alcohols
(b) Sterols. These contain phenanthrene nucleus important examples are cholesterol, ergosterol and stigmasterol.
(c) Alchols having b-ionone ring. These include vitamin A1 and carotenols (е.g., lutein and zeaxanthin).
3. Hydrocarbons. These include aliphatic hydrocarbons, carotenes, and squalene.
4. Certain vitamins. These include vitamins D, E and К.
5. Steroids hormones.
6. Bile acids.
The liver secretes а clear, golden-yellow, viscous fluid known as bile. It is stored in the gall bladder and is mainly useful for digestive system. Bile consists of inorganic (chiefly НСО3–, С1–, Na+, К+ etc.) ions as well as organic compounds. Among organic compounds the main constituents are bile acids, bile pigments, lipids, fatty acids and cholesterol.
