The signaling function is characteristic of such lipids as. Higher alcohols include cholesterol and fat-soluble vitamins – A, D, E

is a group of organic substances that are part of living organisms and are characterized by insolubility in water and solubility in non-polar solvents such as diethylether, chloroform and benzene. This definition combines a large number of compounds of different chemical nature, in particular such as fatty acids, waxes, phospholipids, steroids and many others. The functions of lipids in living organisms are also diverse: fats are a form of energy storage, phospholipids and steroids are part of biological membranes, other lipids contained in cells in smaller quantities can be coenzymes, light-absorbing pigments, electron carriers, hormones, secondary messengers, intracellular transmission time signal, hydrophobic “anchors” that contain membrane proteins, chaperones that promote protein folding, emulsifiers in the gastrointestinal tract.

Humans and other animals have specialized biochemical pathways for the biosynthesis and breakdown of lipids, but some of these substances are essential and must be obtained from food, such as ω-3 and ω-6 unsaturated fatty acids.

Classification of lipids

Traditionally, lipids are divided into simple (esters of fatty acids with alcohols) and complex (which, in addition to the fatty acid residue and alcohol, contain additional groups: hydrocarbons, phosphates and others). The first group includes, in particular, acylglycerols and waxes, the second group includes phospholipids, glycolipids, and lipoproteins can also be included here. This classification does not cover the entire variety of lipids, so some of them will be divided into a separate group of precursors and derivatives of lipids (for example, fatty acids, sterols, some aldehydes, etc.).

Modern nomenclature and classification of lipids, used in research in the field of lipidomics, is based on dividing them into eight main groups, each of which is abbreviated by two English letters:

• Fatty acids (FA)
• Glycerolipids (GL)
• Glycerophospholipids (GP)
• Sphingolipids (SP);
• Steroid lipids (ST);
• Prenolni lipids (PR)
• Sugar lipids (SL)
• Polyketides (PK).

Each group is divided into separate subgroups, designated by a combination of two numbers.

It is also possible to classify lipids based on their biological functions; in this case, groups such as storage, structural, signaling lipids, cofactors, pigments, and the like can be distinguished.

Characteristics of the main classes of lipids

Fatty acid

Fatty acids are carboxylic acids whose molecules contain from four to thirty-six carbon atoms. More than two hundred compounds of this class have been discovered in living organisms, but about twenty have become widespread. The molecules of all natural fatty acids contain an even number of carbon atoms (this is due to the peculiarities of biosynthesis, which occurs by adding dicarbonic units), mainly from 12 to 24. Their hydrocarbon chains are usually unbranched, occasionally they may contain tricarboxylic rings, hydroxyl groups or branches.

Depending on the presence of double bonds between carbon atoms, all fatty acids are divided into saturated, which contain them, and unsaturated, which contain double bonds. The most common saturated fatty acids in the human body are palmitic (C 16) and stearic (C 18).

Unsaturated fatty acids are found in living organisms more often than saturated ones (about 3/4 of the total content). In most of them, a certain pattern is observed in the placement of double bonds: if there is only one such bond, then it is predominantly located between the 9th and 10th carbon atoms, additional double bonds mainly appear in positions between the 12th and 13th and between 15th and 16th carbons (arachidonic acid is an exception to this rule). Double bonds in natural polyunsaturated fatty acids are always isolated, that is, there is at least one methylene group between them (-CH = CH-CH 2 -CH = CH-). In almost all unsaturated fatty acids found in living organisms, double bonds are found in cis configurations. The most common unsaturated fatty acids include oleic, linoleic, linolenic and arachidonic.

Availability cis-Double bonds affect the shape of the fatty acid molecule (making it less compact), and accordingly the physical properties of these substances: unsaturated fatty acids in cis-forms have a lower melting point than the corresponding ones trance isomer and saturated fatty acids.

Fatty acids are found in living organisms primarily as residues in other lipids. However, in small quantities they can also be found in free form. Fatty acid derivatives eicosanoids play an important role as signaling compounds.

Acylglycerides

Acylglycerides (acylglycerols, glycerides) are esters of trihydric alcohol glycerol and fatty acids. Depending on the number of esterified hydroxyl groups in the glycerol molecule, they are divided into triglycerides (triacylglycerols), diglycerides (diacylglycerols) and monoglycerides (monoacylglycerols). The most common triglycerides, which also have the empirical name neutral fats or simply fats.

Fats can be simple, that is, containing three identical fatty acid residues, such as tristearin or triolein, but more often mixed fats are found, containing residues of different fatty acids, for example, 1-palmito-2-oleolinolene. The physical properties of triglycerides depend on the fatty acid composition: the more residues they contain of long unsaturated fatty acids, the higher their melting point, and vice versa - the more short unsaturated fatty acids, the lower it is. In general, vegetable fats (oils) contain about 95% unsaturated fatty acids, and therefore are in a liquid aggregate state at room temperature. Animal fats, on the contrary, contain mainly saturated fatty acids (for example, cow butter consists mainly of tristearin), and therefore are solid at room temperature.

The main function of acylglycerides is that they serve to store energy, and are the most energy-intensive fuel of the cell.

Waxes

Waxes are esters of fatty acids and higher monohydric or dihydric alcohols, with the number of carbon atoms from 16 to 30. Cetyl (C 16 H 33 OH) and myricyl alcohols (C 30 H 61 OH) are often found in waxes. Natural waxes of animal origin include beeswax, spermaceti, lanolin; all of them, in addition to esters, also contain a certain amount of free fatty acids and alcohols, as well as hydrocarbons with a number of carbon atoms of 21-35.

Although some species, such as certain planktonic microorganisms, use waxes as a form of energy storage, they typically serve other functions, such as waterproofing the integument of both animals and plants.

Steroids

Steroids are a group of natural lipids containing a cyclopentane perhydrophenanthrene core. In particular, this class of compounds includes alcohols with a hydroxyl group in the third position - sterols (sterols) and their esters with fatty acids - sterides. The most common sterol in animals is cholesterol, which in non-esterified composition is part of cell membranes.

Steroids perform many important functions in different organisms: some of them are hormones (for example, sex hormones and adrenal hormones in humans), vitamins (vitamin D), emulsifiers (bile acids), etc.

Phospholipids

The main group of structural lipids is phospholipids, which, depending on the alcohol included in their composition, are divided into glycerophospholipids and sphingophospholipids. A common feature of phospholipids is their amphiphilicity: they have hydrophilic and hydrophobic parts. This structure allows them to form micelles and bilayers in an aqueous environment, the latter forming the basis of biological membranes.

Glycerophospholipids

Glycerophospholipids (phosphoglycerides) are derivatives of phosphatidic acid, consisting of glycerol, in which the first two hydroxyl groups are esterified with fatty acids (R 1 and R 2), and the third with phosphate acid. A radical (X), usually containing nitrogen, is added to the phosphate group in the third position. In natural phosphoglycerides, the first position most often contains a saturated fatty acid residue, and the second - an unsaturated fatty acid.

Fatty acid residues are nonpolar, so they form the hydrophobic part of the glycerophospholipid molecule, the so-called hydrophobic tails. The phosphate group in a neutral environment carries a negative charge, while nitrogen-containing compounds have a positive charge (some phosphoglycerides may also contain a negatively charged or neutral radical), so this part of the molecule is polar, it forms a hydrophilic head. In an aqueous solution, phosphoglycerides form micelles in which the heads are turned outward (aqueous phase), and the gyrophobic tails are turned inward.

The most common phosphoglycerides that are part of the membranes of animals and higher plants are phosphatidylcholine (lecithin), in which the X radical is a choline residue, and phosphatidylethanolamine, which contains an ethanolamine residue. Less common are phosphatidylserine, in which the amino acid serine is attached to the phosphate group.

There are also nitrogen-free glycerophospholipids: for example, phosphatididinositols (radical X - the cyclic hexahydric alcohol inositol), involved in cellular signaling, and cardiolipins - double phosphoglycerides (two molecules of phosphatidic acid connected by phosphate), found in the inner membrane of mitochondria.

Glycerophospholipids also include plasmalogens, characteristic feature The structure of these substances is that in them the acyl residue at the first carbon atom is attached NOT by an ester, but by an ester bond. In vertebrates, plasmalogenams, also called ether lipids, are enriched in cardiac muscle tissue. Also belonging to this class of compounds is the biologically active substance platelet activating factor.

Sphingophospholipids

Sphingophospholipids (sphingomyelins) consist of a ceramide containing one residue of the long-chain amino alcohol sphingosine and one fatty acid residue, and a gyrophilic radical attached to sphingosine by a phosphodiesterone bond. Choline or ethanolamine most often acts as a gyrophilic radical. Sphingomyelins are found in the membranes of various cells, but nervous tissue is rich in them, with a particularly high content of these substances in the myelin sheath of axons, hence their name.

Glycolipids

Glycolipids are a class of lipids containing mono- or oligosaccharide residues. They can be either glycerol or sphingosine derivatives.

Glyceroglycolipids

Glyceroglycolipids (glycosylglycerols) are diacylglycerol derivatives in which a mono- or oligosaccharide is attached to the third carbon atom of glycerol by a glycosyl bond. The most common of this class of compounds are galactolipids, containing one or two galactose residues. They constitute 70% to 80% of all thylakoid membrane lipids, making them the most abundant membrane lipids in the biosphere. It is assumed that plants “replaced” phospholipids with glycolipids because the phosphate content in the soil is often a limiting factor, and such replacement reduces the need for it.

Along with galactolipids, plant membranes also contain sulfolipids containing a sulfated glucose residue.

Sphingoglycolipids

Sphingoglycolipids contain ceramide and one or more sugar residues. This class of compounds is divided into several subclasses depending on the structure of the carbohydrate radical:

• Cerebrosides are sphingoglycolipids, the hydrophilic part of which is a monosaccharide residue, usually glucose or galactose. Galactocerebrosides are distributed in neuronal membranes.
• Globosides are oligosaccharide derivatives of ceramides. Together with cerebrosides, they are called neutral glycolipids because at pH 7 they are uncharged.
• Gangliosides are complex glycolipids; their hydrophilic part is represented by oligosaccharides, at the end of which there is always one or more N-acetylneuraminic (sialic) acid residues, so they have acidic properties. Gangliosides are most abundant in the membranes of ganglion neurons.

Main functions

The vast majority of lipids in living organisms belong to one of two groups: reserve ones, which perform the function of storing energy (mainly triacylglycerols), and structural ones, which are involved in the construction of cell membranes (mainly phospholipids and gylcolipids, as well as cholesterol). However, the functions of lipids are not limited to just these two; they can also be hormones or other signaling molecules, pigments, emulsifiers, water repellents of the integument, provide thermal insulation, alter buoyancy, and the like.

Storage lipids

Almost all living organisms store energy in the form of fats. There are two main reasons why these substances are best suited to perform this function. Firstly, fats contain residues of fatty acids, the level of oxidation of which is very low (almost the same as in petroleum hydrocarbons). Therefore, the complete oxidation of fats to water and carbon dioxide allows you to obtain more than twice as much energy as the oxidation of the same mass of carbohydrates. Secondly, fats are hydrophobic compounds, therefore the body, which stores energy in this form, does not have to carry the additional mass of water necessary for hydration, as is the case with polysaccharides, 2 g of water per 1 g. However, triglycerides are a slower source of energy than carbohydrates.

Fats are stored in the form of droplets in the cytoplasm of the cell. Vertebrates have specialized cells, adipocytes, that are almost entirely filled with a large drop of fat. The seeds of many plants are also rich in TG. The mobilization of fats in adipocytes and seed cells that germinate occurs thanks to lipase enzymes, which break them down into glycerol and fatty acids.

In humans, the largest amount of fatty tissue is located under the skin (the so-called subcutaneous tissue), especially in the abdomen and mammary glands. For a mildly obese person (15-20 kg of triglycerides), such reserves may be enough to provide energy for a month, while the entire reserve glycogen will last less than a day.

Adipose tissue, along with providing energy, also performs other functions: protecting internal organs from mechanical damage; thermal insulation, especially important for warm-blooded animals living in very cold conditions, such as seals, penguins, walruses; fats can also be a source of metabolic water; it is for this purpose that desert dwellers use their reserves of triglycerides: camels, kangaroos, rats (Dipodomys).

Structural lipids

All living cells are surrounded by plasma membranes, the main structural element of which is a double layer of lipids (lipid bilayer). 1 micron 2 of a biological membrane contains about a million lipid molecules. All lipids that make up membranes have amphiphilic properties: they have gyrophilic and gyrophobic parts. In an aqueous environment, such molecules spontaneously form micelles and bilayers as a result of hydrophobic interactions; in such structures, the polar heads of the molecules are returned to the outside of the aqueous phase, and the non-polar tails are returned to the inside; the same arrangement of lipids is characteristic of natural membranes. The presence of a hydrophobic layer is very important for membranes to perform their functions, since it is impermeable to ions and polar compounds.

The lipid bilayer of biological membranes is a two-dimensional liquid, that is, individual molecules can move freely relative to each other. The fluidity of membranes depends on their chemical composition: for example, with an increase in the content of lipids, which include polyunsaturated fatty acids, it increases.

The main structural lipids that make up the membranes of animal cells are glycerophospholipids, mainly phosphatidylcholine and phosphatidylethanolamine, as well as cholesterol, which increases their impermeability. Certain tissues may be selectively enriched in other classes of membrane lipids, for example, nervous tissue contains large amounts of sphingophospholipids, in particular sphingomyelin, as well as sphingoglycolipids. There is no cholesterol in the membranes of plant cells, but another steroid, ergosterol, is found. Thylakoid membranes contain large amounts of galactolipids, as well as sulfolipids.

Archaeal membranes are characterized by a unique lipid composition: they consist of the so-called glycerol dialkyl gylcerol tetraether (GDHT). These compounds are built from two long (about 32 carbon atoms) branched hydrocarbons attached at both ends to glycerol residues by an ester bond. The use of an ester linkage instead of an ester bond, characteristic of phospho- and glycolipids, is explained by the fact that it is more resistant to hydrolysis under conditions of low pH and high temperature, which is typical for the environment in which archaea usually live. At each end of GDHT, one hydrophilic group is attached to glycerol. GDHT is on average twice as long as the membrane lipids of bacteria and eukaryotes and can penetrate through the membrane.

Regulatory lipids

Some of the lipids play an active role in regulating the life of individual cells and the body as a whole. In particular, lipids include steroid hormones secreted by the gonads and the adrenal cortex. These substances are carried by the blood throughout the body and affect its functioning.

Lipids also include secondary messengers - substances that take part in transmitting signals from hormones or other biologically active substances inside the cell. In particular, phosphatidylinositol 4,5 biphosphate (PI (4,5) P2) is involved in signaling with the participation of G proteins, phosphatidylinositol 3,4,5 triphosphate initiates the formation of supramolecular complexes of signaling proteins in response to the action of certain extracellular factors, sphingolipids , such as sphingomyelin and cermaid, can regulate protein kinase activity.

Derivatives of arachidonic acid - eicosanoids - are an example of paracrine regulators of lipid nature. Depending on their structural features, these substances are divided into three main groups: prostaglandins, thromboxanes and leukotrienes. They are involved in the regulation of a wide range of physiological functions, in particular eicosanoids necessary for the functioning of the reproductive system, for induction and passage inflammatory process(including providing such aspects as pain and fever), for blood clotting, regulation of blood pressure, and they can also be involved in allergic reactions.

Other features

Some vitamins, that is, substances necessary for the functioning of the body in small quantities, are classified as lipids. They are combined under the name fat-soluble vitamins and are divided into four groups: vitamin A, D, E and K. By chemical nature, all these substances are isoprenoids. Isoprenoids also include the electron carriers ubiquinone and plastoquinone, which are part of the electron transport chains of mitochondria and plastids, respectively.

Most isoprenoids contain conjugated double bonds, which makes electron delocalization possible in their molecules. Such compounds are easily excited by light, causing them to have a color visible to the human eye. Many organisms use isoprenoids as pigments to absorb light (for example, carotenoids included in the light-harvesting complexes of chloroplasts), as well as for communication with individuals of their own or other species (for example, the isoprenoid zeaxanthin gives the feathers of some birds a yellow color).

Lipids in the human diet

Among the lipids in the human diet, triglycerides (neutral fats) predominate; they are a rich source of energy, as well as necessary for the absorption of fat-soluble vitamins. Foods of animal origin are rich in saturated fatty acids: meat, dairy products, as well as some tropical plants such as coconuts. Unsaturated fatty acids enter the human body through the consumption of nuts, seeds, olive and other vegetable oils. The main sources of cholesterol in the diet are meat and animal organs, egg yolks, dairy products and fish. However, about 85% of the cholesterol in the blood is synthesized by the liver.

Organization American Heart Association recommends consuming lipids in an amount of no more than 30% of the total diet, reducing the content of saturated fatty acids in the diet to 10% of total fats and not consuming more than 300 mg (the amount contained in one yolk) of cholesterol per day. The goal of these recommendations is to limit blood cholesterol and triglyceride levels to 20 mg/L.

Fats occupy a high energy value and play an important role in the biosynthesis of lipid structures, primarily cell membranes. Food fats are represented by triglycerides and lipoid substances. Animal fats consist of saturated fatty acids with a high melting point. Vegetable fats contain significant amounts of polyunsaturated fatty acids (PUFAs).

Animal fats contain lard (90-92% fat), butter (72-82%), pork (up to 49%), sausages (20-40% for different varieties), sour cream (20-30%), cheeses (15-30%). Sources of vegetable fats are oils (99.9% fat), nuts (53-65%), oatmeal (6.1%), buckwheat (3.3%).

Essential fatty acids

The liver plays a key role in the metabolism of fatty acids, but it is unable to synthesize some of them. Therefore, they are called essential, these in particular include ω-3 (linolenic) and ω-6 (linoleic) polyunsaturated fatty acids; they are found mainly in vegetable fats. Linolenic acid is a precursor for the synthesis of two other ω-3 acids: eoosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These substances are necessary for brain function and have a positive effect on cognition and behavioral functions.

The ratio of ω-6 ω-3 fatty acids in the diet is also important: recommended proportions range from 1:1 to 4:1. However, studies show that most North Americans consume 10 to 30 times more ω-6 fatty acids. than ω-3. This diet is associated with a risk of cardiovascular disease. But the “Mediterranean diet” is considered much healthier, it is rich in linolenic and other ω-acids, the source of which is green plants (lettuce), fish, garlic, whole grains, fresh vegetables and fruits. It is recommended to consume fish oil as a dietary supplement containing ω-c fatty acids.

Trance-unsaturated fatty acids

Most natural fats contain unsaturated fatty acids with double bonds cis-configurations. If food rich in such fats is in contact with air for a long time, it becomes bitter. This process is associated with the oxidative cleavage of double bonds, which results in the formation of aldehydes and carboxylic acids with lower molecular weight, some of which are volatile substances.

In order to increase the shelf life and resistance to high temperatures of triglycerides with unsaturated fatty acids, a partial hydrogenation procedure is used. The consequence of this process is the transformation of double bonds into single bonds, however side effect there may also be a transition of double bonds with cis- V trance-configurations. Consumption of so-called “trans fats” leads to an increase in the content of low-density lipoproteins (“bad” cholesterol) and a decrease in the content of high-density lipoproteins (“good” cholesterol) in the blood, which leads to an increased risk of cardiovascular diseases, in particular coronary insufficiency. Moreover, “trans fats” contribute to inflammatory processes.

The negative effect of “trans fats” manifests itself when consuming 2-7 g per day; this amount can be contained in one serving of French fries fried in partially hydrogenated oils. Some laws prohibit the use of this oil, such as Denmark, Philadelphia and New York.

Lipids constitute a large and quite heterogeneous in chemical composition group of organic substances that are part of living cells, soluble in low-polar organic solvents (ether, benzene, chloroform, etc.) and insoluble in water. In general, they are considered to be derivatives of fatty acids.

A peculiarity of the structure 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 carry out 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 groups of lipids have two obligatory structural components - higher carboxylic acids and alcohols.

Higher fatty acids (HFAs). Many higher carboxylic acids were first isolated from fats, which is why they are called fatty. Biologically important fatty acids can be saturated(Table 10.1) and unsaturated(Table 10.2). Their general structural features:

They are monocarbon;

Include an even number of carbon atoms in the chain;

Have a cis configuration of double bonds (if present).

Table 10.1.Essential saturated fatty acid 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 the methylene group CH 2.

The IUPAC rules for DRCs allow the use of their trivial names (see Tables 10.1 and 10.2).

Currently, our own nomenclature of unsaturated liquid liquids is also used. In it, the terminal carbon atom, regardless of the length of the chain, is designated 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. Thus, 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, constitute the omega-3 family of liquid 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.

From saturated acids in lipids human body the most important are palmitic C16 and stearic C18 (see Table 10.1), and of the unsaturated ones - oleic C18:1, linoleic C18:2, linolenic and arachidonic C 20:4 (see Table 10.2).

It should be emphasized the role of polyunsaturated linoleic and linolenic acids as compounds irreplaceable for humans (“vitamin F”). They are not synthesized in the body and should 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. Essential unsaturated fatty acid 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 hypolipidemic herbal medicine. Alcohols. Lipids may include:

Higher monohydric alcohols;

Polyhydric alcohols;

Amino alcohols.

In natural lipids, the most common are saturated and less often unsaturated long-chain alcohols (C 16 or more), 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 waxes.

Polyhydric alcohols in most natural lipids are represented by the trihydric alcohol glycerol. Other polyhydric alcohols are found, such as the dihydric alcohols ethylene glycol and 1,2 propanediol, as well as myoinositol (see 7.2.2).

The most important amino alcohols that are part of natural lipids are 2-aminoethanol (colamine), choline, and serine and sphingosine, which also belong to the α-amino acids.

Sphingosine is an unsaturated long-chain dihydric amino alcohol. The double bond in sphingosine has trance-configuration, and the asymmetric atoms C-2 and C-3 - D-configuration.

Alcohols in lipids are acylated with higher carboxylic acids at the corresponding hydroxyl groups 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 production of creams and ointments. An example is palmitic acid cetyl ester(cetin) - main component spermaceti. Spermaceti is secreted from the fat contained in the cavities of the skull of sperm whales. Another example is Palmitic acid melissil ester- 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 - complete esters of glycerol and IVG, although mono- and diacylglycerols are also found and take part in 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 increased level 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, while mixed ones contain residues of different ones.

Triacylglycerols of animal origin usually contain predominantly saturated acid residues. Such triacylglycerols are usually solids. On the contrary, vegetable oils contain mainly residues of unsaturated acids and have a liquid consistency.

Below are examples of neutral triacylglycerols and their systematic and (in parentheses) 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 small quantities in the tissues of plants and animals. 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 groups that allow them to be classified simultaneously into different groups. According to the general classification of lipids (see Diagram 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 remove 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.Classification of phospholipids

Glycerophospholipids. These compounds are the main lipid components of cell membranes.

According to their chemical structure, glycerophospholipids are derivatives l -glycero-3-phosphate.

l-Glycero-3-phosphate contains an asymmetric carbon atom and, therefore, can exist in the form of two stereoisomers.

Natural glycerophospholipids have the same configuration, being derivatives of l-glycero-3-phosphate, formed during metabolism from dihydroxyacetone phosphate.

Phosphatides. Among glycerophospholipids, the most common are phosphatides - ester derivatives of l-phosphatidic acids.

Phosphatidic 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 hydroxyls of phosphoric acid 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 ionic groups in phosphatides are ionized.

Examples of phosphatides are compounds containing phosphatidic acids esterified for phosphate hydroxyl with corresponding alcohols:

Phosphatidylserines, esterifying agent - serine;

Phosphatidylethanolamines, esterifying agent - 2-aminoethanol (in biochemical literature often, but not quite correctly, called ethanolamine);

Phosphatidylcholines, esterifying agent - choline.

These esterifying agents are related 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 amino-containing esterifying agent, contain residues of polyhydric alcohols - glycerol, myoinositol, etc. The phosphatidylglycerols and phosphatidylinositols given below as examples belong to acidic glycerophospholipids, since their structures do not contain fragments of amino alcohols, which give phosphatidylethanolamines and related compounds a neutral character.

Plasmalogens. Less common than ester glycerophospholipids are lipids with an ether linkage, 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).

alcohol linked by an ether bond to the C-1 atom of glycero-3-phosphate, such as plasmalogens with an ethanolamine fragment - L-phosphatidal ethanolamines. Plasmalogens make up up to 10% of all CNS lipids.

10.4.2. Sphingolipids

Sphingolipids are structural analogues of glycerophospholipids in which sphingosine is used instead of glycerol. Another example of sphingolipids is 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 of C-1 ceramide is esterified, as a rule, with choline phosphate (less often with colamine phosphate), so they can also be classified as phospholipids.

10.4.3. Glycolipids

As the name suggests, compounds of this group include carbohydrate residues (usually 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 be considered sphingolipids).

IN 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 containing at least one residue V-acetylneuraminic acid (see Appendix 11-2).

10.5. Properties of lipids

and their structural components

A special feature of complex lipids is their biphilicity, caused by non-polar hydrophobic and highly polar ionized hydrophilic groups. In phosphatidylcholines, for example, the hydrocarbon radicals of fatty acids form two non-polar “tails”, and the carboxyl, phosphate and choline groups form the polar part.

At the interface, such compounds act as excellent emulsifiers. As part of cell membranes, lipid components provide 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, which allows them to penetrate the membranes of nerve cells.

Fatty acids are weak electrolytes( p K a~4.8). They are slightly dissociated in aqueous solutions. At pH< p K a non-ionized form predominates, at pH > p Ka, i.e., under 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 of soap are partially hydrolyzed in water, their solutions have an alkaline reaction.

Natural unsaturated fatty acids that have cis- double bond configuration, have a large supply of internal energy and, therefore, compared to trance-isomers are thermodynamically less stable. Their cis-trans -isomerization occurs easily when heated, especially in the presence of radical reaction initiators. In laboratory conditions, this transformation can be carried out by the action of nitrogen oxides formed during the decomposition of nitric acid when heated.

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

Using the hydrolysis reaction, the structure of lipids is determined, and valuable products (soaps) are obtained. Hydrolysis is the first stage of utilization and metabolism of dietary fats in the body.

Hydrolysis of triacylglycerols is carried out either by exposure to 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 esters, the ether bond is cleaved in an acidic, but not in an alkaline, environment.

10.5.2. Addition reactions

Lipids containing unsaturated acid residues in their structure add hydrogen, halogens, hydrogen halides, and water through double bonds in an acidic environment. Iodine number is a measure of the unsaturation of triacylglycerols. It corresponds to the number of grams of iodine that can add to 100 g of a substance. The composition of natural fats and oils and their iodine numbers vary within fairly wide limits. 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 turn into solid fats.

10.5.3. Oxidation reactions

Oxidative processes involving lipids and their structural components are quite diverse. In particular, the oxidation of unsaturated triacylglycerols by oxygen during storage (auto-oxidation, see 3.2.1), accompanied 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, in radiation sickness).

Structural fragments of unsaturated higher fatty acids in phospholipids serve as targets for attack active forms of oxygen(AFC, see Appendix 03-1).

When attacked, in particular by the hydroxyl radical HO, the most active of ROS, the lipid molecule LH undergoes homolytic rupture S-N connections in the allylic position, as shown in the lipid peroxidation model (Scheme 10.3). The resulting allylic radical L" instantly reacts with molecular oxygen present in the oxidation environment to form the lipid peroxyl radical LOO". From this moment, a chain cascade of lipid peroxidation reactions begins, since the constant formation of allylic lipid radicals L" occurs, renewing 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 to form 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 allylic radical has a mesomeric structure and can further undergo transformations in two directions (see diagram 10.3, paths A And b), leading to intermediate hydroperoxides. Hydroperoxides are unstable and even at ordinary temperatures decompose to form aldehydes, which are further oxidized into acids - the final products of the reaction. The result is generally two monocarboxylic and two dicarboxylic acids with shorter carbon chains.

Unsaturated acids and lipids with residues of unsaturated acids under mild conditions are oxidized with an aqueous solution of potassium permanganate, forming glycols, and in more rigid conditions (with the rupture of carbon-carbon bonds) - the corresponding acids.

What are lipids, what is the classification of lipids, what is their structure and function? The answer to this and many other questions is given by biochemistry, which studies these and other substances that are of great importance for metabolism.

What it is

Lipids are organic substances that are insoluble in water. The functions of lipids in the human body are diverse.

Lipids - this word means "small particles of fat"

This is first of all:

• Energy. Lipids serve as a substrate for storing and using energy. When 1 gram of fat is broken down, approximately 2 times more energy is released than when protein or carbohydrates of the same weight are broken down.
• Structural function. The structure of lipids determines the structure of the membranes of the cells of our body. They are arranged in such a way that the hydrophilic part of the molecule is located inside the cell, and the hydrophobic part is on its surface. Thanks to these properties of lipids, each cell, on the one hand, is an autonomous system, fenced off from the outside world, and on the other hand, each cell can exchange molecules with others and with environment using special transport systems.
• Protective. The surface layer that we have on our skin and serves as a kind of barrier between us and the outside world is also made up of lipids. In addition, they, as part of adipose tissue, provide thermal insulation and protection from harmful external influences.
• Regulatory. They are part of vitamins, hormones and other substances that regulate many processes in the body.

The general characteristics of lipids are based on their structural features. They have dual properties, since they have a soluble and insoluble part in the molecule.

Entry into the body

Lipids partly enter the human body with food, and partly can be synthesized endogenously. Splitting the main part dietary lipids occurs in the duodenum under the influence of pancreatic juice secreted by the pancreas and bile acids in the composition of bile. Having broken down, they are resynthesized again in the intestinal wall and, already as part of special transport particles ─ lipoproteins, ─ are ready to enter the lymphatic system and general blood flow.

A person needs to receive about 50-100 grams of fat from food every day, which depends on the condition of the body and the level of physical activity.

Classification

Classification of lipids depending on their ability to form soaps under certain conditions divides them into the following classes of lipids:

• Saponifiable. This is the name for substances that, in an alkaline environment, form salts of carboxylic acids (soaps). This group includes simple lipids and complex lipids. Both simple and complex lipids are important for the body; they have different structures and, accordingly, lipids perform different functions.
• Unsaponifiable. In an alkaline environment they do not form salts of carboxylic acids. Biological chemistry includes fatty acids, derivatives of polyunsaturated fatty acids - eicosanoids, cholesterol, as the most prominent representative of the main class of sterols-lipids, as well as its derivatives - steroids and some other substances, for example, vitamins A, E, etc.

General classification of lipids

Fatty acid

Substances that belong to the group of so-called simple lipids and are of great importance for the body are fatty acids. Depending on the presence of double bonds in the non-polar (water-insoluble) carbon “tail”, fatty acids are divided into saturated (do not have double bonds) and unsaturated (have one or even more carbon-carbon double bonds). Examples of the first: stearic, palmitic. Examples of unsaturated and polyunsaturated fatty acids: oleic, linoleic, etc.

It is unsaturated fatty acids that are especially important for us and must be supplied with food.

Why? Because they:

• They serve as a component for the synthesis of cell membranes and participate in the formation of many biologically active molecules.
• Help maintain normal functioning of the endocrine and reproductive systems.
• Help prevent or slow down the development of atherosclerosis and many of its consequences.

Fatty acids are divided into two large groups: unsaturated and saturated

Inflammatory mediators and more

Another type of simple lipids are such important mediators of internal regulation as eicosanoids. They have a unique (like almost everything in biology) chemical structure and, accordingly, unique chemical properties. The main basis for the synthesis of eicosanoids is arachidonic acid, which is one of the most important unsaturated fatty acids. It is eicosanoids that are responsible for the course of inflammatory processes in the body.

Their role in inflammation can be briefly described as follows:

• They change the permeability of the vascular wall (namely, they increase its permeability).
• Stimulate the release of leukocytes and other cells immune system in fabric.
• With the help of chemicals, they mediate the movement of immune cells, the release of enzymes and the absorption of particles foreign to the body.

But the role of eicosanoids in the human body does not end there; they are also responsible for the blood coagulation system. Depending on the situation, eicosanoids can dilate blood vessels, relax smooth muscle, reduce aggregation or, if necessary, cause the opposite effects: vasoconstriction, contraction of smooth muscle cells and thrombus formation.

Eicosanoids are a large group of physiologically and pharmacologically active compounds.

Studies have been conducted according to which, people who received sufficient quantities of the main substrate for eicosanoid synthesis ─ arachidonic acid ─ with food (located in fish oil, fish, vegetable oils) suffered less from cardiovascular diseases. Most likely, this is due to the fact that such people have a more advanced eicosanoid metabolism.

Substances of complex structure

Complex lipids are a group of substances that are no less important for the body than simple lipids. The main properties of this group of fats:

• They participate in the formation of cell membranes, along with simple lipids, and also provide intercellular interactions.
• They are part of the myelin sheath of nerve fibers, necessary for the normal transmission of nerve impulses.
• They are one of the important components of surfactant ─ a substance that ensures breathing processes, namely, preventing the alveoli from collapsing during exhalation.
• Many of them play the role of receptors on the surface of cells.
• The significance of some complex fats secreted from cerebrospinal fluid, nervous tissue, and heart muscle is not fully understood.

The simplest representatives of lipids in this group include phospholipids, glyco- and sphingolipids.

Cholesterol

Cholesterol is a substance of lipid nature with the most important significance in medicine, since disruption of its metabolism negatively affects the condition of the entire organism.

Some of the cholesterol is ingested with food, and some is synthesized in the liver, adrenal glands, gonads and skin.

It is also involved in the formation of cell membranes, the synthesis of hormones and other chemically active substances, and is also involved in the metabolism of lipids in the human body. Indicators of cholesterol in the blood are often examined by doctors, as they show the state of lipid metabolism in the human body as a whole.

Lipids have their own special transport forms - lipoproteins. With their help, they can be transported through the bloodstream without causing embolism.

Disorders of fat metabolism are most quickly and clearly manifested by disorders of cholesterol metabolism, the predominance of atherogenic carriers (the so-called low- and very low-density lipoproteins) over anti-atherogenic ones (high-density lipoproteins).

The main manifestation of the pathology of lipid metabolism is the development of atherosclerosis.

It manifests itself by narrowing the lumen of arterial vessels throughout the body. Depending on the predominance of various localizations in the vessels, a narrowing of the lumen of the coronary vessels develops (accompanied by angina pectoris), cerebral vessels (with memory and hearing impairments, possible headaches, noise in the head), kidney vessels, blood vessels lower limbs, vessels of the digestive organs with corresponding symptoms.

Thus, lipids are at the same time an indispensable substrate for many processes in the body and, at the same time, if lipid metabolism is disturbed, they can cause many diseases and pathological conditions. Therefore, fat metabolism requires monitoring and correction when the need arises.

Composition, properties and functions of lipids in the body

The nutritional value oils and fats used in the baking and confectionery industry.

Cyclic lipids. Role in food technology and vital functions of the body.

Simple and complex lipids.

Composition, properties and functions of lipids in the body.

Lipids in raw materials and food products

Lipids combine a large number of fats and fat-like substances of plant and animal origin, which have a number of common characteristics:

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 high molecular weight compounds and consist of several molecules linked to each other. Lipids may contain alcohols and linear chains of a number of carboxylic acids. In some cases, their individual blocks may consist of high molecular weight acids, various phosphoric acid residues, carbohydrates, nitrogenous bases and other components.

Lipids, together with proteins and carbohydrates, make up the bulk of organic substances in all living organisms, being an essential component of every cell.

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. A mixture of lipids and compounds soluble in them, extracted from natural objects, is called “crude” fat.

Main components of crude fat

Substances accompanying lipids play an important role in food technology and 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 65-72. The lipid content in them depends not only on the individual characteristics of the plants, but also on the variety, location, and growing conditions. Lipids play an important role in the vital processes of the body.

Their functions are very diverse: their role is important in energy processes, in the body’s defense reactions, in its maturation, aging, etc.

Lipids are part of all structural elements cells and primarily cell membranes, influencing their permeability. They are involved in the transmission of nerve impulses, provide intercellular contact, active transport of nutrients across membranes, transport of fats in the blood plasma, protein synthesis and various enzymatic processes.

According to their functions in the body, they are conventionally divided into two groups: spare and structural. Spare ones (mainly acylglycerols) have a high calorie content, are the body's energy reserve and are used by it in case of lack of nutrition and diseases.

Storage lipids are storage substances that help the body endure adverse environmental influences. Most plants (up to 90%) contain storage lipids, mainly in the 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 oil seeds). These are difficult to extract “bound” lipids.

Natural fatty acids that are part of lipids in animals and plants have many common properties. They usually contain a clear number of carbon atoms and have an unbranched chain. Conventionally, fatty acids are divided into three groups: saturated, monounsaturated and polyunsaturated. Unsaturated fatty acids in 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 have some common structural features, but a strict classification of lipids does not yet exist. One of the approaches to the classification of lipids is chemical, according to which lipids include derivatives of alcohols and higher fatty acids.

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 are called oils and fats. Fat contains mainly triglycerides, but also contains 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 found in almost all natural oils and fats. Erucic acid is part of rapeseed oil. Most of the most common oils contain unsaturated acids containing 1-3 double bonds. Some acids in natural oils and fats tend to have a cis configuration, i.e. the substituents are distributed on one side of the double bond plane.

Acids with branched carbohydrate chains containing hydroxy, keto and other groups are usually found in small quantities in lipids. The exception is racinolic acid in castor oil. In natural plant triacylglycerols, positions 1 and 3 are preferentially occupied by saturated fatty acid residues, and position 2 is unsaturated. In animal fats the picture is the opposite.

The position of fatty acid residues in triacylglycerols significantly affects their physicochemical properties.

Acylglycerols are liquid or solid substances with low melting points and fairly high boiling points, with 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 coefficient, shows how many free fatty acids are contained in the 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 serves as 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.

Iodine value is an indicator of fat unsaturation. O is determined by the number of grams of iodine added to 100 g of fat. The higher the iodine value, the more unsaturated the fat is.

Waxes. Waxes are esters of higher fatty acids and high-molecular alcohols (18-30 carbon atoms). The fatty acids that make up waxes are the same as those for fats, but there are also specific ones that are characteristic only of waxes.

For example: carnauba;

cerotinic;

montanova

The general formula of waxes can be written as follows:

Waxes are widespread in nature, covering leaves, stems, and fruits of plants with a thin layer, they protect them from wetting with water, drying out, and the action of microorganisms. The wax content in grains and fruits is low.

Complex lipids. Complex lipids have multicomponent molecules, the individual parts of which are connected by chemical bonds of various types. These include phospholipids, consisting of fatty acid residues, glycerol and other polyhydric alcohols, phosphoric acid and nitrogenous bases. In the structure of glycolipids, along with polyhydric alcohols and high-molecular fatty acids, there are also carbohydrates (usually galactose, glucose, mannose residues).

There are also two groups of lipids, which include simple and complex lipids. These are diol lipids, which are simple and complex lipids of dihydric alcohols and high molecular weight fatty acids, in some cases containing phosphoric acid and nitrogenous bases.

Ormitinolipids are built from fatty acid residues, the amino acid ormitine or lysine, and in some cases including dihydric alcohols. The most important and widespread group of complex lipids are phospholipids. Their molecule is built from residues of alcohols, high-molecular 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.

Scheme 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 are isolated as by-products during 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 participate 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, playing an important role in the prevention of atherosclerosis.

Lipids- fat-like organic compounds, insoluble in water, but highly soluble in non-polar solvents (ether, gasoline, benzene, chloroform, etc.). Lipids belong to the simplest biological molecules. Chemically, most lipids are esters of higher carboxylic acids and a number of alcohols. The most famous among them fats. Each fat molecule is formed by a molecule of the triatomic alcohol glycerol and the ester bonds of three molecules of higher carboxylic acids attached to it. According to the accepted nomenclature, fats are called triacylglycerols.

When fats are hydrolyzed (that is, broken down by the introduction of H + and OH - into ester bonds), they break down into glycerol and free higher carboxylic acids, each containing an even number of carbon atoms.

Carbon atoms in molecules of higher carboxylic acids can be connected to each other by both simple and double bonds. Among the saturated (saturated) higher carboxylic acids most often found in fats are:

• palmitic CH 3 - (CH 2) 14 - COOH or C 15 H 31 COOH;
• stearic CH 3 - (CH 2) 16 - COOH or C 17 H 35 COOH;
• arachine CH 3 - (CH 2) 18 - COOH or C 19 H 39 COOH;

among the unlimited:

• oleic CH 3 - (CH 2) 7 - CH = CH - (CH 2) 7 - COOH or C 17 H 33 COOH;
• linoleic CH 3 - (CH 2) 4 - CH = CH - CH 2 - CH - (CH 2) 7 - COOH or C 17 H 31 COOH;
• linolenic CH 3 - CH 2 - CH = CH - CH 2 - CH = CH - CH 2 - CH = CH - (CH 2) 7 - COOH or C 17 H 29 COOH.

The degree of unsaturation and the length of chains of higher carboxylic acids (i.e., the number of carbon atoms) determines the physical properties of a particular fat.

Fats containing short and unsaturated carbon chains in fatty acid residues have a low melting point. At room temperature these are liquids (oils) or ointment-like substances. Conversely, fats with long and saturated chains of higher carboxylic acids are solids at room temperature. This is why, when hydrogenation (saturation of acid chains with hydrogen atoms at double bonds), liquid peanut butter, for example, turns into a homogeneous, spreadable peanut butter, and sunflower oil into margarine. The bodies of animals living in cold climates, such as fish from the Arctic seas, usually contain more unsaturated triacylglycerols than those living in southern latitudes. For this reason, their body remains flexible even at low temperatures.

There are:

Phospholipids- amphiphilic compounds, i.e. they have polar heads and non-polar tails. The groups forming the polar head group are hydrophilic (soluble in water), while the non-polar tail groups are hydrophobic (insoluble in water).

The dual nature of these lipids determines their key role in the organization of biological membranes.

Wax- esters of monohydric (with one hydroxyl group) high molecular weight (having a long carbon skeleton) alcohols and higher carboxylic acids.

Another group of lipids consists of steroids. These substances are based on cholesterol alcohol. Steroids are very poorly soluble in water and do not contain higher carboxylic acids.

These include bile acids, cholesterol, sex hormones, vitamin D, etc.

Close to steroids terpenes(plant growth substances - gibberellins; phytol, which is part of chlorophyll; carotenoids - photosynthetic pigments; plant essential oils - menthol, camphor, etc.).

Lipids can form complexes with other biological molecules.

Lipoproteins- complex formations containing triacylglycerols, cholesterol and proteins, the latter not having covalent bonds with lipids.

Glycolipids is a group of lipids built on the basis of the alcohol sphingosine and containing, in addition to the residue of higher carboxylic acids, one or more sugar molecules (most often glucose or galactose).

Functions of lipids

Structural. Phospholipids together with proteins form biological membranes. The membranes also contain sterols.

Energy. When 1 g of fat is oxidized, 38.9 kJ of energy is released, which goes towards the formation of ATP. A significant portion of the body's energy reserves are stored in the form of lipids, which are consumed when there is a lack of nutrients. Hibernating animals and plants accumulate fats and oils and use them to maintain vital processes. The high lipid content in seeds provides energy for the development of the embryo and seedling until it begins to feed itself. The seeds of many plants (coconut palm, castor bean, sunflower, soybean, rapeseed, etc.) serve as raw materials for producing oil industrially.

Protective and thermal insulation. Accumulating in the subcutaneous fatty tissue and around some organs (kidneys, intestines), the fat layer protects the body from mechanical damage. In addition, due to low thermal conductivity, the layer of subcutaneous fat helps retain heat, which allows, for example, many animals to live in cold climates. In whales, in addition, it plays another role - it promotes buoyancy.

Lubricant and water repellent. Waxes cover skin, wool, feathers, make them more elastic and protect them from moisture. The leaves and fruits of plants are covered with a waxy coating; wax is used by bees in the construction of honeycombs.

Regulatory. Many hormones are derivatives of cholesterol, such as sex hormones (testosterone in men and progesterone in women) and corticosteroids (aldosterone).

Metabolic. Cholesterol derivatives, vitamin D play a key role in the metabolism of calcium and phosphorus. Bile acids are involved in the processes of digestion (emulsification of fats) and absorption of higher carboxylic acids.

Lipids are a source of metabolic water. The oxidation of fat produces approximately 105 g of water. This water is very important for some desert inhabitants, in particular for camels, which can do without water for 10-12 days: the fat stored in the hump is used precisely for this purpose. Bears, marmots and other hibernating animals obtain the water they need for life as a result of fat oxidation.