Lipids constitute a large and rather heterogeneous group of organic substances that are part of living cells, soluble in low-polarity organic solvents (ether, benzene, chloroform, etc.) and insoluble in water. AT general view they are considered as derivatives of fatty acids.
A structural feature of lipids is the presence in their molecules of both polar (hydrophilic) and non-polar (hydrophobic) structural fragments, which gives lipids an affinity for both water and the non-aqueous phase. Lipids are biphilic substances, which allows them to perform their functions at the interface.
10.1. Classification
Lipids are divided into simple(two-component), if the products of their hydrolysis are alcohols and carboxylic acids, and complex(multicomponent), when, as a result of their hydrolysis, other substances are also formed, for example, phosphoric acid and carbohydrates. Simple lipids include waxes, fats and oils, as well as ceramides, complex lipids include phospholipids, sphingolipids and glycolipids (Scheme 10.1).
Scheme 10.1.General classification of lipids
10.2. Structural components of lipids
All lipid groups have two obligatory structural components - higher carboxylic acids and alcohols.
Higher fatty acids (HFAs). Many higher carboxylic acids were first isolated from fats, hence the name fatty. Biologically important fatty acids can be rich(Table 10.1) and unsaturated(Table 10.2). Their common structural features are:
They are monocarboxylic;
Include an even number of carbon atoms in the chain;
have a cis configuration double bonds(if they are present).
Table 10.1.Major saturated fatty acids of lipids
In natural acids, the number of carbon atoms ranges from 4 to 22, but acids with 16 or 18 carbon atoms are more common. Unsaturated acids contain one or more double bonds in the cis configuration. The double bond closest to the carboxyl group is usually located between the C-9 and C-10 atoms. If there are several double bonds, then they are separated from each other by a methylene group CH 2.
The IUPAC rules for VZhK allow the use of their trivial names (see Tables 10.1 and 10.2).
Currently, a proprietary nomenclature of unsaturated HFAs is also used. In it, the terminal carbon atom, regardless of the chain length, is denoted by the last letter of the Greek alphabet ω (omega). The position of double bonds is counted not as usual from the carboxyl group, but from the methyl group. So, linolenic acid is designated as 18:3 ω-3 (omega-3).
Linoleic acid itself and unsaturated acids with a different number of carbon atoms, but with the arrangement of double bonds also at the third carbon atom, counting from the methyl group, make up the omega-3 family of fatty acids. Other types of acids form similar families of linoleic (omega-6) and oleic (omega-9) acids. For normal human life, the correct balance of lipids of three types of acids is of great importance: omega-3 ( linseed oil, fish oil), omega-6 (sunflower, corn oils) and omega-9 (olive oil) in the diet.
Of the saturated acids in the lipids of the human body, palmitic C 16 and stearic C 18 are the most important (see Table 10.1), and of the unsaturated acids, oleic C18: 1, linoleic С18:2 , linolenic and arachidonic C 20:4 (see table 10.2).
The role of polyunsaturated linoleic and linolenic acids should be emphasized as compounds indispensable for humans ("vitamin F"). They are not synthesized in the body and must be supplied with food in an amount of about 5 g per day. In nature, these acids are found mainly in vegetable oils. They contribute
Table 10 .2. Major unsaturated fatty acids of lipids
* Included for comparison. ** For cis isomers.
normalization of the lipid profile of blood plasma. Linetol, which is a mixture of ethyl esters of higher unsaturated fatty acids, is used as a lipid-lowering drug of plant origin. Alcohols. Lipids may include:
Higher monohydric alcohols;
Polyhydric alcohols;
Amino alcohols.
In natural lipids, saturated and less often unsaturated long-chain alcohols (C 16 and more) are most often found, mainly with an even number of carbon atoms. As an example of higher alcohols, cetyl CH 3 (CH 2 ) 15 OH and melissil CH 3 (CH 2) 29 OH alcohols that are part of the waxes.
Polyhydric alcohols in most natural lipids are represented by the trihydric alcohol glycerol. Other polyhydric alcohols are encountered, such as the dihydric alcohols ethylene glycol and propanediol-1,2, and myoinositol (see 7.2.2).
The most important amino alcohols that are part of natural lipids are 2-aminoethanol (colamine), choline, which also belongs to the α-amino acids serine and sphingosine.
Sphingosine is an unsaturated long chain dihydric amino alcohol. The double bond in sphingosine has trance-configuration, and asymmetric С-2 and С-3 atoms - D-configuration.
Alcohols in lipids are acylated with higher carboxylic acids at the corresponding hydroxyl or amino groups. In glycerol and sphingosine, one of the alcohol hydroxyls can be esterified with a substituted phosphoric acid.
10.3. Simple lipids
10.3.1. Waxes
Waxes are esters of higher fatty acids and higher monohydric alcohols.
Waxes form a protective lubricant on the skin of humans and animals and protect plants from drying out. They are used in the pharmaceutical and perfume industries in the manufacture of creams and ointments. An example is palmitic acid cetyl ester(cetin) - the main component spermaceti. Spermaceti is secreted from the fat contained in the cavities of the skull of sperm whales. Another example is melisyl ester of palmitic acid- component of beeswax.
10.3.2. Fats and oils
Fats and oils are the most common group of lipids. Most of them belong to triacylglycerols - full esters of glycerol and VFA, although mono- and diacylglycerols also occur and take part in the metabolism.
Fats and oils (triacylglycerols) are esters of glycerol and higher fatty acids.
In the human body, triacylglycerols play the role of a structural component of cells or a reserve substance (“fat depot”). Their energy value is approximately twice that of proteins.
or carbohydrates. However, an elevated level of triacylglycerols in the blood is one of the additional risk factors for the development of coronary heart disease.
Solid triacylglycerols are called fats, liquid triacylglycerols are called oils. Simple triacylglycerols contain residues of the same acids, mixed - different.
In the composition of triacylglycerols of animal origin, saturated acid residues usually predominate. Such triacylglycerols are generally solids. In contrast, vegetable oils contain mostly unsaturated acid residues and have a liquid consistency.
Below are examples of neutral triacylglycerols and their systematic and (in brackets) commonly used trivial names based on the names of their constituent fatty acids.
10.3.3. Ceramides
Ceramides are N-acylated derivatives of the alcohol sphingosine.
Ceramides are present in trace amounts in plant and animal tissues. Much more often they are part of complex lipids - sphingomyelins, cerebrosides, gangliosides, etc.
(see 10.4).
10.4. Complex lipids
Some complex lipids are difficult to classify unambiguously, since they contain groupings that allow them to be simultaneously assigned to different groups. According to the general classification of lipids (see Scheme 10.1), complex lipids are usually divided into three large groups: phospholipids, sphingolipids and glycolipids.
10.4.1. Phospholipids
The group of phospholipids includes substances that split off phosphoric acid during hydrolysis, for example, glycerophospholipids and some sphingolipids (Scheme 10.2). In general, phospholipids are characterized by a fairly high content of unsaturated acids.
Scheme 10.2.Phospholipid classification
Glycerophospholipids. These compounds are the main lipid components of cell membranes.
By chemical structure Glycerophospholipids are derivatives l -glycero-3-phosphate.
l-Glycero-3-phosphate contains an asymmetric carbon atom and therefore can exist as two stereoisomers.
Natural glycerophospholipids have the same configuration, being derivatives of l-glycero-3-phosphate, which is formed during metabolism from dihydroxyacetone phosphate.
Phosphatides. Among glycerophospholipids, phosphatides are the most common - ester derivatives of l-phosphatidic acids.
Phosphatic acids are derivatives l -glycero-3-phosphate, esterified with fatty acids at alcohol hydroxyl groups.
As a rule, in natural phosphatides in position 1 of the glycerol chain there is a residue of a saturated acid, in position 2 - an unsaturated acid, and one of the phosphoric acid hydroxyls is esterified with a polyhydric alcohol or amino alcohol (X is the residue of this alcohol). In the body (pH ~ 7.4), the remaining free hydroxyl of phosphoric acid and other ionogenic groups in phosphatides are ionized.
Examples of phosphatides are compounds containing phosphatidic acids esterified on phosphate hydroxyl with the corresponding alcohols:
Phosphatidylserines, esterifying agent - serine;
Phosphatidylethanolamines, esterifying agent - 2-aminoethanol (often, but not quite correctly, called ethanolamine in the biochemical literature);
Phosphatidylcholines, esterifying agent - choline.
These esterifying agents are interrelated because the ethanolamine and choline moieties can be metabolized from the serine moiety by decarboxylation and subsequent methylation with S-adenosylmethionine (SAM) (see 9.2.1).
A number of phosphatides instead of an amine-containing esterifying agent contain residues polyhydric alcohols- glycerol, myoinositol, etc. The phosphatidylglycerols and phosphatidylinositols given below as an example are acidic glycerophospholipids, since their structures do not contain amino alcohol fragments, which give phosphatidylethanolamines and related compounds a neutral character.
Plasmalogens. Less common compared to ester glycerophospholipids are lipids with a simple ether bond, in particular plasmalogens. They contain an unsaturated residue
* For convenience, the way of writing the configuration formula of the myoinositol residue in phosphatidylinositols has been changed from that given above (see 7.2.2).
an alcohol linked by an ether bond to the C-1 atom of glycero-3-phosphate, such as, for example, plasmalogens with an ethanolamine fragment - L-phosphatidalethanolamines. Plasmalogens make up to 10% of all CNS lipids.
10.4.2. sphingolipids
Sphingolipids are structural analogs of glycerophospholipids that use sphingosine instead of glycerol. Another example of sphingolipids are the ceramides discussed above (see 10.3.3).
An important group of sphingolipids are sphingomyelins, first discovered in nervous tissue. In sphingomyelins, the hydroxyl group at C-1 of ceramide is usually esterified with choline phosphate (less often with colamine phosphate), so they can also be classified as phospholipids.
10.4.3. Glycolipids
As the name suggests, the compounds of this group include carbohydrate residues (more often D-galactose, less often D-glucose) and do not contain a phosphoric acid residue. Typical representatives of glycolipids - cerebrosides and gangliosides - are sphingosine-containing lipids (therefore, they can also be considered sphingolipids).
AT cerebrosides the ceramide residue is linked to D-galactose or D-glucose by a β-glycosidic bond. Cerebrosides (galactocerebrosides, glucocerebrosides) are part of the membranes of nerve cells.
Gangliosides- carbohydrate-rich complex lipids - were first isolated from the gray matter of the brain. Structurally, gangliosides are similar to cerebrosides, differing in that instead of a monosaccharide, they contain a complex oligosaccharide, including at least one residue V-acetylneuraminic acid (see Appendix 11-2).
10.5. Lipid Properties
and their structural components
A feature of complex lipids is their bifilality, due to non-polar hydrophobic and highly polar ionized hydrophilic groups. In phosphatidylcholines, for example, hydrocarbon radicals of fatty acids form two non-polar "tails", and carboxyl, phosphate and choline groups form a polar part.
At the interface, such compounds act as excellent emulsifiers. As part of cell membranes, lipid components provide a high electrical resistance of the membrane, its impermeability to ions and polar molecules, and permeability to non-polar substances. In particular, most anesthetic drugs are highly lipid soluble, allowing them to penetrate nerve cell membranes.
Fatty acids are weak electrolytes( p K a~4.8). They are slightly dissociated into aqueous solutions. At pH< p K a the non-ionized form predominates, at pH > p K a , i.e. in physiological conditions, the ionized form RCOO - predominates. Soluble salts of higher fatty acids are called soaps. Sodium salts of higher fatty acids are solid, potassium salts are liquid. As salts of weak acids and strong bases, soaps are partially hydrolyzed in water, their solutions are alkaline.
Natural unsaturated fatty acids cis-double bond configuration, have a large supply of internal energy and, therefore, in comparison with trance-isomers are thermodynamically less stable. Them cis-trans -isomerization easily takes place when heated, especially in the presence of initiators of radical reactions. Under laboratory conditions, this transformation can be carried out by the action of nitrogen oxides formed during the decomposition of nitric acid upon heating.
Higher fatty acids exhibit general Chemical properties carboxylic acids. In particular, they easily form the corresponding functional derivatives. Fatty acids with double bonds exhibit the properties of unsaturated compounds - they add hydrogen, hydrogen halides and other reagents to the double bond.
10.5.1. Hydrolysis
With the help of the hydrolysis reaction, the structure of lipids is established, and valuable products (soaps) are also obtained. Hydrolysis is the first step in the utilization and metabolism of dietary fats in the body.
The hydrolysis of triacylglycerols is carried out either by the action of superheated steam (in industry) or by heating with water in the presence of mineral acids or alkalis (saponification). In the body, lipid hydrolysis occurs under the action of lipase enzymes. Some examples of hydrolysis reactions are given below.
In plasmalogens, as in ordinary vinyl ethers, the ether bond is cleaved in an acidic but not in an alkaline medium.
10.5.2. Addition reactions
Lipids containing unsaturated acid residues in the structure add hydrogen, halogens, hydrogen halides, and water via double bonds in an acidic medium. Iodine number is a measure of the unsaturation of triacylglycerols. It corresponds to the number of grams of iodine that can be added to 100 g of a substance. The composition of natural fats and oils and their iodine numbers vary within a fairly wide range. As an example, we give the interaction of 1-oleoyl-distearoylglycerol with iodine (the iodine number of this triacylglycerol is 30).
Catalytic hydrogenation (hydrogenation) of unsaturated vegetable oils is an important industrial process. In this case, hydrogen saturates the double bonds and liquid oils are converted into solid fats.
10.5.3. Oxidation reactions
Oxidative processes involving lipids and their structural components are quite diverse. In particular, the oxidation by atmospheric oxygen of unsaturated triacylglycerols during storage (autoxidation, see 3.2.1), followed by hydrolysis, is part of the process known as rancidity of oil.
The primary products of the interaction of lipids with molecular oxygen are hydroperoxides formed as a result of a chain free radical process (see 3.2.1).
lipid peroxidation - one of the most important oxidative processes in the body. It is the main cause of damage to cell membranes (for example, with radiation sickness).
Structural fragments of unsaturated higher fatty acids in phospholipids serve as a target for attack reactive oxygen species(AFK, see Appendix 03-1).
When attacked, in particular, by the hydroxyl radical HO", the most active of the ROS, the lipid LH molecule undergoes a homolytic cleavage S-N connections in the allyl position, as shown in the lipid peroxidation model (Scheme 10.3). The resulting allyl-type radical L" instantly reacts with molecular oxygen in the oxidation medium to form the lipid peroxyl radical LOO". From this moment, a chain cascade of lipid peroxidation reactions begins, since there is a constant formation of allyl lipid radicals L", resuming this process.
Lipid peroxides LOOH are unstable compounds and can spontaneously or with the participation of metal ions of variable valence (see 3.2.1) decompose with the formation of lipidoxyl radicals LO", capable of initiating further oxidation of the lipid substrate. Such an avalanche-like process of lipid peroxidation poses a danger of destruction of membrane structures cells.
The intermediately formed allyl-type radical has a mesomeric structure and can further undergo transformations in two directions (see Scheme 10.3, paths a and b) leading to intermediate hydroperoxides. Hydroperoxides are unstable and decompose already at ordinary temperatures to form aldehydes, which are further oxidized to acids, the end products of the reaction. The result is generally two monocarboxylic and two dicarboxylic acids with shorter carbon chains.
Under mild conditions, unsaturated acids and lipids with residues of unsaturated acids are oxidized with an aqueous solution of potassium permanganate, forming glycols, and under more rigid conditions (with breaking of carbon-carbon bonds), the corresponding acids.
Lipids (from the Greek lipos - ether) are a complex mixture of ether-like organic compounds with similar physical and chemical properties. Lipids are widely used in the production of many food products, they are important components food products, largely determining their nutritional and biological usefulness and taste.
In plants, lipids accumulate mainly in seeds and fruits and vary from a few percent in cereals and cereals to tens of percent in oilseeds. In animals and fish, lipids are concentrated in the subcutaneous, brain, and nervous tissues. The lipid content in fish varies from 8 to 25%, in carcasses of terrestrial animals it varies greatly: 33% (pork), 9.8% (beef). in milk various kinds animal lipid content ranges from 1.7% in mare's milk to 34.5% in the milk of female reindeer.
Lipids are insoluble in water (hydrophobic*), readily soluble in organic solvents (gasoline, diethyl ether, chloroform, etc.).
According to the chemical structure, lipids are derivatives of fatty acids, alcohols, aldehydes, built using ester, ether, phosphoester, glycosidic bonds. Lipids are divided into two main groups: simple and complex lipids. Simple neutral lipids include derivatives of higher fatty acids and alcohols: glycerolipids, waxes, cholesterol esters, glycolipids and other compounds. Molecules of complex lipids contain not only residues of high molecular weight carboxylic acids, but also phosphoric, sulfuric acid or nitrogen.
The most important and widespread group of simple neutral lipids are acylglycerols (or glycerides). These are esters of glycerol and higher carboxylic acids. They make up the bulk of lipids (sometimes up to 95%) and, in fact, they are called fats or oils. The composition of fats includes mainly triacylglycerols (I), less often diacylglycerols (II) and monoacylglycerols (III):
The most important representatives of complex lipids are phospholipids- obligatory components of plants (0.3-1.7%). Their molecules are built from residues of alcohols (glycerol, sphingosine), fatty acids, phosphoric acid (H 3 PO 4), and also contain nitrogenous bases, amino acid residues and some other compounds.
The molecules of most phospholipids are built according to a general principle. They include, on the one hand, hydrophobic, characterized by low affinity for water, on the other hand, hydrophilic groups (residues of phosphoric acid and nitrogenous base). They are called "polar heads". Due to this property (amphiphilicity), phospholipids often form an interface (membrane) between water and the hydrophobic phase in living organism systems and foods.
Lipids perform not only an energy function (free lipids), but also perform a structural function: together with proteins and carbohydrates, they are part of cell membranes and cellular structures. In terms of mass, structural lipids make up a much smaller group of lipids (3-5% in oilseeds). These are hard-to-remove "bound" and "strongly bound" lipids.
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Lipids combine a large number of fats and fat-like substances of plant and animal origin, which have a number of common features:
a) insolubility in water (hydrophobicity and good solubility in organic solvents, gasoline, diethyl ether, chloroform, etc.);
b) the presence in their molecules of long-chain hydrocarbon radicals and esters
groupings().
Most lipids are not macromolecular compounds and consist of several molecules linked to each other. Lipids can include alcohols and linear chains of a number of carboxylic acids. In some cases, their individual blocks may consist of macromolecular acids, various phosphoric acid residues, carbohydrates, nitrogenous bases, and other components.
Lipids, together with proteins and carbohydrates, make up the bulk of organic matter in all living organisms, being an indispensable component of every cell.
Simple and complex lipids
When lipids are isolated from oilseed raw materials, a large group of accompanying fat-soluble substances passes into the oil: steroids, pigments, fat-soluble vitamins and some other compounds. Extracted from natural objects a mixture consisting of lipids and compounds soluble in them is called "crude" fat.
Main components of crude fat
Substances associated with lipids play an important role in food technology, affect the nutritional and physiological value of the resulting food products. Vegetative parts of plants accumulate no more than 5% of lipids, mainly in seeds and fruits. For example, the lipid content in various plant products is (g / 100g): sunflower 33-57, cocoa (beans) 49-57, soybeans 14-25, hemp 30-38, wheat 1.9-2.9, peanuts 54- 61, rye 2.1-2.8, flax 27-47, corn 4.8-5.9, coconut palm 65-72. The content of lipids in them depends not only on the individual characteristics of plants, but also on the variety, place, and growing conditions. Lipids play an important role in the life processes of the body.
Their functions are very diverse: their role in energy processes is important, in defensive reactions organism, in its maturation, aging, etc.
Lipids are part of all structural elements of the cell and, first of all, cell membranes, affecting their permeability. They are involved in the transmission of a nerve impulse, provide intercellular contact, active transfer of nutrients through membranes, transport of fats in blood plasma, protein synthesis and various enzymatic processes.
According to their functions in the body, they are conditionally divided into two groups: spare and structural. Spare (mainly acylglycerols) have a high calorie content, are the body's energy reserve and are used by it in case of malnutrition and diseases.
Spare lipids are reserve substances that help the body to endure adverse effects. external environment. Most plants (up to 90%) contain storage lipids, mainly in seeds. They are easily extracted from fat-containing material (free lipids).
Structural lipids (primarily phospholipids) form complex complexes with proteins and carbohydrates. They are involved in a variety of complex processes occurring in the cell. By weight, they constitute a significantly smaller group of lipids (3-5% in oilseeds). These are hard-to-remove “bound” lipids.
Natural fatty acids found in lipids, animals and plants share many properties. They contain, as a rule, a clear number of carbon atoms and have an unbranched chain. Fatty acids are conventionally divided into three groups: saturated, monounsaturated and polyunsaturated. Unsaturated fatty acids of animals and humans usually contain a double bond between the ninth and tenth carbon atoms, the remaining carboxylic acids that make up fats are as follows:
Most lipids share some common structural features, but there is no strict classification of lipids yet. One of the approaches to the classification of lipids is chemical, according to which derivatives of alcohols and higher fatty acids belong to lipids.
Lipid classification scheme.
simple lipids. Simple lipids are represented by two-component substances, esters of higher fatty acids with glycerol, higher or polycyclic alcohols.
These include fats and waxes. The most important representatives of simple lipids are acylglycerides (glycerols). They make up the bulk of lipids (95-96%) and they are called oils and fats. The composition of fats includes mainly triglycerides, but there are mono- and diacylglycerols:
The properties of specific oils are determined by the composition of the fatty acids involved in the construction of their molecules and the position occupied by the residues of these acids in the molecules of oils and fats.
Up to 300 carboxylic acids of various structures have been found in fats and oils. However, most of them are present in small quantities.
Stearic and palmitic acids are part of almost all natural oils and fats. Erucic acid is found in rapeseed oil. Most of the most common oils contain unsaturated acids containing 1-3 double bonds. Some acids in natural oils and fats are usually in the cis configuration, i.e. substituents are distributed on one side of the plane of the double bond.
Branched carbohydrate acids containing hydroxy, keto and other groups are usually found in small amounts in lipids. The exception is racinoleic acid in castor oil. In natural plant triacylglycerols, positions 1 and 3 are preferably occupied by saturated fatty acid residues, and position 2 is unsaturated. In animal fats, the picture is reversed.
The position of fatty acid residues in triacylglycerols significantly affects their physicochemical properties.
Acylglycerols are liquids or solids with low melting points and fairly high boiling points, high viscosity, colorless and odorless, lighter than water, non-volatile.
Fats are practically insoluble in water, but form emulsions with it.
In addition to the usual physical indicators, fats are characterized by a number of physicochemical constants. These constants for each type of fat and its grade are provided by the standard.
The acid number, or acidity index, indicates how much free fatty acids are in a fat. It is expressed as the number of mg of KOH required to neutralize free fatty acids in 1 g of fat. The acid number is an indicator of the freshness of the fat. On average, it varies for different types of fat from 0.4 to 6.
The saponification number, or saponification coefficient, determines the total amount of acids, both free and bound in triacylglycerols, found in 1 g of fat. Fats containing residues of high molecular weight fatty acids have a lower saponification number than fats formed by low molecular weight acids.
The iodine number is an indicator of the unsaturation of fat. O is determined by the number of grams of iodine added to 100 g of fat. The higher the iodine number, the more unsaturated the fat is.
Waxes. Waxes are esters of higher fatty acids and high molecular weight alcohols (18-30 carbon atoms). The fatty acids that make up waxes are the same as for fats, but there are also specific ones that are characteristic only for waxes.
For example: carnauba;
cerotinic;
montana.
General formula waxes can be written like this:
Waxes are widely distributed in nature, covering the leaves, stems, fruits of plants with a thin layer, they protect them from wetting with water, drying out, and the action of microorganisms. The content of wax in grains and fruits is low.
complex lipids. Complex lipids have multicomponent molecules, individual parts of which are connected by chemical bonds of various types. These include phospholipids, consisting of residues of fatty acids, glycerol and other polyhydric alcohols, phosphoric acid and nitrogenous bases. In the structure of glycolipids, along with polyhydric alcohols and high molecular weight fatty acids, there are also carbohydrates (usually residues of galactose, glucose, mannose).
There are also two groups of lipids, which include both simple and complex lipids. These are diol lipids, which are simple and complex lipids of dihydric alcohols and high molecular weight fatty acids, containing in some cases phosphoric acid, nitrogenous bases.
Ormitinolipids are built from fatty acid residues, the amino acid ormitin or lysine, and in some cases include dihydric alcohols. The most important and widespread group of complex lipids are phospholipids. Their molecule is built from residues of alcohols, high molecular weight fatty acids, phosphoric acid, nitrogenous bases, amino acids and some other compounds.
The general formula of phospholipids (phosphotides) is as follows:
Therefore, the phospholipid molecule has two types of groups: hydrophilic and hydrophobic.
Phosphoric acid residues and nitrogenous bases act as hydrophilic groups, and hydrocarbon radicals act as hydrophobic groups.
Schematic diagram of the structure of phospholipids
Rice. 11. Phospholipid molecule
The hydrophilic polar head is a residue of phosphoric acid and a nitrogenous base.
Hydrophobic tails are hydrocarbon radicals.
Phospholipids have been isolated as by-products in the production of oils. They are surfactants that improve the baking properties of wheat flour.
They are also used as emulsifiers in the confectionery industry and in the production of margarine products. They are an essential component of cells.
Together with proteins and carbohydrates, they are involved in the construction of cell membranes and subcellular structures that perform the functions of supporting membrane structures. They promote better absorption of fats and prevent fatty liver by playing important role in the prevention of atherosclerosis.
The transformation of lipids and their impact on the quality of products during storage and processing:
a) hydrolysis
b) hydrogenation
c) interesterification
d) autoxidation and enzymatic oxidation (rancidity).
A group of organic substances, including fats and fat-like substances (lipoids), is called lipids. Fats are found in all living cells, act as a natural barrier, limiting the permeability of cells, and are part of hormones.
Structure
Lipids are chemically one of the three types vital organic matter. They practically do not dissolve in water; are hydrophobic compounds, but form an emulsion with H 2 O. Lipids decompose in organic solvents - benzene, acetone, alcohols, etc. By physical properties fats are colorless, tasteless and odorless.
By structure, lipids are compounds of fatty acids and alcohols. When additional groups (phosphorus, sulfur, nitrogen) are attached, complex fats are formed. A fat molecule necessarily includes carbon, oxygen and hydrogen atoms.
Fatty acids are aliphatic, i.e. not containing cyclic carbon bonds, carboxylic (-COOH group) acids. They differ in the number of -CH2- groups.
Producing acids:
- unsaturated - include one or more double bonds (-CH=CH-);
- rich - do not contain double bonds between carbon atoms
Rice. 1. The structure of fatty acids.
In cells, they are stored in the form of inclusions - drops, granules, in multicellular organism- in the form of adipose tissue, consisting of adipocytes - cells capable of accumulating fats.
Classification
Lipids are complex compounds that occur in various modifications and perform various functions. Therefore, the classification of lipids is extensive and is not limited to one feature. The most complete classification by structure is given in the table.
The lipids described above are saponifiable fats - when they are hydrolyzed, soap is formed. Separately, in the group of unsaponifiable fats, i.e. do not interact with water, release steroids.
They are divided into subgroups depending on the structure:
- sterols - steroid alcohols that are part of animal and plant tissues (cholesterol, ergosterol);
- bile acids - derivatives of cholic acid, containing one group -COOH, contribute to the dissolution of cholesterol and the digestion of lipids (cholic, deoxycholic, lithocholic acids);
- steroid hormones - contribute to the growth and development of the body (cortisol, testosterone, calcitriol).
Rice. 2. Scheme for the classification of lipids.
Lipoproteins are isolated separately. These are complex complexes of fats and proteins (apolipoproteins). Lipoproteins are classified as complex proteins, not fats. They include a variety of complex fats - cholesterol, phospholipids, neutral fats, fatty acids.
There are two groups:
- soluble - are part of the blood plasma, milk, yolk;
- insoluble - are part of the plasmalemma, the sheath of nerve fibers, chloroplasts.
Rice. 3. Lipoproteins.
Plasma lipoproteins have been studied the most. They vary in density. The more fat, the lower the density.
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Lipids are classified according to their physical structure into solid fats and oils. By being in the body, reserve (non-permanent, dependent on nutrition) and structural (genetically determined) fats are isolated. By origin, fats can be vegetable and animal.
Meaning
Lipids must be ingested with food and participate in metabolism. Depending on the type of fats perform in the body various functions:
- triglycerides keep the body warm;
- subcutaneous fat protects internal organs;
- phospholipids are part of the membranes of any cell;
- adipose tissue is a reserve of energy - the breakdown of 1 g of fat gives 39 kJ of energy;
- glycolipids and a number of other fats perform a receptor function - they bind cells, receiving and conducting signals received from the external environment;
- phospholipids are involved in blood clotting;
- waxes cover the leaves of plants, at the same time protecting them from drying out and getting wet.
An excess or deficiency of fats in the body leads to a change in metabolism and a violation of the functions of the body as a whole.
What have we learned?
Fats have complex structure, are classified according to different criteria and perform various functions in the body. Lipids are made up of fatty acids and alcohols. When additional groups are attached, complex fats are formed. Proteins and fats can form complex complexes - lipoproteins. Fats are part of the plasmalemma, blood, tissues of plants and animals, perform heat-insulating and energy functions.
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organic matter. General characteristics. Lipids
organic matter are complex carbon-containing compounds. These include proteins, fats, carbohydrates, enzymes, hormones, vitamins and products of their transformations present in living organisms.
Name " organic compounds”appeared at an early stage in the development of chemistry and speaks for itself: scientists of that era believed that living beings consist of special organic compounds.
Among all chemical elements carbon most closely associated with living organisms. More than a million different molecules based on it are known. interesting unique ability carbon atoms enter into covalent bond with each other, forming long chains, complex rings and other structures.
Most organic compounds in nature are formed as a result of the process of photosynthesis - from carbon dioxide and water with the participation of solar radiation energy in chlorophyll-containing organisms.
Low molecular weight organic compounds They got their name because of their small molecular weight. These include amino acids, lipids, organic acids, vitamins, coenzymes (vitamin derivatives that determine the activity of enzymes) and others.
Low molecular weight organic compounds make up 0.1 - 0.5% of the cell mass.
High molecular weight organic compounds (biopolymers)
A macromolecule made up of monomers is calledpolymer(from Greek poly - "lot"). Therefore, a polymer is a multi-link chain in which a relatively simple substance is a link.
Polymers- These are molecules consisting of repeating structural units - monomers.
The properties of biopolymers depend on the number and variety of monomeric units that form the polymer. If you combine 2 types of monomers together BUT and B, then you can get a variety of polymers, the structure and properties of which will depend on the number, ratio and order of alternation of monomers in the chains.
Let's say there are 16 units in paraffin. You won’t repeat methylene - methylene - methylene 16 times ... For such long word there is a simplification - "hexadecane". What if there are a thousand units in a molecule? We speak simplistically poly- "lot". For example, take a thousand links ethylene, connect, get everyone familiar polyethylene.
Homopolymers (or regular) are built from monomers of the same type (for example, glycogen, starch and cellulose made up of molecules glucose).
Heteropolymers(or irregular) are built from different monomers (for example, proteins, consisting of 20 amino acids, and nucleic acids, built from 8 nucleotides).
Each of the monomers determines some property of the polymer. For example, BUT- high strength, B- electrical conductivity. Alternating them in different ways, you can get huge number polymers with different properties. This principle underlies the diversity of life on our planet.
Lipids, their structure, properties and functions
Lipids are esters of the trihydric alcohol glycerol and higher fatty acids. Each of them has an acidic COOH residue, which, losing a hydrogen atom, combines with glycerol, and a carbon chain is connected to the residue. Lipids are low molecular weight hydrophobic organic compounds.
« Bold» acids are named because some of the high molecular weight members of this group are part of fats. General formula of fatty acids: CH 3 - (CH 2) p - COOH. Most of the fatty acids are even number carbon atoms (from 14 to 22).
Fatty acids are synthesized from cholesterol in the liver, then they enter the duodenum with bile, where they contribute to the digestion of fats, emulsifying them, thereby stimulating their absorption.
Lipids include fats, waxes, steroids, phospholipids, terpenes, glycolipids, lipoproteins.
Lipids are usually divided into fats and oils, depending on whether they remain solid at 20 ° C (fats) or have a liquid consistency at this temperature (oils).
Pure fat is always white, and pure oil is always colorless. The yellow, orange and brown color of the oil is due to the presence of carotene or similar compounds. Olive oil, on the other hand, sometimes has a greenish tint: it contains a little chlorophyll.
Fats have a high boiling point. Thanks to this, it is convenient to fry food on fats. They do not evaporate from a hot pan, they begin to burn only at a temperature of 200 - 300 0 C.
Neutral Fats(triglycerides) are compounds of high molecular weight fatty acids and the trihydric alcohol glycerol. In the cytoplasm of cells, triglycerides are deposited in the form of fat droplets.
Excess fat can cause fatty degeneration. The main sign of the appearance of fatty degeneration is an increase and thickening of the liver due to the accumulation of fat in hepatocytes (liver cells).
Waxes- plastic substances with water-repellent properties. In insects, they serve as material for building honeycombs. Wax coating on the surface of leaves, stems, fruits protects plants from mechanical damage, ultraviolet radiation and plays an important role in the regulation of water balance.
Phospholipids- representatives of the class of fat-like substances, which are esters of glycerol and fatty acids, containing a residue of phosphoric acid.
They form the basis of all biological membranes. In their structure, phospholipids are similar to fats, but in their molecule one or two fatty acid residues are replaced by a phosphoric acid residue.
Glycolipids- substances formed as a result of the combination of carbohydrates and lipids. The carbohydrate components of glycolipid molecules are polar, and this determines their role: like phospholipids, glycolipids are part of cell membranes.
To fat-like substances (lipoids) include precursors and derivatives of simple and complex lipids: cholesterol, bile acids, fat-soluble vitamins, steroid hormones, glycerol other.
General properties of lipids:
1) have a high energy intensity;
2) have a density lower than that of water;
3) have an advantageous boiling point;
4) high-calorie substances.
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Variety lipids |
Role in plant and animal organisms |
|
Fats and oils |
1. Serve as an energy depot. |
|
Wax |
Mainly used as a water-repellent coating: 1) forms an additional protective layer on the cuticle of the epidermis of some plant organs, such as leaves, fruits and seeds (mainly in xerophytes); Bees build honeycombs from wax. |
|
Phospholipids |
membrane components. |
|
Steroids |
Bile acids, such as cholic acid, are part of bile. |
|
Terpenes |
Substances on which the aroma of essential oils of plants depends, for example, menthol in mint, camphor. Gibberellins are the growth substances of plants. Phyton is part of chlorophyll. Carotenoids are photosynthetic pigments. |
|
Lipoproteins |
Membranes are made up of lipoproteins. |
|
Glycolipids |
Components of cell membranes, especially in the myelin sheath of nerve fibers and on the surface of nerve cells, as well as components of chloroplast membranes. |
General functions of lipids
| Function | Explanation |
|---|---|
| Energy | When splitting 1 g of triglycerides, 38.9 kJ of energy is released |
| Structural | Phospholipids and glycolipids are involved in the formation of cell membranes |
| Reserve | Fats and oils are the most important reserve substances. Fats are deposited in the cells of adipose tissue of animals and serve as a source of energy during hibernation, migration or hunger. Plant seed oils provide energy for future seedlings |
| Source of metabolic water | When 1 g of fat is oxidized, 1.1 g of water is formed |
| Protective | Layers of fat provide depreciation of animal organs, and subcutaneous fatty tissue creates a heat-insulating layer. Wax serves as a water-repellent coating for plants |
| Regulatory | Steroid hormones regulate fundamental processes in animal organisms - growth, differentiation, reproduction, adaptation, etc. |
| catalytic | Fat soluble vitamins A, D, E, K are cofactors of enzymes, and although they do not have catalytic activity on their own, enzymes cannot perform their functions without them |