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What are lipid substances?

Lipids are one of the groups of organic compounds that are of great importance for living organisms. According to the chemical structure, all lipids are divided into simple and complex. A simple lipid molecule is composed of alcohol and bile acids, while a complex lipid contains other atoms or compounds.

In general, lipids are of great importance for humans. These substances are included in a significant part of food products, are used in medicine and pharmacy, and play an important role in many industries. In a living organism, lipids in one form or another are part of all cells. From a nutritional point of view, it is a very important source of energy.

What is the difference between lipids and fats?

In principle, the term "lipids" comes from the Greek root meaning "fat", however, these definitions still have some differences. Lipids are a broader group of substances, while only certain types of lipids are understood as fats. A synonym for "fats" are "triglycerides", which are obtained from the combination of glycerol alcohol and carboxylic acids. Both lipids in general and triglycerides in particular play a significant role in biological processes.

Lipids in the human body

Lipids are part of almost all tissues of the body. Their molecules are in any living cell, and life is simply impossible without these substances. There are many different lipids found in the human body. Each type or class of these compounds has its own functions. Many biological processes depend on the normal intake and formation of lipids.

From the point of view of biochemistry, lipids are involved in the following important processes:

• body's production of energy;

• cell division;
• transmission of nerve impulses;
• the formation of blood components, hormones and other important substances;
• protection and fixation of some internal organs;
• cell division, respiration, etc.

Thus, lipids are vital chemical compounds. A significant part of these substances enters the body with food. After that, the structural components of lipids are absorbed by the body, and cells produce new lipid molecules.

The biological role of lipids in a living cell

Lipid molecules perform a huge number of functions not only on the scale of the whole organism, but also in each living cell individually. In fact, a cell is a structural unit of a living organism. It is the assimilation and synthesis ( education) of certain substances. Some of these substances are used to maintain the life of the cell itself, some - for cell division, some - for the needs of other cells and tissues.

In a living organism, lipids perform the following functions:

• energy;
• reserve;
• structural;
• transport;
• enzymatic;
• storage;
• signal;
• regulatory.

energy function

The energy function of lipids is reduced to their breakdown in the body, during which a large amount of energy is released. Living cells need this energy to maintain various processes ( respiration, growth, division, synthesis of new substances). Lipids enter the cell with blood flow and are deposited inside ( in the cytoplasm) in the form of small drops of fat. If necessary, these molecules are broken down, and the cell receives energy.

Reserve ( storage) function

The reserve function is closely related to the energy one. In the form of fats inside cells, energy can be stored "in reserve" and released as needed. Special cells, adipocytes, are responsible for the accumulation of fats. Most of their volume is occupied by a large drop of fat. It is from adipocytes that adipose tissue in the body consists. The largest reserves of adipose tissue are in the subcutaneous fat, the greater and lesser omentum ( in the abdominal cavity). With prolonged starvation, adipose tissue gradually disintegrates, since lipid reserves are used for energy.

Also, adipose tissue deposited in the subcutaneous fat provides thermal insulation. Tissues rich in lipids generally conduct heat worse. This allows the body to maintain a constant body temperature and not so quickly cool or overheat in various environmental conditions.

Structural and barrier functions ( membrane lipids)

Lipids play an important role in the structure of living cells. In the human body, these substances form a special double layer that forms the cell wall. Thanks to this, a living cell can perform its functions and regulate the metabolism with the external environment. The lipids that make up the cell membrane also help keep the shape of the cell.

Why do lipid monomers form a double layer ( bilayer)?

Monomers are chemical substances ( in this case, molecules), which are able, when combined, to form more complex compounds. The cell wall consists of a double layer ( bilayer) lipids. Each molecule that forms this wall has two parts - hydrophobic ( not in contact with water) and hydrophilic ( in contact with water). The double layer is obtained due to the fact that lipid molecules are deployed by hydrophilic parts inside the cell and outward. The hydrophobic parts are practically in contact, as they are located between the two layers. Other molecules can also be located in the thickness of the lipid bilayer ( proteins, carbohydrates, complex molecular structures), which regulate the passage of substances through the cell wall.

transport function

The transport function of lipids is of secondary importance in the body. It is performed only by some connections. For example, lipoproteins, consisting of lipids and proteins, carry certain substances in the blood from one organ to another. However, this function is rarely distinguished, not considering it the main one for these substances.

Enzymatic function

In principle, lipids are not part of the enzymes involved in the breakdown of other substances. However, without lipids, organ cells will not be able to synthesize enzymes, the end product of life. In addition, certain lipids play a significant role in the absorption of dietary fats. Bile contains significant amounts of phospholipids and cholesterol. They neutralize excess pancreatic enzymes and prevent them from damaging intestinal cells. It also dissolves in bile emulsification) exogenous lipids from food. Thus, lipids play a huge role in digestion and help in the work of other enzymes, although they are not enzymes themselves.

Signal function

Part of the complex lipids performs a signaling function in the body. It consists in maintaining various processes. For example, glycolipids in nerve cells are involved in the transmission of a nerve impulse from one nerve cell to another. Besides, great importance have signals within the cell itself. She needs to "recognize" the substances coming from the blood in order to transport them inside.

Regulatory function

The regulatory function of lipids in the body is secondary. Blood lipids themselves have little effect on the course of various processes. However, they are part of other substances that are of great importance in the regulation of these processes. First of all, these are steroid hormones ( adrenal and sex hormones). They play an important role in metabolism, growth and development of the body, reproductive function, and affect the functioning of the immune system. Lipids are also part of prostaglandins. These substances are produced during inflammatory processes and affect some processes in nervous system (e.g. perception of pain).

Thus, lipids themselves do not perform a regulatory function, but their deficiency can affect many processes in the body.

Biochemistry of lipids and their relationship with other substances ( proteins, carbohydrates, ATP, nucleic acids, amino acids, steroids)

Lipid metabolism is closely related to the metabolism of other substances in the body. First of all, this connection can be traced in human nutrition. Any food consists of proteins, carbohydrates and lipids, which must be ingested in certain proportions. In this case, a person will receive both enough energy and enough structural elements. Otherwise ( for example, with a lack of lipids) proteins and carbohydrates will be broken down to produce energy.

Lipids are also to some extent associated with the metabolism of the following substances:

• Adenosine triphosphoric acid ( ATP). ATP is a kind of unit of energy within the cell. When lipids are broken down, part of the energy goes to the production of ATP molecules, and these molecules take part in all intracellular processes ( transport of substances, cell division, neutralization of toxins, etc.).
• Nucleic acids. Nucleic acids are the building blocks of DNA and are found in the nuclei of living cells. The energy generated during the breakdown of fats goes partly into cell division. During division, new strands of DNA are formed from nucleic acids.
• Amino acids. Amino acids are the structural components of proteins. In combination with lipids, they form complex complexes, lipoproteins, which are responsible for the transport of substances in the body.
• Steroids. Steroids are a type of hormone containing a significant amount of lipids. With poor absorption of lipids from food, the patient may begin problems with the endocrine system.

Thus, the metabolism of lipids in the body, in any case, must be considered in combination, from the point of view of the relationship with other substances.

Digestion and absorption of lipids ( metabolism, metabolism)

Digestion and absorption of lipids is the first step in the metabolism of these substances. The main part of lipids enters the body with food. In the oral cavity, food is crushed and mixed with saliva. Next, the lump enters the stomach, where the chemical bonds are partially destroyed by the action of hydrochloric acid. Also, some chemical bonds in lipids are destroyed by the action of the enzyme lipase, contained in saliva.

Lipids are insoluble in water, so they are not immediately digested by enzymes in the duodenum. First, the so-called emulsification of fats occurs. After that, chemical bonds are cleaved under the action of lipase coming from the pancreas. In principle, for each type of lipid, its own enzyme is now defined, which is responsible for the breakdown and assimilation of this substance. For example, phospholipase breaks down phospholipids, cholesterol esterase breaks down cholesterol compounds, etc. All these enzymes are contained in pancreatic juice in one quantity or another.

The split fragments of lipids are individually absorbed by the cells of the small intestine. In general, the digestion of fats is a very complex process, which is regulated by many hormones and hormone-like substances.

What is lipid emulsification?

Emulsification is the incomplete dissolution of fatty substances in water. In the food bolus that enters the duodenum, fats are contained in the form of large drops. This prevents their interaction with enzymes. In the process of emulsification, large fat droplets are "crushed" into smaller droplets. As a result, the area of ​​contact between the fat droplets and the surrounding water-soluble substances increases, and the breakdown of lipids becomes possible.

The process of lipid emulsification in the digestive system takes place in several stages:

• At the first stage, the liver produces bile, which will emulsify fats. It contains salts of cholesterol and phospholipids, which interact with lipids and contribute to their "crushing" into small drops.
• Bile secreted from the liver accumulates in the gallbladder. Here it is concentrated and released as needed.
• When fatty foods are consumed, the smooth muscles of the gallbladder receive a signal to contract. As a result, a portion of bile is secreted through the bile ducts into the duodenum.
• In the duodenum, fats are actually emulsified and interact with pancreatic enzymes. The contractions of the walls of the small intestine contribute to this process by "mixing" the contents.

Some people may have trouble absorbing fats after having their gallbladder removed. Bile enters the duodenum continuously, directly from the liver, and is not enough to emulsify all the lipids if too much is eaten.

Enzymes for splitting lipids

For the digestion of each substance in the body there are enzymes. Their task is to break chemical bonds between molecules ( or between atoms in molecules) so that nutrients can be properly absorbed by the body. Different enzymes are responsible for the breakdown of different lipids. Most of them are found in the juice secreted by the pancreas.

The following groups of enzymes are responsible for the breakdown of lipids:

• lipases;
• phospholipases;
• cholesterol esterase, etc.

What vitamins and hormones are involved in lipid regulation?

The level of most lipids in human blood is relatively constant. It can fluctuate within certain limits. It depends on the biological processes occurring in the body itself, and on a number of external factors. The regulation of blood lipid levels is a complex biological process involving many different organs and substances.

The following substances play the greatest role in the assimilation and maintenance of a constant level of lipids:

• Enzymes. A number of pancreatic enzymes are involved in the breakdown of lipids that enter the body with food. With a lack of these enzymes, the level of lipids in the blood may decrease, since these substances simply will not be absorbed in the intestines.
• Bile acids and their salts. Bile contains bile acids and a number of their compounds, which contribute to the emulsification of lipids. Without these substances, normal absorption of lipids is also impossible.
• Vitamins. Vitamins have a complex strengthening effect on the body and directly or indirectly also affect lipid metabolism. For example, with a lack of vitamin A, cell regeneration in the mucous membranes deteriorates, and the digestion of substances in the intestine also slows down.
• intracellular enzymes. The cells of the intestinal epithelium contain enzymes that, after absorption of fatty acids, convert them into transport forms and direct them into the bloodstream.
• Hormones. A number of hormones affect the metabolism in general. For example, high insulin levels can greatly affect blood lipid levels. That is why for patients with diabetes, some norms have been revised. Thyroid hormones, glucocorticoid hormones, or norepinephrine can stimulate the breakdown of adipose tissue to release energy.

Thus, maintaining a normal level of lipids in the blood is a very complex process, which is directly or indirectly affected by various hormones, vitamins and other substances. In the process of diagnosis, the doctor needs to determine at what stage this process was violated.

Biosynthesis ( education) and hydrolysis ( decay) lipids in the body ( anabolism and catabolism)

Metabolism is the totality of metabolic processes in the body. All metabolic processes can be divided into catabolic and anabolic. Catabolic processes include the breakdown and breakdown of substances. With respect to lipids, this is characterized by their hydrolysis ( break down into simpler substances) in the gastrointestinal tract. Anabolism combines biochemical reactions aimed at the formation of new, more complex substances.

Lipid biosynthesis occurs in the following tissues and cells:

• Cells of the intestinal epithelium. Absorption of fatty acids, cholesterol and other lipids occurs in the intestinal wall. Immediately after this, new, transport forms of lipids are formed in the same cells, which enter the venous blood and are sent to the liver.
• Liver cells. In the liver cells, some of the transport forms of lipids will break down, and new substances are synthesized from them. For example, cholesterol compounds and phospholipids are formed here, which are then excreted in the bile and contribute to normal digestion.
• Cells of other organs. Part of the lipids enters with the blood into other organs and tissues. Depending on the type of cells, lipids are converted into certain types of compounds. All cells, one way or another, synthesize lipids to form a cell wall ( lipid bilayer). In the adrenal glands and gonads, steroid hormones are synthesized from a part of lipids.

The combination of the above processes is the lipid metabolism in the human body.

Resynthesis of lipids in the liver and other organs

Resynthesis is the process of formation of certain substances from simpler ones that were assimilated earlier. In the body, this process takes place in the internal environment of some cells. Resynthesis is necessary in order for tissues and organs to receive all the necessary types of lipids, and not just those that were consumed with food. Resynthesized lipids are called endogenous. For their formation, the body expends energy.

At the first stage, lipid resynthesis occurs in the intestinal walls. Here, the fatty acids that come with food are converted into transport forms that will go with the blood to the liver and other organs. Part of the resynthesized lipids will be delivered to the tissues, while the other part will form the substances necessary for vital activity ( lipoproteins, bile, hormones, etc.), the excess is converted into adipose tissue and stored "in reserve".

Are lipids part of the brain?

Lipids are a very important component of nerve cells not only in the brain, but throughout the nervous system. As you know, nerve cells control various processes in the body by transmitting nerve impulses. At the same time, all nerve pathways are “isolated” from each other so that the impulse comes to certain cells and does not affect other nerve pathways. This "isolation" is possible due to the myelin sheath of nerve cells. Myelin, which prevents the chaotic propagation of impulses, is approximately 75% lipid. As in cell membranes ah, here they form a double layer ( bilayer), which is wrapped several times around the nerve cell.

The composition of the myelin sheath in the nervous system includes the following lipids:

• phospholipids;
• cholesterol;
• galactolipids;
• glycolipids.

Neurological problems are possible in some congenital disorders of lipid formation. This is due precisely to the thinning or interruption of the myelin sheath.

lipid hormones

Lipids play an important structural role, including being present in the structure of many hormones. Hormones that contain fatty acids are called steroid hormones. In the body, they are produced by the gonads and adrenal glands. Some of them are also present in adipose tissue cells. Steroid hormones are involved in the regulation of many vital processes. Their imbalance can affect body weight, the ability to conceive a child, the development of any inflammatory processes, and the functioning of the immune system. The key to normal production of steroid hormones is a balanced intake of lipids.

Lipids are part of the following vital hormones:

• corticosteroids ( cortisol, aldosterone, hydrocortisone, etc.);
• male sex hormones - androgens ( androstenedione, dihydrotestosterone, etc.);
• female sex hormones - estrogen estriol, estradiol, etc.).

Thus, the lack of certain fatty acids in food can seriously affect the functioning of the endocrine system.

The role of lipids for skin and hair

Lipids are of great importance for the health of the skin and its appendages ( hair and nails). The skin contains the so-called sebaceous glands, which secrete a certain amount of secretion rich in fats to the surface. This substance performs many useful functions.

For hair and skin, lipids are important for the following reasons:

• a significant part of the substance of the hair consists of complex lipids;
• skin cells are rapidly changing, and lipids are important as an energy resource;
• secret ( excreted substance a) sebaceous glands moisturizes the skin;
• thanks to fats, elasticity, elasticity and smoothness of the skin are maintained;
• a small amount of lipids on the surface of the hair give them a healthy shine;
• lipid layer on the surface of the skin protects it from the aggressive effects of external factors ( cold, sun rays, microbes on the surface of the skin, etc.).

In skin cells, as well as in hair follicles, lipids come with blood. Thus, normal nutrition ensures healthy skin and hair. Use of shampoos and creams containing lipids ( especially essential fatty acids) is also important, because some of these substances will be absorbed from the surface of the cells.

Lipid classification

In biology and chemistry, there are quite a few different classifications of lipids. The main one is chemical classification, according to which lipids are divided depending on their structure. From this point of view, all lipids can be divided into simple ( consisting only of oxygen, hydrogen and carbon atoms) and complex ( containing at least one atom of other elements). Each of these groups has corresponding subgroups. This classification is the most convenient, since it reflects not only the chemical structure of substances, but also partially determines the chemical properties.

Biology and medicine have their own additional classifications using other criteria.

Exogenous and endogenous lipids

All lipids in the human body can be divided into two large groups - exogenous and endogenous. The first group includes all substances that enter the body from the external environment. The greatest amount of exogenous lipids enters the body with food, but there are other ways. For example, when using various cosmetics or drugs, the body can also receive some lipids. Their action will be predominantly local.

After entering the body, all exogenous lipids are broken down and absorbed by living cells. Here, from their structural components, other lipid compounds that the body needs will be formed. These lipids, synthesized by one's own cells, are called endogenous. They may have a completely different structure and function, but they consist of the same "structural components" that entered the body with exogenous lipids. That is why, with a lack of certain types of fats in food, various diseases can develop. Part of the components of complex lipids cannot be synthesized by the body on its own, which affects the course of certain biological processes.

Fatty acid

Fatty acids are a class of organic compounds that are the structural part of lipids. Depending on which fatty acids are included in the composition of the lipid, the properties of this substance may change. For example, triglycerides, the most important source of energy for the human body, are derivatives of the alcohol glycerol and several fatty acids.

In nature, fatty acids are found in a variety of substances - from oil to vegetable oils. They enter the human body mainly with food. Each acid is a structural component for certain cells, enzymes or compounds. After absorption, the body converts it and uses it in various biological processes.

The most important sources of fatty acids for humans are:

• animal fats;
• vegetable fats;
• tropical oils ( citrus, palm, etc.);
• fats for the food industry margarine, etc.).

In the human body, fatty acids can be stored in adipose tissue as triglycerides or circulate in the blood. They are found in the blood both in the free form and in the form of compounds ( various fractions of lipoproteins).

Saturated and unsaturated fatty acids

All fatty acids are divided into saturated and unsaturated according to their chemical structure. Saturated acids are less beneficial to the body, and some of them are even harmful. This is due to the fact that there are no double bonds in the molecule of these substances. These are chemically stable compounds, and they are less absorbed by the body. Some saturated fatty acids have now been shown to be associated with the development of atherosclerosis.

Unsaturated fatty acids are divided into two large groups:

• Monounsaturated. These acids have one double bond in their structure and are thus more active. It is believed that eating them can lower cholesterol levels and prevent the development of atherosclerosis. The largest amount of monounsaturated fatty acids is found in a number of plants ( avocado, olives, pistachios, hazelnuts) and, accordingly, in the oils obtained from these plants.
• Polyunsaturated. Polyunsaturated fatty acids have several double bonds in their structure. Distinctive feature of these substances is that the human body is not able to synthesize them. In other words, if polyunsaturated fatty acids are not supplied to the body with food, over time this will inevitably lead to certain disorders. The best sources of these acids are seafood, soybean and linseed oils, sesame seeds, poppy seeds, wheat germ, etc.

Phospholipids

Phospholipids are complex lipids containing a phosphoric acid residue in their composition. These substances, along with cholesterol, are the main component of cell membranes. Also, these substances are involved in the transport of other lipids in the body. From a medical point of view, phospholipids can also play a signaling role. For example, they are part of bile, as they contribute to emulsification ( dissolution) other fats. Depending on which substance is more in bile, cholesterol or phospholipids, it is possible to determine the risk of developing cholelithiasis.

Glycerin and triglycerides

Chemically, glycerol is not a lipid, but it is an important structural component of triglycerides. This is a group of lipids that play a huge role in the human body. The most important function of these substances is the supply of energy. Triglycerides that enter the body with food are broken down into glycerol and fatty acids. As a result, a very large amount of energy is released, which goes to the work of the muscles ( skeletal muscles, heart muscles, etc.).

Adipose tissue in the human body is represented mainly by triglycerides. Most of these substances, before being deposited in adipose tissue, undergo some chemical transformations in the liver.

Beta lipids

Beta lipids are sometimes referred to as beta lipoproteins. The duality of the name is explained by differences in classifications. This is one of the fractions of lipoproteins in the body, which plays an important role in the development of certain pathologies. First of all, we are talking about atherosclerosis. Beta-lipoproteins transport cholesterol from one cell to another, but due to the structural features of the molecules, this cholesterol often "gets stuck" in the walls of blood vessels, forming atherosclerotic plaques and preventing normal blood flow. Before use, you should consult with a specialist.

LIPIDS - this is a heterogeneous group of natural compounds, completely or almost completely insoluble in water, but soluble in organic solvents and in each other, giving high molecular weight fatty acids upon hydrolysis.

In a living organism, lipids perform a variety of functions.

Biological functions of lipids:

1) Structural

Structural lipids form complex complexes with proteins and carbohydrates, from which cell membranes and cell structures are built, and participate in various processes occurring in the cell.

2) Spare (energy)

Spare lipids (mainly fats) are the energy reserve of the body and are involved in metabolic processes. In plants, they accumulate mainly in fruits and seeds, in animals and fish - in subcutaneous adipose tissues and tissues surrounding internal organs, as well as liver, brain and nervous tissues. Their content depends on many factors (type, age, nutrition, etc.) and in some cases is 95-97% of all lipids released.

Calorie content of carbohydrates and proteins: ~ 4 kcal / gram.

Calorie content of fat: ~ 9 kcal / gram.

The advantage of fat as an energy reserve, unlike carbohydrates, is hydrophobicity - it is not associated with water. This ensures the compactness of fat reserves - they are stored in an anhydrous form, occupying a small volume. On average, a person has a supply of pure triacylglycerols of approximately 13 kg. These stocks could be enough for 40 days of fasting in conditions of moderate physical activity. For comparison: the total glycogen stores in the body are approximately 400 g; during starvation, this amount is not enough even for one day.

3) Protective

Subcutaneous fatty tissues protect animals from cooling, and internal organs from mechanical damage.

The formation of fat reserves in the human body and some animals is considered as an adaptation to an irregular diet and to living in a cold environment. A particularly large supply of fat is in animals falling into long hibernation (bears, marmots) and adapted to living in cold conditions (walruses, seals). The fetus has practically no fat, and appears only before birth.

A special group in terms of their functions in a living organism is made up of protective plant lipids - waxes and their derivatives, covering the surface of leaves, seeds and fruits.

4) An important component of food raw materials

Lipids are an important component of food, largely determining its nutritional value and palatability. The role of lipids in various processes of food technology is exceptionally great. Damage to grain and products of its processing during storage (rancidity) is primarily associated with a change in its lipid complex. Lipids isolated from a number of plants and animals are the main raw materials for obtaining the most important food and technical products (vegetable oil, animal fats, including butter, margarine, glycerin, fatty acids, etc.).

2 Lipid classification

There is no generally accepted classification of lipids.

It is most expedient to classify lipids depending on their chemical nature, biological functions, and also in relation to some reagents, for example, alkalis.

According to their chemical composition, lipids are usually divided into two groups: simple and complex.

Simple lipids - Esters of fatty acids and alcohols. These include fats , waxes And steroids .

Fats - esters of glycerol and higher fatty acids.

Waxes - esters of higher alcohols of the aliphatic series (with a long carbohydrate chain of 16-30 C atoms) and higher fatty acids.

Steroids - esters of polycyclic alcohols and higher fatty acids.

Complex lipids - in addition to fatty acids and alcohols, they contain other components of various chemical nature. These include phospholipids and glycolipids .

Phospholipids - these are complex lipids in which one of the alcohol groups is associated not with fatty acids, but with phosphoric acid (phosphoric acid can be combined with an additional compound). Depending on which alcohol is included in the composition of phospholipids, they are divided into glycerophospholipids (containing glycerol alcohol) and sphingophospholipids (containing sphingosine alcohol).

Glycolipids - these are complex lipids in which one of the alcohol groups is associated not with fatty acids, but with a carbohydrate component. Depending on which carbohydrate component is included in the composition of glycolipids, they are divided into cerebrosides (they contain any monosaccharide, disaccharide or a small neutral homooligosaccharide as a carbohydrate component) and gangliosides (they contain acidic heterooligosaccharide as a carbohydrate component).

Sometimes in an independent group of lipids ( minor lipids ) secrete fat-soluble pigments, sterols, fat-soluble vitamins. Some of these compounds can be classified as simple (neutral) lipids, while others are complex.

According to another classification, lipids, depending on their relationship to alkalis, are divided into two large groups: saponifiable and unsaponifiable.. The group of saponifiable lipids includes simple and complex lipids, which, when interacting with alkalis, are hydrolyzed to form salts of macromolecular acids, called "soaps". The group of unsaponifiable lipids includes compounds that are not subject to alkaline hydrolysis (sterols, fat-soluble vitamins, ethers, etc.).

According to their functions in a living organism, lipids are divided into structural, reserve and protective.

Structural lipids are mainly phospholipids.

Spare lipids are mainly fats.

Protective lipids of plants - waxes and their derivatives, covering the surface of leaves, seeds and fruits, animals - fats.

FATS

The chemical name for fats is acylglycerols. These are esters of glycerol and higher fatty acids. "Acyl-" means "fatty acid residue".

Depending on the number of acyl radicals, fats are divided into mono-, di- and triglycerides. If the molecule contains 1 fatty acid radical, then the fat is called MONOACYLGLYCEROL. If there are 2 fatty acid radicals in the molecule, then the fat is called DIACYLGLYCERIN. Triacylglycerols predominate in humans and animals (they contain three fatty acid radicals).

The three hydroxyls of glycerol can be esterified with either only one acid, such as palmitic or oleic, or with two or three different acids:

Natural fats contain mainly mixed triglycerides, including residues of various acids.

Since the alcohol in all natural fats is the same - glycerol, the differences observed between fats are due solely to the composition of fatty acids.

Over four hundred carboxylic acids of various structures have been found in fats. However, most of them are present only in small quantities.

The acids contained in natural fats are monocarboxylic, built from unbranched carbon chains containing an even number of carbon atoms. Acids containing an odd number of carbon atoms, having a branched carbon chain, or containing cyclic fragments are present in minor amounts. The exceptions are isovaleric acid and a number of cyclic acids found in some very rare fats.

The most common fatty acids contain between 12 and 18 carbon atoms and are often referred to as fatty acids. The composition of many fats includes low molecular weight acids (C 2 -C 10) in a small amount. Acids with more than 24 carbon atoms are present in waxes.

The glycerides of the most common fats contain a significant amount of unsaturated acids containing 1-3 double bonds: oleic, linoleic and linolenic. Animal fats contain arachidonic acid containing four double bonds; acids with five, six or more double bonds have been found in fish and marine animal fats. Most unsaturated lipid acids have a cis-configuration, their double bonds are isolated or separated by a methylene (-CH 2 -) group.

Of all the unsaturated acids found in natural fats, oleic acid is the most common. In very many fats, oleic acid makes up more than half of the total mass of acids, and only a few fats contain less than 10%. Two other unsaturated acids - linoleic and linolenic - are also very widespread, although they are present in much smaller quantities than oleic acid. Significant amounts of linoleic and linolenic acids are found in vegetable oils; for animal organisms, they are essential acids.

Of the saturated acids, palmitic acid is almost as widespread as oleic acid. It is present in all fats, with some containing 15-50% of the total acid content. Stearic and myristic acids are widely distributed. Stearic acid is found in large quantities (25% or more) only in the reserve fats of some mammals (for example, in sheep fat) and in the fats of some tropical plants, for example, in cocoa butter.

It is advisable to divide the acids contained in fats into two categories: major and minor acids. The main acids of fat are considered to be acids, the content of which in fat exceeds 10%.

Physical properties of fats

As a rule, fats do not withstand distillation and decompose, even if they are distilled under reduced pressure.

The melting point, and, accordingly, the consistency of fats depend on the structure of the acids that make up their composition. Solid fats, i.e., fats that melt at a relatively high temperature, consist mainly of glycerides of saturated acids (stearic, palmitic), and oils that melt at a lower temperature and are thick liquids contain significant amounts of glycerides of unsaturated acids (oleic , linoleic, linolenic).

Since natural fats are complex mixtures of mixed glycerides, they do not melt at a certain temperature, but in a certain temperature range, and they are first softened. To characterize fats, it is usually used solidification temperature, which does not coincide with the melting point - it is somewhat lower. Some natural fats are solids; others are liquids (oils). The solidification temperature varies widely: -27 ° C for linseed oil, -18 ° C for sunflower oil, 19-24 ° C for cow fat and 30-38 ° C for beef fat.

The solidification temperature of fat is determined by the nature of its constituent acids: it is the higher, the greater the content of saturated acids.

Fats dissolve in ether, polyhalogen derivatives, carbon disulfide, aromatic hydrocarbons (benzene, toluene) and gasoline. Solid fats are hardly soluble in petroleum ether; insoluble in cold alcohol. Fats are insoluble in water, but they can form emulsions which are stabilized in the presence of surfactants (emulsifiers) such as proteins, soaps and some sulfonic acids, especially in slightly alkaline media. Milk is a natural emulsion of fat stabilized by proteins.

Chemical properties of fats

Fats enter into all chemical reactions characteristic of esters, however, in their chemical behavior there are a number of features associated with the structure of fatty acids and glycerol.

Among the chemical reactions involving fats, several types of transformations are distinguished.

Lipids- These are fat-like organic compounds that are insoluble in water, but readily 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 trihydric alcohol glycerol and ester bonds of three molecules of higher carboxylic acids attached to it. According to the accepted nomenclature, fats are called triacylglycerols.

Carbon atoms in the molecules of higher carboxylic acids can be connected to each other by both single and double bonds. Of the limiting (saturated) higher carboxylic acids, most often in the composition of fats are palmitic, stearic, arachidic; from unsaturated (unsaturated) - oleic and linoleic.

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

Fats with short and unsaturated acid chains have a low melting point. At room temperature, these are liquids (oils) or greasy substances (fats). Conversely, fats with long and saturated chains of higher carboxylic acids become solid at room temperature. That's why hydrogenation (saturation of acid chains with hydrogen atoms in double bonds) liquid peanut butter, for example, becomes greasy, and sunflower oil turns into solid margarine. Compared with the inhabitants of the southern latitudes, the body of animals living in a cold climate (for example, fish in the Arctic seas) usually contains more unsaturated triacylglycerols. For this reason, their body remains flexible even at low temperatures.

IN phospholipids one of the extreme chains of higher carboxylic acids of triacylglycerol is replaced by a group containing phosphate. Phospholipids have polar heads and non-polar tails. The groups forming the polar head are hydrophilic, while the non-polar tail groups are hydrophobic. The dual nature of these lipids determines their key role in the organization biological membranes.

Another group of lipids are steroids (sterols). These substances are built on the basis of cholesterol alcohol. Sterols are poorly soluble in water and do not contain higher carboxylic acids. These include bile acids, cholesterol, sex hormones, vitamin D, etc.

Lipids are also terpenes(growth substances of plants - gibberellins; carotenoids - photosynthetic pigments; essential oils of plants, as well as waxes).

Lipids can form complexes with other biological molecules such as proteins and sugars.

Functions of lipids the following:

1. Structural. Phospholipids together with proteins form biological membranes. The membranes also contain sterols.
2. Energy. When fats are oxidized, a large amount of energy is released, which goes to the formation of ATP. In the form of lipids, a significant part of the body's energy reserves are stored, which are consumed when there is a lack of nutrients. Hibernating animals and plants accumulate fats and oils and use them to maintain life processes. The high content of lipids in plant seeds ensures the development of the embryo and seedling before their transition to independent nutrition. Seeds of many plants (coconut palm, castor bean, sunflower, soybean, rapeseed, etc.) serve as raw materials for industrial production of vegetable oil.
3. Protective and heat-insulating. Accumulating in the subcutaneous tissue and around certain organs (kidneys, intestines), the fat layer protects the animal body and its individual organs from mechanical damage. In addition, due to its low thermal conductivity, the layer of subcutaneous fat helps to retain heat, which allows, for example, many animals to live in cold climates. In whales, in addition, it plays another role - it contributes to buoyancy.
4. Lubricating and water repellent. Wax coats the skin, wool, feathers, makes them more elastic and protects them from moisture. Leaves and fruits of many plants have a wax coating.
5. Regulatory. Many hormones are derivatives of cholesterol, such as sex hormones (testosterone at men and progesterone in women) and corticosteroids (aldosterone). Derivatives of cholesterol, vitamin D play a key role in the exchange of calcium and phosphorus. Bile acids are involved in the processes of digestion (emulsification of fats) and absorption of higher carboxylic acids.

Lipids are also a source of metabolic water formation. Oxidation of 100 g of fat gives approximately 105 g of water. This water is very important for some desert dwellers, in particular for camels, who can go without water for 10-12 days: the fat stored in the hump is used precisely for this purpose. Bears, marmots and other hibernating animals receive the water necessary for life as a result of fat oxidation.

In the myelin sheaths of axons of nerve cells, lipids are insulators during the conduction of nerve impulses.

Wax is used by bees in building honeycombs.

A source : ON THE. Lemeza L.V. Kamlyuk N.D. Lisov "Biology manual for applicants to universities"

Composition, properties and functions of lipids in the body

Nutritional value of oils and fats used in the baking and confectionery industries.

cyclic lipids. Role in food technology and the life 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 features:

a) insolubility in water (hydrophobicity and good solubility in organic solvents, gasoline, diethyl ether, chloroform, etc.);

b) the presence in their molecules of long-chain hydrocarbon radicals and esters

groupings().

Most lipids are not macromolecular compounds and consist of several molecules linked to one another. Lipids can include alcohols and linear chains of a number of carboxylic acids. In some cases, their individual blocks may consist of macromolecular acids, various phosphoric acid residues, carbohydrates, nitrogenous bases, and other components.

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

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. The mixture extracted from natural objects, consisting of lipids and compounds soluble in them, was called "raw" fat.

Main components of crude fat

Substances associated with lipids play an important role in food technology, affect the nutritional and physiological value of the resulting food products. Vegetative parts of plants accumulate no more than 5% of lipids, mainly in seeds and fruits. For example, the lipid content in various plant products is (g / 100g): sunflower 33-57, cocoa (beans) 49-57, soybeans 14-25, hemp 30-38, wheat 1.9-2.9, peanuts 54- 61, rye 2.1-2.8, flax 27-47, corn 4.8-5.9, coconut palm 65-72. The content of lipids in them depends not only on the individual characteristics of plants, but also on the variety, place, and growing conditions. Lipids play an important role in the life processes of the body.

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

Lipids are part of all structural elements of the cell and, first of all, cell membranes, affecting their permeability. They are involved in the transmission of a nerve impulse, provide intercellular contact, active transfer of nutrients through membranes, transport of fats in blood plasma, protein synthesis and various enzymatic processes.

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

Storage lipids are storage substances that help the body to tolerate adverse effects external environment. Most plants (up to 90%) contain storage lipids, mainly in seeds. They are easily extracted from fat-containing material (free lipids).

Structural lipids (primarily phospholipids) form complex complexes with proteins and carbohydrates. They are involved in a variety of complex processes occurring in the cell. By weight, they constitute a significantly smaller group of lipids (3-5% in oilseeds). These are hard-to-remove “bound” lipids.

Natural fatty acids, which are part of lipids, animals and plants, have many common properties. They contain, as a rule, a clear number of carbon atoms and have an unbranched chain. Fatty acids are conventionally divided into three groups: saturated, monounsaturated and polyunsaturated. Unsaturated fatty acids of animals and humans usually contain a double bond between the ninth and tenth carbon atoms, the remaining carboxylic acids that make up fats are as follows:

Most lipids share some common structural features, but there is no strict classification of lipids yet. One of the approaches to the classification of lipids is chemical, according to which derivatives of alcohols and higher fatty acids belong to lipids.

Lipid classification scheme.

simple lipids. Simple lipids are represented by two-component substances, esters of higher fatty acids with glycerol, higher or polycyclic alcohols.

These include fats and waxes. The most important representatives of simple lipids are acylglycerides (glycerols). They make up the bulk of lipids (95-96%) and they are called oils and fats. The composition of fats includes mainly triglycerides, but there are mono- and diacylglycerols:

The properties of specific oils are determined by the composition of the fatty acids involved in the construction of their molecules and the position occupied by the residues of these acids in the molecules of oils and fats.

Up to 300 carboxylic acids of various structures have been found in fats and oils. However, most of them are present in small quantities.

Stearic and palmitic acids are part of almost all natural oils and fats. Erucic acid is found in rapeseed oil. Most of the most common oils contain unsaturated acids containing 1-3 double bonds. Some acids in natural oils and fats are usually in the cis configuration, i.e. substituents are distributed on one side of the plane of the double bond.

Branched carbohydrate acids containing hydroxy, keto and other groups are usually found in small amounts in lipids. The exception is racinoleic acid in castor oil. In natural plant triacylglycerols, positions 1 and 3 are preferably occupied by saturated fatty acid residues, and position 2 is unsaturated. In animal fats, the picture is reversed.

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

Acylglycerols are liquids or solids with low melting points and fairly high boiling points, high viscosity, colorless and odorless, lighter than water, non-volatile.

Fats are practically insoluble in water, but form emulsions with it.

In addition to the usual physical indicators, fats are characterized by a number of physicochemical constants. These constants for each type of fat and its grade are provided by the standard.

The acid number, or acidity index, indicates how much free fatty acids are in a fat. It is expressed as the number of mg of KOH required to neutralize free fatty acids in 1 g of fat. The acid number is an indicator of the freshness of the fat. On average, it varies for different types of fat from 0.4 to 6.

The saponification number, or saponification coefficient, determines the total amount of acids, both free and bound in triacylglycerols, found in 1 g of fat. Fats containing residues of high molecular weight fatty acids have a lower saponification number than fats formed by low molecular weight acids.

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

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

For example: carnauba;

cerotinic;

montana.

The general formula for waxes can be written as follows:

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

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

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

Ormitinolipids are built from fatty acid residues, the amino acid ormitin or lysine, and in some cases include dihydric alcohols. The most important and widespread group of complex lipids are phospholipids. Their molecule is built from residues of alcohols, high molecular weight fatty acids, phosphoric acid, nitrogenous bases, amino acids and some other compounds.

The general formula of phospholipids (phosphotides) is as follows:

Therefore, the phospholipid molecule has two types of groups: hydrophilic and hydrophobic.

Phosphoric acid residues and nitrogenous bases act as hydrophilic groups, and hydrocarbon radicals act as hydrophobic groups.

Schematic diagram of the structure of phospholipids

Rice. 11. Phospholipid molecule

The hydrophilic polar head is a residue of phosphoric acid and a nitrogenous base.

Hydrophobic tails are hydrocarbon radicals.

Phospholipids have been isolated as by-products in the production of oils. They are surfactants that improve the baking properties of wheat flour.

They are also used as emulsifiers in the confectionery industry and in the production of margarine products. They are an essential component of cells.

Together with proteins and carbohydrates, they are involved in the construction of cell membranes and subcellular structures that perform the functions of supporting membrane structures. They promote better absorption of fats and prevent fatty liver, playing an important role in the prevention of atherosclerosis.

In addition to dividing into simple and complex, lipids can be divided into saponifiable and unsaponifiable.

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.

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 should also be noted 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 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. Molecular structure 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.