Among glycolipids, galactosylacylglycerols are especially widespread.

These compounds are found in a wide variety of plant tissues. They are found in mitochondria, chloroplasts and localized in membranes; found in algae, some photosynthetic bacteria.

The main form of glycolipids in animal tissues, especially in nervous tissue, in particular in the brain, are glycosphingolipids. The latter contains a ceramide consisting of the sphingosine alcohol and a fatty acid residue, and one or more sugar residues. The most important glycosphingolipids are cerobrosides and gangliosides.

The simplest cerobrosides are galactosylceramides and glucosylceramides. The composition of galactosylceramides includes D-galactose, which is linked by an ether bond to the hydroxyl group of the amino alcohol sphingosine. In addition, galactosylceramide contains a fatty acid. Most often, lignoceric, nervonic or cerebronic acid, i.e. fatty acids having 24 carbon atoms.

Sphingosine

									C HC (CH2)21
							H2 C
	CH2 OH
									Fatty acid (ex.
									cerebronic acid)
				HOH

HOH

β-D-galactose

Figure 5 - The structure of galactosylceramide

There are sulphogalactosylceramides, which differ from galactosylceramides by the presence of a sulfuric acid residue attached to the third carbon atom of the hexose.

Glucosylceramides, unlike galactosylceramides, have a glucose residue instead of a galactose residue.

More complex glycosphingolipids are gangliosides. One of the simplest gangliosides is hematoside, isolated from the stroma of erythrocytes. It contains ceramide, one molecule each of galactose, glucose and N-acetylneuraminic acid. Gangliosides are found in large quantities in the nervous tissue. They perform receptor and other functions.

1.6 Unsaponifiable lipids

Lipids that are not hydrolyzed to release fatty acids and are unable to form soaps during alkaline hydrolysis are called unsaponifiables.

mi. The classification of unsaponifiable lipids is based on their division into two groups - steroids and terpenes.

1.6.1 Steroids

Steroids are widely distributed compounds in nature. These are derivatives of tetracyclic triterpenes. The basis of their structure is the core:

				10B

Cyclopentanperhydrophenanthrene

Steroids include sterols (sterols) - high molecular weight cyclic alcohols and sterides - esters of sterols and higher fatty acids. Sterides do not dissolve in water, but are highly soluble in all fatty solvents and are part of crude fat. Sterids form the saponifiable fraction of lipids. Sterols, however, during saponification of fat remain in the unsaponifiable fraction, making up the largest part of it.

In humans and animals, the main representative of sterols (sterols) is cholesterol:

			CH3 CH 2	CH2	CH3
				CH2
	CH3				CH3

CH 3 13 17

OH 3 5 6

cholesterol (cholesterol)

Cholesterol plays an important role in the life of the animal organism

– involved in the construction of biological membranes. Being in the composition of cell membranes, together with phospholipids and proteins, it provides selective permeability of the cell membrane, has a regulatory effect on the state of the membrane and on the activity of enzymes associated with it;

– It is a precursor to the formation of bile acids in the body, as well as steroid hormones. These hormones include testosterone (male sex hormone), estradiol (one of the female hormones), aldesterone (formed in the adrenal cortex and regulating water-salt balance);

– is a provitamin of vitamins of group D. Cholesterol under the action of UV-

rays in the skin turns into vitamin D3 (cholecalciferol), which in turn serves as a precursor of a hormone involved in the regulation of calcium metabolism and bone mineralization. It should also be noted that in case of violation

metabolism, cholesterol is deposited on the walls of blood vessels, leading to a serious illness - atherosclerosis.

Plants and yeast contain ergosterol (ergosterol):

			CH3 CH	CH2	CH3
	CH3				CH3

CH 3 13 17

10 8 OH 3 5 6 7

Ergosterol (ergosterol)

When ergosterol is irradiated with UV, vitamin D2 (ergocalciferol) is formed from it. Yeast is used for the industrial production of vitamins of group D (antirachitic vitamins), they contain over 2% of sterides and sterols per dry matter.

Vegetable oils (soybean, corn, wheat germ oils) usually contain from two to four different sterols, differing from each other in quantity, arrangement of double bonds and side chain structure, and β-sitosterol is an obligatory component:

CH3

CH3

CH2

CH2

CH3

CH3

C2 H5

CH3

10 OH 3 5 6

β-sitosterol

In corn, the share of β-sitosterol is 86% of all sterols, and in wheat it is 66%.

1.6.2 Terpenes

The structure of terpenes is based on the isoprene molecule:

H2 CC CHCH2

This is the monomer from which the oligomeric or polymeric chains of unsaponifiable lipids are built. Terpenes whose molecules are compounds of 2, 3, 4, 6, 8 isoprene molecules are called mono-, sesqui-, di-, tri- and tetraterpenes, respectively. Terpene molecules can have a linear or cyclic structure, contain hydroxyl, carbonyl and carboxyl groups.

Monoterpenes. These are volatile liquid substances with a pleasant smell. They are the main components of fragrant essential oils obtained from plant tissues - flowers, leaves, fruits.

As a typical representative of aliphatic monoterpenes is myrcene. From 30 to 50% of myrcene is found in the essential oil of hops. Representatives of oxygen derivatives of aliphatic terpenes are linalool, geraniol and citronellol. All of them are alcohols. Linalool is found in lily of the valley flowers, orange and coriander oil. Apparently, the aroma of peaches is due to various esters of linalool - acetic acid, formic acid, etc. Geraniol is found in eucalyptus oil. Citronellol has a rose scent and is found in rose, geranium and other oils.

Among the monocyclic terpenes, the most common and important are limonene, menthol, and carvone. Limonene is found in turpentine, caraway oil; menthol is the main (up to 70%) essential oil of peppermint, and carvone is found in the essential oils of cumin and dill.

Sesquiterpenes. This group of terpenes is also found in essential oils. One of the most interesting compounds is the aromatic sesquiterpene dimer gossypol, a specific pigment in cottonseeds.

Diterpenes. The compounds that are part of many biologically important compounds are most widely represented. So, diterpene alcohol phytol is a part of chlorophyll.

Chlorophyll is the pigment that gives plants their green color. It is found in leaves and stems, ears and grains. Chlorophyll is found in special formations in protoplasm called chloroplasts. There are two types of chlorophyll in plants: chlorophyll a (blue-green) and chlorophyll b (yellow-green).

		OCH3			OCH3
C32 H30 OH4 Mg			С 32Н 28О 2N 4 Mg
		OS 20H 39	Chlorophyll in		OS 20H 39
Chlorophyll a
		alcohol phytol			alcohol phytol

Of great interest is the similarity of the structure of chlorophyll with the blood coloring substance hemin. The composition of chlorophyll and hemin includes four pyrrole residues connected in the form of a porphyrin series, which in hemin is associated with iron, and in chlorophyll with magnesium. Chlorophyll takes an active part in the process of photosynthesis. As a result of this process, carbon dioxide, under the influence of sunlight absorbed by chlorophyll, is reduced to hexose and free oxygen is released. Photosynthesis is the only process during which the radiant energy of the sun is stored in organic compounds in the form of chemical bonds.

Diterpene chains are part of vitamins E and K1; Vitamin A is a monocyclic diterpene. The tricyclic diterpene is abietic acid, the main component of resin acids, known in the art as rosin.

Sodium rosin salts are one of the components of laundry soap. Many diterpenes are components of essential oils - camphorene, kaurene, steviol and agate.

Triterpenes. Represented by the most famous triterpene squalene. Squalene is the parent compound from which steroids, such as cholesterol, are synthesized in animals and yeast. The triterpene chain is part of vitamin K2. More complex triterpenes include limonin and cucurbitacin A, compounds that cause the bitter taste of lemon and pumpkin.

Tetraterpenes. These pigments are carotenoids. They give the plants a yellow or orange color of different shades. The most famous representatives of carotenoids are carotene, lutein, zeaxanthin and cryptoxanthin.

Carotenes were first isolated from carrots (from the Latin “karota” – carrots). Three types of carotenes are known: α-, β- and γ-carotenes, which differ both in chemical structure and biological functions. β-carotene has the highest biological activity, since it contains two β-ionone rings and during its hydrolytic decomposition under the action of the carotene enzyme, two molecules of vitamin A1 are formed:

C 1"

β - carotene

carotenoses

(carotene - dioxygenase)

vitamin A1

(retinol)

During the hydrolytic cleavage of α- and γ-carotene, one molecule of vitamin A is formed, since they each contain one β-ionone ring. The degree of digestibility of carotenoids and free vitamin A depends on the fat content of the food. β-carotene gives carrots, pumpkins, oranges, peaches and other vegetables and fruits their characteristic color. Carotenes, along with chlorophyll, are found in all green parts of plants.

Lutein is a yellow pigment found along with carotenes in the green parts of plants. The color of yellow corn seeds depends on the carotenes and carotenoids present in them, called zeaxanthin and cryptoxanthin. The color of tomato fruits is due to the carotenoid lycopene.

Lutein, zeaxanthin and cryptoxanthin also show vitamin A activity.

Carotenoids play an important role in the metabolism of plants, participating in the process of photosynthesis. Also, carotenoids are of great importance in the food industry. Pigmentation of cereal grains by carotenoids affects

The classification of lipids allows you to understand the nuances of the participation of these microelements in a variety of biological processes of human life. The biochemistry and structure of each such substance that is part of cells still cause a lot of controversy among scientists and experimenters.

General description of lipids

Lipids, as you know, are natural compounds that include various fats in their composition. The difference between these substances and other representatives of this organic group is that they are practically not utilized in water. Being active esters of acids with a high level of fat content, they are not able to completely eliminate themselves with the help of inorganic type solvents.

Lipids are present in the human body. Their share reaches an average of 10-15% of the total body. The importance of lipids cannot be underestimated: they serve as a direct supplier of unsaturated fatty acids. From the outside, substances enter the body with vitamin F, which is extremely important for the proper functioning of the digestive system.

In addition, lipid is a hidden fluid resource in the human body. Oxidized, 100 g of fats are able to form 106 g of water. One of the main purposes of these elements is to perform the function of a natural solvent. It is thanks to her that in the intestines there is a continuous absorption of valuable fatty acids and vitamins that dissolve in organic solvents. Almost half of the entire mass of the brain belongs to lipids. In the composition of other tissues and organs, their number is also large. In the layers of subcutaneous fat can be up to 90% of all lipids.

The main types of lipid compounds

The biochemistry of fatty organic substances and their structure predetermine class differences. The table allows you to visually demonstrate what lipids are.

Each fat-containing substance belongs to one of two categories of lipids:

• saponifiable;
• unsaponifiable.

If salts of high fat acids have been formed by hydrolysis using alkali, saponification may occur. In this case, potassium and sodium salts are called soaps. Saponifiable substances are the largest group of lipids.

In turn, the group of saponifiable elements can be conditionally divided into two groups:

• simple (consisting only of oxygen, carbon dioxide and hydrogen atoms);
• complex (they are simple compounds in combination with phosphorus bases, glycerol residues or two-volume unsaturated sphingosine).

Simple lipids

Biochemistry classifies various fatty acids and alcohol esters as simple lipids. Among the latter substances, the most common are cholesterol (the so-called cyclic alcohol), glycerol and oleic alcohol.

One of the esters of glycerol can be called triaciglycerol, which consists of several molecules of high fat acids. In fact, simple compounds are part of the apodocytes of adipose tissues. It is also worth noting that ester contacts with fatty acids can occur at three points at once, since glycerol is a trihydric alcohol. In this case, compounds formed from the above-mentioned bond arise:

• triacylglycerides;
• diacylglycerides;
• monoacylglycerides.

The predominant part of these neutral-type fats is present in the body of warm-blooded animals. Their structure contains most of the residues of palmitic, stearic acids of high fat content. In addition, neutral fats in some tissues can differ significantly in their content from fats in other organs within the same organism. For example, human subcutaneous tissue is enriched with such acids by an order of magnitude higher than the liver, which consists of unsaturated fats.

Neutral Fats

Both types of acids, regardless of saturation, belong to the type of aliphatic carboxylic acids. Biochemistry makes it possible to understand how important these substances are for lipids by comparing micronutrients with building blocks. Thanks to them, each lipid is built.
If we talk about the first type, about saturated acids, then in the human body you can most often find palmitic and stearic acids. Much less often, lignocerine is involved in biochemical processes, the structure of which is more complex (24 carbon atoms). At the same time, saturated acids, having less than 10 atoms in their composition, are practically absent in animal lipids.

The most common atomic set of unsaturated acids are compounds consisting of 18 carbon atoms. The following types of unsaturated acids are considered indispensable, having from 1 to 4 double bonds:

• oleic;
• linoleic;
• linolenic;
• arachidonic.

Prostaglandids and waxes

To a greater or lesser extent, they all possess in the body of mammals. Derivatives of unsaturated acids, which are prostaglandids, are of great importance. Synthesized by all cells and tissues, except for erythrocytes, they have a tremendous effect on the functioning of the main structures and processes of the human body:

• circulatory system and heart;
• metabolism and electrolyte exchange;
• central and peripheral nervous systems;
• digestive organs;
• reproductive function.

In a separate group are esters of complex acids and alcohols with one or two atoms in the chain - waxes. The total number of carbon particles in them can reach 22. Due to the hard texture, these substances are perceived by lipids as protectors. Among the natural waxes synthesized by organisms, the most common are beeswax, lanolin and an element that covers the surface of the leaves.

Complex lipids

Lipid classes are represented by groups of complex compounds. Biochemistry includes:

• phospholipids;
• glycolipids;
• sulfolipids.

Phospholipids are biological constructs with a complex structure. They necessarily include phosphorus, nitrogenous compounds, alcohols and much more. For the body, they play a significant role, being a fundamental component of the construction process of biological membranes. Phospholipids are present in the heart, liver, and brain.

The subclass of complex lipids also includes glycolipids - these are compounds that contain sphingosine alcohol, and hence carbohydrates. More than any other tissue in the body, nerve sheaths are rich in glycolipids.

A variety of glycolipids containing sulfuric acid residues are considered sulfolipids. Meanwhile,
the classification of lipids always implies the allocation of these substances to a separate group. The main difference between the two complex compounds lies in the features of their structure. In place of the galactose of the third carbon atom of the glycolipid, there is a sulfuric acid residue.

Group of unsaponifiable lipids

In contrast to the group of saponifiable lipids, which is impressive in terms of the number of varieties, unsaponifiable lipids completely release fatty acids and do not undergo hydrolysis by alkaline action. These substances are of two types:

• higher alcohols;
• higher hydrocarbons.

The first category includes vitamins that differ in fat-soluble qualities - A, E, D. The most famous representative of the second type of sterols - higher alcohols - is cholesterol. Scientists managed to isolate the element from gallstones by isolating monohydric alcohol several centuries ago.

Cholesterol cannot be found in plants, while in mammals it is present in absolutely all cells. Its presence is an important condition for the full functioning of the digestive, hormonal and genitourinary systems.

When considering higher hydrocarbons, which are also unsaponifiables, it is important to refer to the definition given by biochemistry. These elements are scientifically the components produced by isoprene. The molecular structure of hydrocarbons is based on the combination of isoprene particles.

As a rule, these elements are present in plant cells of especially fragrant species. In addition, the well-known natural rubber - polyterpene - belongs to the group of unsaponifiable higher hydrocarbons.

Lipids are a large class of organic substances with their own special properties and structure. Different groups of complex compounds perform specific functions in the body.

It is known that almost all living organisms are composed of three types of chemicals: carbohydrates, proteins and fats. It is the latter that should be given special attention, because they are the most diverse classes. What are lipid compounds, what is their structure and why are they needed?

Lipids are a large class of chemicals that include compounds such as fats, waxes, and some hormones. Lipids are insoluble in polar solvents (for example, in water), but are readily soluble in organic ones (acetone, chloroform).

What is the structure of most lipids? There are two main types: saponifiable and unsaponifiable fats, which have different structures.

Saponifiable lipids

Saponifiable lipids include complex compounds, the structural parts of which are united by an ether bond. This class of fats is easily hydrolyzed in solution by the action of alkalis.

Saponifiable lipids are a large class of substances consisting of separate groups:

• esters;
• glycolipids;
• phospholipids.

Esters

This group includes:

• fats (composed of glycerol and fatty acids);
• waxes (derivatives of fatty alcohol and acid);
• esters of sterols.

Esters arise from the interaction of an organic acid containing a carboxyl functional group and an alcohol whose properties are associated with a hydroxyl group. The reaction between them leads to the formation of a compound that has an ester bond.

Glycolipids

Among the saponifiable lipids, glycolipids deserve special attention - complex substances, the molecule of which is a combination of a lipid and a carbohydrate. These include:

• cerebrosides;
• gangliosides.

Glycolipids are usually based on a molecule of a special organic alcohol - sphingosine. They also contain a phosphate group, as in phospholipids, but it is no longer a “head”, as it binds to rather long molecules of polymeric carbohydrates. Just like other saponifiable lipids, glycolipids contain organic acids in their composition.

Phospholipids

The group includes the following substances:

• phosphatidic acids;
• phosphatides;
• sphingolipids.

Phospholipids, as the name implies, are related to phosphorus. Indeed, in their structure there is a phosphate functional group (a residue of orthophosphoric acid). In addition to it, the lipids of this group also contain an organic alcohol and one or two organic acids.

Together, these components create something similar to a tadpole: the polar phosphate group interacts well with water, forming a "head", while non-polar organic acids interact poorly with water, and form a kind of "tail". These features of phospholipids just allow them to perform their important functions in the body, which will be discussed a little later.

Unsaponifiable lipids

Lipids incapable of interacting with alkalis constitute a separate group of substances - unsaponifiable lipids. These compounds are long chain alcohols, cyclic alcohols, and also carotenoids.

There is no single classification of unsaponifiable lipids; among all their abundance, several distinct groups can be outlined.

1. Long-chain organic acids (the sequence of carbon atoms is more than 16 atoms, ends with a carboxyl group).
2. Long chain organic alcohols (a long sequence of carbon atoms that ends in a hydroxyl functional group).
3. Eicosanoids (derivatives of fatty acids formed by partial cyclization and the appearance of intramolecular bonds).
4. Cyclic alcohols (polycyclic compounds, which are characterized by a large number of hydroxyl groups).
5. Steroids (derivatives of cyclic alcohols formed by the appearance of additional functional groups).
6. Carotenoids (long carbon chains, often ending in cyclic alkanes).

All of the above substances have their own characteristics, but they are united by some chemical properties. Among them: high molecular weight, poor ability to interact with water, solubility in organic substances, the ability to penetrate biological membranes.

Functions

Lipids in a living organism perform a wide range of tasks. Since these complex substances are fundamentally different in structure, the functionality of each group of fats lies in different areas. Below is a table with the functions that are most often found in nature.

energy function

Lipids are one of the most important sources of energy in the body. The fat molecule, which is mainly used as a reserve, contains much more stored energy than a similarly sized glycogen or starch molecule. Being oxidized in mitochondria to carbon dioxide and water, fats allow the formation of large amounts of ATP (the universal energy carrier in the body).

structural function

Some lipids (phospholipids, sphingolipids) act as a building material for cell membranes. These complex compounds are stacked in a double layer, turning the polar "heads" outward from the "wall", and non-polar "tails" hiding inward. In a similar way, a lipid bilayer is created - the basis of all membrane structures of the cell.

Insulation

Subcutaneous deposits of fatty substances, as well as their deposits around the internal organs, reliably protect the body from hypothermia. In addition, such a shell around the "inhabitants" of the abdominal cavity does not allow their collision.

Protective and lubricating function

It is especially found in nature in birds. Wax, covering the bird's beak, prevents it from drying out and cracking, and feathers impregnated with a fatty substance repel water. These properties of lipids help birds to easily float on the water without soaking their plumage in it, and improve the flow of water around the beak during spearfishing.

Change in membrane fluidity

Biological membranes are complex structures consisting mainly of phospholipids. Including between their molecules, cholesterol shows its properties: it increases the ability of the membrane to fluctuate, thereby improving the mobility of its various sections.

Metabolic regulation

The metabolic pathways of the body are complex and therefore require precise regulation. This function is performed by steroid hormones, which can easily penetrate the cell membrane. Inside, the steroid reacts with the corresponding receptor, causing certain changes in the cell.

Lipids are a large and diverse class of organic compounds, without which the life of any organism would be impossible, because each group of substances has its own unique properties that allow them to perform various functions in the body.

Let us consider the features of the chemical structure and biochemical functions of the most important representatives of unsaponifiable lipids - steroids and terpenes.

Steroids.

Steroids include an extensive class of natural substances, the molecules of which are based on a condensed backbone called sterane. Cholesterol is the most common among numerous biological compounds of a steroid nature.

Cholesterol- monohydric alcohol (cholesterol); it exhibits the properties of a secondary alcohol and an alkene. About 30% of cholesterol in the body is found in free form, the rest is in the composition of acylcholesterols, i.e. esters with higher carboxylic acids, both saturated (palmitic and stearic) and unsaturated (linoleic, arachidonic, etc.), i.e. in the form of acylcholesterols. The total cholesterol content in the human body is 210-250 g. It is found in large quantities in the brain and spinal cord, and is a component of biomembranes.

The most important biochemical function of cholesterol is due to the fact that it plays the role of an intermediate product in the synthesis of many steroid compounds: in the placenta, testes, corpus luteum and adrenal glands, cholesterol is converted into the hormone progesterone, which is the initial substrate of a complex chain of biosynthesis of steroid sex hormones and corticosteroids.

Other ways of using cholesterol in the body are associated with the formation of vitamin D and the bile acids necessary for digestion - cholic and 7-deoxycholic.

In the body, cholic acid, forming amides at the carbonyl group with glycine and taurine, is converted into glycinecholic and taurocholic acids.

The anions of these acids are effective surfactants. In the intestines, they are involved in the emulsification of fats and thus contribute to their absorption and digestion.

Bile acids are used as drugs to prevent the formation and dissolution of existing gallstones, which are made up of cholesterol and bilirubin.

The transport of lipids insoluble in body fluids, including cholesterol, is carried out as part of special particles - lipoproteins, which are complex complexes with proteins.

Several forms of lipoproteins have been found in the blood, which differ in density: chylomicrons, very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL). Lipoproteins can be separated using ultracentrifugation.

Lipoproteins are spherical particles, the hydrophilic surface of which is a layer of oriented phospholipids and proteins, and the core is formed by hydrophobic molecules of triacylglycerols and cholesterol esters.

Triacylglycerols and cholesterol under the action of specific enzymes (lipoprotein lipase) are released from chylomicrons and then consumed by adipose tissue, liver, heart and other organs.

With some metabolic disorders or a high concentration of cholesterol in the blood, the concentration of VLDL and LDL increases, which leads to their deposition on the walls of blood vessels (atherosclerosis), including in the arteries of the heart muscle (ischemic heart disease and myocardial infarction).

Terpenes.

Terpenes are a series of biologically active hydrocarbons and their oxygen-containing derivatives, the carbon skeleton of which consists of several units of isoprene C 5 H 8 . Therefore, the general formula for most terpenes is (C 5 H 8) n. Terpenes can have an acyclic or cyclic (bi-, tri- and polycyclic) structure. Structures of terpenes with the general formula C 10 H 16 - myrcene and limonene:

The composition of essential oils includes derivatives of terpenes containing hydroxyl, aldehyde or keto groups - terpenoids. Among them, menthol (contained in mint oil, from which it got its name, from Latin menta - mint), linalool (a liquid with the smell of lily of the valley), citral, camphor are of great use.

Terpenes include resin acids, which have the general formula C 20 H 30 O 2 and make up 4/5 of the resin of coniferous plants (resin). During the processing of resin, a solid residue of resin acids is obtained - rosin, which serves as a raw material for many industries. In addition, terpene groups (isoprenoid chains) are included in the structure of many complex biologically active compounds, such as carotenoids, phytol, etc.

Phytol is not found in free form in nature, but is part of the molecules of chlorophyll, vitamins A and E and other biocompounds.

Rubber and gutta are polyterpenes in which isoprene residues are linked head-to-tail.

The lipids discussed above are often referred to as saponifiable, since when they are heated, soaps are formed (as a result of the elimination of fatty acids). Cells also contain, albeit in smaller quantities, lipids of another class, which are called unsaponifiable , because they are not hydrolyzed to release fatty acids. There are two main types of unsaponifiable lipids: steroids and terpenes . These chemical compounds belong to two different classes, but they have a number of very similar features, which are due to the fact that they are all built from the same five-carbon building blocks.

Steroids

Steroids are derivatives of the core containing three fused cyclohexane rings. The most common sterol in animal tissues is cholesterol - is found in the body, both in free and esterified form. Crystalline cholesterol is a white, optically active substance that melts at 150°C. It is insoluble in water, but is easily extracted from cells with chloroform, ether, benzene, or hot alcohol.

The plasma membranes of many animal cells are rich in cholesterol. An important intermediate in cholesterol biosynthesis is lanosterol, part of lanolin (sheep wool fat).

Cholesterol has not been found in plants. Plants have other sterols known collectively as phytosterols.

Terpenes

Among the lipid components found in cells in a relatively small amount are terpenes , whose molecules are built by combining several molecules of a five-carbon hydrogen carbon isoprene(2-methyl-1,3-butadiene). Terpenes containing two isoprene groups are called monoterpenes, and containing three such groupings - sesquiterpenes ; terpenes containing 4, 6 and 8 isoprene groups are called respectively diterpenes, tri-terpenes and mempamepenes. Terpene molecules can have a linear or cyclic structure; there are also terpenes, in the molecules of which there are both linear and cyclic components.

A very large number of mono- and sesquiterpenes have been found in plants. Thus, the monoterpenes geraniol, limonene, menthol, pinene, camphor and carvone are the main components of geranium, lemon, mint, turpentine, camphor and caraway oils, respectively. An example of sesquiterpenes is farnesol. Diterpenes include phytol, which is a component of the photosynthetic pigment chlorophyll, as well as vitamin A. Triterpenes include squalene and lanosterol, which play the role of important precursors in the biosynthesis of cholesterol. Other higher terpenes include carotenoids belonging to the group of tetraterpenes.

Lipoproteins

Polar lipids associate with certain specific proteins to form lipoproteins of which the transport lipoproteins present in the blood plasma of mammals are the best known. In such complex proteins, the interactions between the lipid(s) and protein components are carried out without the participation of the shunallunt ones. connections. Lipoproteins usually contain both polar and neutral lipids, as well as cholesterol and its esters. They serve as the form in which lipids are transported from the small intestine to the liver and from the liver to adipose tissue, as well as to various other tissues. Several classes of lipoproteins have been found in blood plasma; the classification of these lipoproteins is based on differences in their density.

SAHARA

Carbohydrates or saccharides are called polyhydroxyaldehydes and polyoxy-ketones with the general formula (CH 2 O) P., as well as derivatives of these compounds. monosaccharides, or simple sugars , consist of one polyhydroxyaldehyde or polyoxyketone unit. The most common monosaccharide is the six-carbon sugar D-glucose; it is the parent monosaccharide from which all other saccharides are derived. D-glucose molecules serve as the main type of cellular fuel in most organisms and act as building blocks, or precursors, of the most abundant polysaccharides.

Oligosaccharides contain from 2 to 10 monosaccharide units connected by a glycosidic bond. molecules polysaccharides are: very long chains built from many monosaccharide units; chains can be either linear or branched. Most polysaccharides contain repeating monosaccharide units of the same species or two alternating species; therefore, they cannot play the role of informational macromolecules.

There are probably more carbohydrates in the biosphere than all other organic compounds combined. This is mainly due to the ubiquity of two D-glucose polymers, namely cellulose and starch, in large quantities. Cellulose is the main extracellular structural component of fibrous and lignified plant tissues. Starch is also found in plants in extremely large quantities; it serves as the main form in which cellular fuel is stored.