Fat has always been regarded as a component of food that is harmful to the body, and some nutritionists are of the opinion that it is better to limit the intake of fat. But are fats really that bad for us?
In fact, fats perform several very important functions for our body, and first of all, fat is an important supplier of energy for us. We can highlight the fact that 1g of fat supplies more calories than proteins and carbohydrates in double quantity. The body does not burn all fats at once, but puts some of them in storage as a reserve to be used in the future as needed. We bring you information about fats that will help you look at fats in a new way.
Why is fat necessary for our body?
Fats supply essential fatty acids for the functioning of our body, which are involved in metabolism and are energy suppliers. In addition, fats are part of cell membranes, for example, nerve cells have membranes that are 60% fat. Thus, several important functions of fats can be identified:
Fats are suppliers of energy material - approximately 30% of energy comes from fats,
By forming subcutaneous fatty tissue, they protect organs and tissues from mechanical damage, and also prevent heat loss,
They are carriers for vitamins A, D, E, K, as well as for minerals, since without fats their absorption in the body is impossible,
They are part of cell membranes (mainly cholesterol). Without them, the cell loses its function and collapses,
Fats produce female sex hormones, which is especially important in postmenopause, when ovarian function has practically disappeared. They also play an important role during the reproductive period, as they maintain hormonal levels at the proper level. If the level of adipose tissue in the body is below 10-15%, then hormonal imbalance up to the cessation of the menstrual cycle,
Omega-6 unsaturated acid (also known as arachidonic acid) is involved in the activation of blood coagulation and anticoagulation systems.
Almost 35% daily diet must consist of fats. In this case, the type of fat plays a significant role.
Which fats are healthy and which are not?
Depending on their chemical structure, fats are divided into saturated and unsaturated fatty acids. Saturated fatty acids contain a large number of hydrogen ions and are part of food products of animal origin. These are precisely the fats that are deposited on the stomach, thighs, and buttocks. This is a kind of energy reserve of the body. Saturated fats hinder growth muscle mass because they reduce the effect of insulin. But at the same time, they are the basis for the production of testosterone. When they are excluded from food, the level of this hormone, important for men, also decreases. The same can be achieved by consuming them excessively. Therefore, they are also important for the body, but in moderation.
Unsaturated fatty acids (Omega-3 and Omega-6) contain few hydrogen ions and are found mainly in animal products, such as olive or vegetable oil, fish oil. These fats are not stored in the body, but are completely burned. They are a beneficial nutritional component for the body and a raw material for the production of hormones.
There are also so-called trans fats, or artificial fats. They are filled with hydrogen ions and are found in candies and cookies, as well as in foods fast food(fast food). They are used primarily for food storage and increase the risk of developing oncological diseases and diseases of the cardiovascular system.
Omega-3 and Omega-6 are unsaturated fatty acids.
Of all types of fats, these fatty acids are the most valuable for our body. They are found in sunflower and corn oils, and rapeseed oil contains them in an ideal ratio.
Omega-3 fatty acids that are beneficial for the body are also found in flaxseed, nut and soybean oils. Salmon, mackerel and herring also contain them sufficient quantity.
Omega-3 and Omega-6 fatty acids:
Reduce the risk of developing atherosclerosis, thus preventing the development of cardiovascular diseases
Reduces cholesterol levels,
Strengthens the walls of blood vessels,
Reduce blood viscosity, thus preventing the development of blood clots,
Improves blood supply to organs and tissues, restoration nerve cells.
Ideally, you need to mix saturated and unsaturated fats, for example, season meat dishes and salads with canola oil.
Which is better, margarine or butter?
Unlike butter, margarine contains more unsaturated fatty acids. But according to new teachings, this does not mean that the oil is more harmful. In terms of calories, both products are almost equal. But margarine contains harmful trans fats, which contribute to the growth of several diseases.
If you are a fan of margarine, then it is better to choose high-quality types with low content solid fats.
Do fats lead to obesity?
Although fat contains more calories, there is no proven link between fat intake and increased weight.
Excess calories lead to obesity: those who consume more calories than they burn gain weight. Food containing sufficient amounts of fat satiates us for a long period and allows us to eat less.
Those who, on the contrary, try to save on fats, often eat more carbohydrates. Cereal products such as White bread And pasta increase blood sugar levels, and with it insulin, which leads to the growth of adipose tissue. In addition, saturation of the body occurs quickly, but does not last long, resulting in more frequent food consumption.
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, yielding high molecular weight fatty acids upon hydrolysis.
In a living organism, lipids perform various functions.
Biological functions of lipids:
1) Structural
Structural lipids form complex complexes with proteins and carbohydrates, from which the membranes of cells and cellular structures are built, and participate in a variety of processes occurring in the cell.
2) Spare (energy)
Reserve lipids (mainly fats) are the body's energy reserve and participate in metabolic processes. In plants they accumulate mainly in fruits and seeds, in animals and fish - in subcutaneous fatty 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 accounts for 95-97% of all secreted lipids.
Calorie content of carbohydrates and proteins: ~ 4 kcal/gram.
Caloric content of fat: ~ 9 kcal/gram.
The advantage of fat as an energy reserve, unlike carbohydrates, is its hydrophobicity - it is not associated with water. This ensures compactness of fat reserves - they are stored in anhydrous form, occupying a small volume. The average person's supply of pure triacylglycerols is approximately 13 kg. These reserves could be enough for 40 days of fasting in moderate conditions. physical activity. For comparison: total reserves glycogen in the body - approximately 400 g; when fasting, this amount is not enough even for one day.
3) Protective
Subcutaneous adipose tissue protects animals from cooling, and internal organs from mechanical damage.
The formation of fat reserves in the body of humans and some animals is considered an adaptation to irregular nutrition and living in a cold environment. Animals that hibernate for a long time (bears, marmots) and are adapted to living in cold conditions (walruses, seals) have a particularly large reserve of fat. The fetus has virtually no fat and appears only before birth.
A special group in terms of their functions in a living organism are the protective lipids of plants - 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 food, largely determining its nutritional value and taste. The role of lipids in various food technology processes is extremely important. Spoilage of grain and its processed products during storage (rancidity) is primarily associated with changes 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 Classification of lipids
There is no generally accepted classification of lipids.
It is most appropriate to classify lipids depending on their chemical nature, biological functions, as well as in relation to some reagents, for example, alkalis.
Based on 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 natures. These include phospholipids and glycolipids .
Phospholipids - these are complex lipids in which one of the alcohol groups is associated not with FA, but with phosphoric acid (phosphoric acid can be connected to an additional compound). Depending on which alcohol is included in the phospholipids, they are divided into glycerophospholipids (contain the alcohol glycerol) and sphingophospholipids (contain the alcohol sphingosine).
Glycolipids – these are complex lipids in which one of the alcohol groups is associated not with FA, but with a carbohydrate component. Depending on which carbohydrate component is part of the glycolipids, they are divided into cerebrosides (they contain a monosaccharide, disaccharide or a small neutral homooligosaccharide as a carbohydrate component) and gangliosides (they contain an acidic heterooligosaccharide as a carbohydrate component).
Sometimes into an independent group of lipids ( minor lipids ) secrete fat-soluble pigments, sterols, and fat-soluble vitamins. Some of these compounds can be classified as simple (neutral) lipids, others - complex.
According to another classification, lipids, depending on their relationship to alkalis, are divided into two large groups: saponifiable and unsaponifiable. The group of saponified lipids includes simple and complex lipids, which, when interacting with alkalis, hydrolyze to form salts of high molecular weight 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, storage and protective.
Structural lipids are mainly phospholipids.
Storage 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 of 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 the molecule contains 2 fatty acid radicals, then the fat is called DIACYLGLYCEROL. In the human and animal body, TRIACYLGLYCEROLS predominate (contain three fatty acid radicals).
The three hydroxyls of glycerol can be esterified either with 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.
More than four hundred were found in fats carboxylic acids of various structures. However, most of them are present only in small quantities.
The acids contained in natural fats are monocarboxylic acids, 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 moieties are present in small quantities. The exceptions are isovaleric acid and a number of cyclic acids found in some very rare fats.
The most common acids in fats contain 12 to 18 carbon atoms and are often called fatty acids. Many fats contain small amounts of low molecular weight acids (C 2 -C 10). Acids with more than 24 carbon atoms are present in waxes.
The glycerides of the most common fats contain significant quantities of unsaturated acids containing 1-3 double bonds: oleic, linoleic and linolenic. Arachidonic acid containing four double bonds is present in animal fats; acids with five, six or more double bonds are found in fats of fish and marine animals. Majority unsaturated acids lipids have a cis configuration, their double bonds are isolated or separated by a methylene (-CH 2 -) group.
Of all the unsaturated acids contained in natural fats, oleic acid is the most common. In 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 acid - are also very widespread, although they are present in much smaller quantities than oleic acid. Linoleic and linolenic acids are found in noticeable quantities 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 used. Stearic acid is found in large quantities (25% or more) only in the storage fats of some mammals (for example, in sheep fat) and in the fats of some tropical plants, such as 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 acids whose content 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 therefore the consistency of fats, depends on the structure of the acids that make up them. Solid fats, i.e. fats that melt at a relatively high temperature, consist predominantly 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 slightly 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 lard and 30-38 °C for beef lard.
The solidification temperature of fat is determined by the nature of its constituent acids: the higher the content of saturated acids, the higher it is.
Fats are soluble in ether, polyhalogen derivatives, carbon disulfide, aromatic hydrocarbons (benzene, toluene) and gasoline. Solid fats are poorly soluble in petroleum ether; insoluble in cold alcohol. Fats are insoluble in water, but they can form emulsions that are stabilized in the presence of surfactants (emulsifiers) such as proteins, soaps and some sulfonic acids, mainly in a slightly alkaline environment. Milk is a natural fat emulsion stabilized by proteins.
Chemical properties of fats
Fats undergo all chemical reactions characteristic of esters, but their chemical behavior has 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- substances that are very heterogeneous in their chemical structure, characterized by varying solubility in organic solvents and, as a rule, insoluble in water. They play an important role in life processes. Being one of the main components of biological membranes, lipids affect their permeability, participate in the transmission of nerve impulses, and the creation of intercellular contacts.
Other functions of lipids are the formation of an energy reserve, the creation of protective water-repellent and thermally insulating covers in animals and plants, and the protection of organs and tissues from mechanical stress.
CLASSIFICATION OF LIPIDS
Depending on their chemical composition, lipids are divided into several classes.
- Simple lipids include substances whose molecules consist only of fatty acid (or aldehyde) residues and alcohols. These include
- fats (triglycerides and other neutral glycerides)
- waxes
- Complex lipids
- orthophosphoric acid derivatives (phospholipids)
- lipids containing sugar residues (glycolipids)
- sterols
- steroids
IN this section Lipid chemistry will be discussed only to the extent necessary to understand lipid metabolism.
If an animal or plant tissue treated with one or more (usually sequentially) organic solvents, for example chloroform, benzene or petroleum ether, then some of the material goes into solution. The components of such a soluble fraction (extract) are called lipids. The lipid fraction contains substances of various types, most of which are presented in the diagram. Note that due to the heterogeneity of the components included in the lipid fraction, the term “lipid fraction” cannot be considered as a structural characteristic; it is only a working laboratory name for the fraction obtained during the extraction of biological material with low-polarity solvents. However, most lipids have some common structural features, determining their important biological properties and similar solubility.
Fatty acid
Fatty acids - aliphatic carboxylic acids - can be found in the body in a free state (trace amounts in cells and tissues) or act as building blocks for most classes of lipids. Over 70 different fatty acids have been isolated from the cells and tissues of living organisms.
Fatty acids found in natural lipids contain an even number of carbon atoms and have predominantly straight carbon chains. Below are the formulas for the most commonly found naturally occurring fatty acids.
Natural fatty acids, although somewhat arbitrarily, can be divided into three groups:
- saturated fatty acids [show]
- monounsaturated fatty acids [show]
Monounsaturated (with one double bond) fatty acids:
- polyunsaturated fatty acids [show]
Polyunsaturated (with two or more double bonds) fatty acids:
In addition to these main three groups, there is also a group of so-called unusual natural fatty acids [show] .
Fatty acids that are part of the lipids of animals and higher plants have many general properties. As already noted, almost all natural fatty acids contain an even number of carbon atoms, most often 16 or 18. Unsaturated fatty acids in animals and humans involved in the construction of lipids usually contain a double bond between the 9th and 10th carbons; additional double bonds, such as usually occur in the area between the 10th carbon and the methyl end of the chain. The counting starts from the carboxyl group: the C-atom closest to the COOH group is designated as α, the one next to it is designated as β, and the terminal carbon atom in the hydrocarbon radical is designated as ω.
The peculiarity of the double bonds of natural unsaturated fatty acids is that they are always separated by two simple bonds, that is, there is always at least one methylene group between them (-CH=CH-CH 2 -CH=CH-). Such double bonds are referred to as “isolated.” Natural unsaturated fatty acids have a cis configuration and trans configurations are extremely rare. It is believed that in unsaturated fatty acids with several double bonds, the cis configuration gives the hydrocarbon chain a bent and shortened appearance, which has biological meaning(especially considering that many lipids are part of membranes). In microbial cells, unsaturated fatty acids usually contain one double bond.
Long chain fatty acids are practically insoluble in water. Their sodium and potassium salts (soaps) form micelles in water. In the latter, the negatively charged carboxyl groups of fatty acids face the aqueous phase, and the nonpolar hydrocarbon chains are hidden inside the micellar structure. Such micelles have a total negative charge and remain suspended in solution due to mutual repulsion (Fig. 95).
Neutral fats (or glycerides)
Neutral fats are esters of glycerol and fatty acids. If all three hydroxyl groups of glycerol are esterified with fatty acids, then such a compound is called a triglyceride (triacylglycerol), if two are esterified, a diglyceride (diacylglycerol) and, finally, if one group is esterified, a monoglyceride (monoacylglycerol).
Neutral fats are found in the body either in the form of protoplasmic fat, which is structural component cells, or in the form of spare, reserve fat. The role of these two forms of fat in the body is not the same. Protoplasmic fat has a constant chemical composition and is contained in tissues in a certain amount, which does not change even with morbid obesity, while the amount of reserve fat undergoes large fluctuations.
The bulk of natural neutral fats are triglycerides. The fatty acids in triglycerides can be saturated or unsaturated. The most common fatty acids are palmitic, stearic and oleic acid. If all three acid radicals belong to the same fatty acid, then such triglycerides are called simple (for example, tripalmitin, tristearin, triolein, etc.), but if they belong to different fatty acids, then they are mixed. The names of mixed triglycerides are derived from the fatty acids they contain; in this case, numbers 1, 2 and 3 indicate the connection of the fatty acid residue with the corresponding alcohol group in a glycerol molecule (for example, 1-oleo-2-palmitostearin).
The fatty acids that make up triglycerides practically determine them physicochemical characteristics. Thus, the melting point of triglycerides increases with increasing number and length of saturated fatty acid residues. In contrast, the higher the content of unsaturated or short-chain fatty acids, the lower the melting point. Animal fats (lard) usually contain a significant amount of saturated fatty acids (palmitic, stearic, etc.), due to which they room temperature hard. Fats, which contain many mono- and polyunsaturated acids, are liquid at ordinary temperatures and are called oils. Thus, in hemp oil, 95% of all fatty acids are oleic, linoleic and linolenic acids, and only 5% are stearic and palmitic acid. Note that human fat, which melts at 15°C (it is liquid at body temperature), contains 70% oleic acid.
Glycerides are capable of participating in all chemical reactions characteristic of esters. Highest value has a saponification reaction, as a result of which glycerol and fatty acids are formed from triglycerides. Saponification of fat can occur either through enzymatic hydrolysis or through the action of acids or alkalis.
Alkaline breakdown of fat under the action of caustic soda or caustic potassium is carried out during the industrial production of soap. Let us remember that soap is sodium or potassium salts of higher fatty acids.
The following indicators are often used to characterize natural fats:
- iodine number - the number of grams of iodine that is in certain conditions binds 100 g of fat; given number characterizes the degree of unsaturation of fatty acids present in fats, the iodine number of beef fat is 32-47, lamb fat 35-46, pork fat 46-66;
- acid number - the number of milligrams of potassium hydroxide required to neutralize 1 g of fat. This number indicates the amount of free fatty acids present in the fat;
- saponification number - the number of milligrams of potassium hydroxide used to neutralize all fatty acids (both those included in triglycerides and free ones) contained in 1 g of fat. This number depends on the relative molecular weight fatty acids that make up fat. The saponification number for the main animal fats (beef, lamb, pork) is almost the same.
Waxes are esters of higher fatty acids and higher monohydric or dihydric alcohols with the number of carbon atoms from 20 to 70. Their general formulas are presented in the diagram, where R, R" and R" are possible radicals.
Waxes can be part of the fat covering the skin, wool, and feathers. In plants, 80% of all lipids that form a film on the surface of leaves and trunks are waxes. Waxes are also known to be normal metabolites of certain microorganisms.
Natural waxes (eg. beeswax, spermaceti, lanolin) usually contain, in addition to the mentioned esters, a certain amount of free higher fatty acids, alcohols and hydrocarbons with a number of carbon atoms of 21-35.
Phospholipids
This class of complex lipids includes glycerophospholipids and sphingolipids.
Glycerophospholipids are derivatives of phosphatidic acid: they contain glycerol, fatty acids, phosphoric acid and usually nitrogen-containing compounds. General formula glycerophospholipids are presented in the diagram, where R 1 and R 2 are radicals of higher fatty acids, and R 3 is a radical of a nitrogenous compound.
A characteristic feature of all glycerophospholipids is that one part of their molecule (radicals R 1 and R 2) exhibits pronounced hydrophobicity, while the other part is hydrophilic due to the negative charge of the phosphoric acid residue and the positive charge of the R 3 radical.
Of all lipids, glycerophospholipids have the most pronounced polar properties. When glycerophospholipids are placed in water, only a small part of them passes into the true solution, while the bulk of the “dissolved” lipid is in water systems in the form of micelles. There are several groups (subclasses) of glycerophospholipids.
- [show]
.
- Phosphatidylserines [show]
.
In the phosphatidylserine molecule, the nitrogenous compound is the amino acid residue serine.
Phosphatidylserines are much less widespread than phosphatidylcholines and phosphatidylethanolamines, and their importance is determined mainly by the fact that they participate in the synthesis of phosphatidylethanolamines.
- Plasmalogens (acetal phosphatides) [show]
.
They differ from the glycerophospholipids discussed above in that instead of one higher fatty acid residue, they contain a fatty acid aldehyde residue, which is linked to the hydroxyl group of glycerol by an unsaturated ester bond:
Thus, plasmalogen, upon hydrolysis, breaks down into glycerol, higher fatty acid aldehyde, fatty acid, phosphoric acid, choline or ethanolamine.
[show]
.
Unlike triglycerides, in the phosphatidylcholine molecule, one of the three hydroxyl groups of glycerol is associated not with fatty acid, but with phosphoric acid. In addition, phosphoric acid, in turn, is connected by an ester bond to the nitrogenous base [HO-CH 2 -CH 2 -N+=(CH 3) 3 ] - choline. Thus, the phosphatidylcholine molecule contains glycerol, higher fatty acids, phosphoric acid and choline
[show] .
The main difference between phosphatidylcholines and phosphatidylethanolamines is that the latter contain the nitrogenous base ethanolamine (HO-CH 2 -CH 2 -NH 3 +) instead of choline.
Of the glycerophospholipids in the body of animals and higher plants, phosphatidylcholines and phosphatidylethanolamines are found in the largest quantities. These two groups of glycerophospholipids are metabolically related to each other and are the main lipid components of cell membranes.
The R3 radical in this group of glycerophospholipids is the six-carbon sugar alcohol - inositol:
Phosphatidylinositols are quite widespread in nature. They are found in animals, plants and microbes. In animals, they are found in the brain, liver and lungs.
[show] . 
It should be noted that free phosphatidic acid occurs in nature, although in relatively small quantities compared to other glycerophospholipids.
Cardiolylin belongs to glycerophospholipids, more precisely to polyglycerol phosphates. The backbone of the cardiolipin molecule includes three glycerol residues connected to each other by two phosphodiester bridges through positions 1 and 3; the hydroxyl groups of the two outer glycerol residues are esterified with fatty acids. Cardiolipin is part of mitochondrial membranes. In table 29 summarizes data on the structure of the main glycerophospholipids.
Among the fatty acids that make up glycerophospholipids, both saturated and unsaturated fatty acids are found (usually stearic, palmitic, oleic and linoleic).
It has also been established that most phosphatidylcholines and phosphatidylethanolamines contain one saturated higher fatty acid, esterified in position 1 (at the 1st carbon atom of glycerol), and one unsaturated higher fatty acid, esterified in position 2. Hydrolysis of phosphatidylcholines and phosphatidylethanolamines with the participation of special enzymes contained , for example, in cobra venom, which belong to phospholipases A 2, leads to the cleavage of unsaturated fatty acids and the formation of lysophosphatidylcholines or lysophosphatidylethanolamines, which have a strong hemolytic effect.
Sphingolipids
Glycolipids
Complex lipids containing carbohydrate groups in the molecule (usually a D-galactose residue). Glycolipids play an essential role in the functioning of biological membranes. They are found primarily in brain tissue, but are also found in blood cells and other tissues. There are three main groups of glycolipids:
- cerebrosides
- sulfatides
- gangliosides
Cerebrosides contain neither phosphoric acid nor choline. They contain a hexose (usually D-galactose), which is linked by an ester bond to the hydroxyl group of the amino alcohol sphingosine. In addition, Cerebroside contains a fatty acid. Among these fatty acids, the most common are lignoceric, nervonic and cerebronic acids, i.e. fatty acids having 24 carbon atoms. The structure of cerebrosides can be represented by a diagram. Cerebrosides can also be classified as sphingolipids, since they contain the alcohol sphingosine.
The most studied representatives of cerebrosides are nervon, containing nervonic acid, cerebron, which includes cerebronic acid, and kerazin, containing lignocyric acid. The content of cerebrosides is especially high in the membranes of nerve cells (in the myelin sheath).
Sulfatides differ from cerebrosides in that they contain a sulfuric acid residue in the molecule. In other words, the sulfatide is a cerebroside sulfate in which the sulfate is esterified at the third carbon atom of the hexose. In the mammalian brain, sulfatides, like n cerebrosides, are found in the white matter. However, their content in the brain is much lower than that of cerebrosides.
When hydrolyzing gangliosides, one can detect higher fatty acid, sphingosine alcohol, D-glucose and D-galactose, as well as amino sugar derivatives: N-acetylglucosamine and N-acetylneuraminic acid. The latter is synthesized in the body from glucosamine.
Structurally, gangliosides are largely similar to cerebrosides, the only difference being that instead of a single galactose residue they contain a complex oligosaccharide. One of the simplest gangliosides is hematoside, isolated from the stroma of erythrocytes (scheme)
Unlike cerebrosides and sulfatides, gangliosides are found predominantly in gray matter brain and are concentrated in the plasma membranes of nerve and glial cells.
All the lipids discussed above are usually called saponified, since their hydrolysis produces soaps. However, there are lipids that do not hydrolyze to release fatty acids. These lipids include steroids.
Steroids are compounds widespread in nature. They are derivatives of a core containing three fused cyclohexane rings and one cyclopentane ring. Steroids include numerous substances of a hormonal nature, as well as cholesterol, bile acids and other connections.
In the human body, the first place among steroids is occupied by sterols. The most important representative of sterols is cholesterol:
It contains an alcohol hydroxyl group at C3 and a branched aliphatic chain of eight carbon atoms at C17. The hydroxyl group at C 3 can be esterified with a higher fatty acid; in this case, cholesterol esters (cholesterides) are formed:
Cholesterol plays a role as a key intermediate in the synthesis of many other compounds. The plasma membranes of many animal cells are rich in cholesterol; it is found in significantly less quantity in mitochondrial membranes and in the endoplasmic reticulum. Note that there is no cholesterol in plants. Plants have other sterols, collectively known as phytosterols.
Lipids - these are fat-like organic compounds, insoluble in water, but highly soluble in non-polar solvents (ether, gasoline, benzene, chloroform, etc.). Lipids belong to the simplest biological molecules.Chemically, most lipids are esters of higher carboxylic acids and a number of alcohols. The most famous among them are fats. Each fat molecule is formed by a molecule of the triatomic alcohol glycerol and the ester bonds of three molecules of higher carboxylic acids attached to it. According to the accepted nomenclature, fats are called triacyl glycerols.
Carbon atoms in molecules of higher carboxylic acids can be connected to each other by both simple and double bonds. Of the saturated (saturated) higher carboxylic acids, palmitic, stearic, and arachidic acids are most often found in fats; 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 ointment-like substances (fats). Conversely, fats with long and saturated chains of higher carboxylic acids become solid at room temperature. This is why, when hydrogenation (saturation of acid chains with hydrogen atoms at double bonds), liquid peanut butter, for example, becomes spreadable, and sunflower oil turns into solid margarine. Compared to the inhabitants of southern latitudes, in the body of animals living in cold climates (for example, in fish arctic seas), usually contains more unsaturated triacylglycerols. For this reason, their body remains flexible even when 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 nonpolar tails. The groups forming the polar head group are hydrophilic, while the non-polar tail groups are hydrophobic. The dual nature of these lipids determines their key role in the organization of biological membranes.
Another group of lipids consists of steroids (sterols). These substances are based on 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 also include terpenes (plant growth substances - gibberellins; carotenoids - photosynthetic pigments; essential oils plants, as well as wax).
Lipids can form complexes with other biological molecules - proteins and sugars.
The functions of lipids are as follows:
Structural. Phospholipids together with proteins form biological membranes. The membranes also contain sterols.
Energy. When fats are oxidized, a large amount of energy is released, which goes towards the formation of ATP. A significant portion is stored in the form of lipids energy reserves organism, which are consumed due to lack of nutrients. Hibernating animals and plants accumulate fats and oils and use them to maintain vital processes. High content Lipids in plant seeds ensure the development of the embryo and seedling before they transition to independent nutrition. The seeds of many plants (coconut palm, castor oil, sunflower, soybean, rapeseed, etc.) serve as raw materials for producing vegetable oil industrially.
Protective and thermal insulating. Accumulating in subcutaneous tissue and around some organs (kidneys, intestines), the fat layer protects the animal’s body and its individual organs from mechanical damage. In addition, due to low thermal conductivity, the layer of subcutaneous fat helps retain heat, which allows, for example, many animals to live in cold climates. In whales, in addition, it plays another role - it promotes buoyancy.
Lubricating and water repellent. Wax covers the skin, wool, feathers, makes them more elastic and protects them from moisture. The leaves and fruits of many plants have a waxy coating.
Regulatory. Many hormones are derivatives of cholesterol, such as sex hormones (testosterone in men and progesterone in women) and corticosteroids (aldosterone). Cholesterol derivatives, vitamin D play a key role in the metabolism of calcium and phosphorus. Bile acids are involved in the processes of digestion (emulsification of fats) and absorption of higher carboxylic acids.
Lipids are also a source of metabolic water. Oxidation of 100 g of fat produces approximately 105 g of water. This water is very important for some desert inhabitants, in particular for camels, which can do without water for 10-12 days: the fat stored in the hump is used precisely for these purposes. Bears, marmots and other hibernating animals obtain the water they need for life as a result of fat oxidation.
In the myelin sheaths of the axons of nerve cells, lipids are insulators during the conduction of nerve impulses.
Wax is used by bees to build honeycombs.
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The site provides background information for informational purposes only. Diagnosis and treatment of diseases must be carried out under the supervision of a specialist. All drugs have contraindications. Consultation with a specialist is required!
What kind of substances are lipids?
Lipids represent one of the groups organic compounds, having great value for living organisms. According to their chemical structure, all lipids are divided into simple and complex. Simple lipids are made up of alcohol and bile acids, while complex lipids contain other atoms or compounds. In general, lipids are of great importance to 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?
Basically, the term "lipids" comes from a Greek root meaning "fat", but there are still some differences between these definitions. Lipids are a larger group of substances, while fats refer to only certain types of lipids. A synonym for “fats” are “triglycerides,” which are obtained from a 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 present in any living cell, and without these substances life is simply impossible. 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 supply and formation of lipids. From a biochemical point of view, lipids take part in the following important processes:
- energy production by the body;
- cell division;
- transmission of nerve impulses;
- formation of blood components, hormones and other important substances;
- protection and fixation of some internal organs;
- cell division, respiration, etc.
Biological role of lipids in a living cell
Lipid molecules perform great amount functions not only on the scale of the entire organism, but also in each living cell individually. In essence, a cell is a structural unit of a living organism. It is where assimilation and synthesis occurs ( education) certain substances. Some of these substances go to maintaining the life of the cell itself, some to cell division, and some to the needs of other cells and tissues.In a living organism, lipids perform the following functions:
- energy;
- reserve;
- structural;
- transport;
- enzymatic;
- storing;
- 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 ( storing) function
The reserve function is closely related to the energy function. 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 adipocytes that make up adipose tissue in the body. The largest reserves of adipose tissue are located in the subcutaneous fat, the greater and lesser omentum ( V abdominal cavity ). During prolonged fasting, adipose tissue gradually breaks down, as lipid reserves are used to obtain energy.Also, adipose tissue deposited in subcutaneous fat provides thermal insulation. Tissues rich in lipids are generally poorer conductors of heat. This allows the body to maintain a constant body temperature and not cool down or overheat so quickly. different conditions external environment.
Structural and barrier functions ( membrane lipids)
Lipids play a huge role in the structure of living cells. In the human body, these substances form a special double layer that forms the cell wall. Thereby living cell can perform its functions and regulate metabolism with the external environment. Lipids that form the cell membrane also help maintain the shape of the cell.Why do lipid monomers form a double layer ( bilayer)?
Monomers are chemical substances ( V in this case– molecules), which are capable of combining 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 the lipid molecules are deployed with hydrophilic parts inside and outside the cell. The hydrophobic parts practically touch, as they are located between the two layers. Other molecules may also be located in the depth 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. Only some connections do this. For example, lipoproteins, consisting of lipids and proteins, transport certain substances in the blood from one organ to another. However, this function is rarely isolated, without considering it to be 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 vital activity. In addition, some 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. Dissolution also occurs in bile ( emulsification) exogenous lipids coming 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
Some complex lipids perform a signaling function in the body. It consists of maintaining various processes. For example, glycolipids in nerve cells take part in the transmission of nerve impulses from one nerve cell to another. In addition, signals within the cell itself are of great importance. She needs to “recognize” substances entering the blood in order to transport them inside.Regulatory function
The regulatory function of lipids in the body is secondary. The lipids themselves in the blood 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 hormones and sex hormones). They play an important role in metabolism, growth and development of the body, reproductive function, 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 (for example, pain perception).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 enter the body 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.Also, lipids are, to one degree or another, associated with the metabolism of the following substances:
- Adenosine triphosphoric acid ( ATP). ATP is a unique unit of energy inside a cell. When lipids are broken down, part of the energy goes into 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 structural elements DNA is found in the nuclei of living cells. The energy generated during the breakdown of fats is partially used for cell division. During division, new DNA chains are formed from nucleic acids.
- Amino acids. Amino acids are structural components of proteins. In combination with lipids, they form complex complexes, lipoproteins, responsible for the transport of substances in the body.
- Steroids. Steroids are a type of hormone that contains significant amounts of lipids. If lipids from food are poorly absorbed, the patient may experience problems with the endocrine system.
Digestion and absorption of lipids ( metabolism, metabolism)
Digestion and absorption of lipids is the first stage in the metabolism of these substances. The main part of lipids enters the body with food. IN oral cavity food is crushed and mixed with saliva. Next, the lump enters the stomach, where the chemical bonds are partially destroyed by hydrochloric acid. Also, some chemical bonds in lipids are destroyed by the enzyme lipase contained in saliva.Lipids are insoluble in water, so they are not immediately broken down by enzymes in the duodenum. First, the so-called emulsification of fats occurs. After this, the chemical bonds are broken down by lipase coming from the pancreas. In principle, each type of lipid now has its own enzyme responsible for the breakdown and absorption of this substance. For example, phospholipase breaks down phospholipids, cholesterol esterase breaks down cholesterol compounds, etc. All these enzymes are contained in varying quantities in pancreatic juice.
The split lipid fragments are absorbed individually by the cells of the small intestine. In general, the digestion of fats is a very difficult 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 a bolus of food entering duodenum, fats are contained in the form of large drops. This prevents them from interacting with enzymes. During the emulsification process, large fat droplets are “crushed” into smaller droplets. As a result, the contact area between fat droplets and surrounding water-soluble substances increases, and lipid breakdown becomes possible.The process of emulsification of lipids 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 droplets.
- Bile secreted from the liver accumulates in gallbladder. Here it is concentrated and released as needed.
- When consumed fatty foods, a signal is sent to the smooth muscles of the gallbladder to contract. As a result, a portion of bile is released through the bile ducts into the duodenum.
- In the duodenum, fats are actually emulsified and interact with pancreatic enzymes. Contractions in the walls of the small intestine facilitate this process by “mixing” the contents.
Enzymes for lipid breakdown
To digest each substance, the body has its own enzymes. Their task is to break chemical bonds between molecules ( or between atoms in molecules), to useful material could be normally absorbed by the body. Different enzymes are responsible for breaking down different lipids. Most of them are contained 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 the regulation of lipid levels?
The levels of most lipids in human blood are relatively constant. It can fluctuate within certain limits. This depends on the biological processes occurring in the body itself, and on a number of external factors. Regulating blood lipid levels is complex biological process, in which many take part various organs and substances.The following substances play the greatest role in the absorption and maintenance of constant lipid levels:
- Enzymes. A number of pancreatic enzymes take part in the breakdown of lipids entering 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 also directly or indirectly 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 intestines also slows down.
- Intracellular enzymes. The intestinal epithelial cells contain enzymes that, after absorption of fatty acids, convert them into transport forms and send them into the bloodstream.
- Hormones. A number of hormones affect metabolism in general. For example, high level Insulin can greatly affect blood lipid levels. That is why some standards have been revised for patients with diabetes. Thyroid hormones, glucocorticoid hormones, or norepinephrine can stimulate the breakdown of fat tissue to release energy.
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. In relation to lipids, this is characterized by their hydrolysis ( breakdown into simpler substances) V gastrointestinal tract. Anabolism combines biochemical reactions aimed at the formation of new, more complex substances.Lipid biosynthesis occurs in the following tissues and cells:
- Intestinal epithelial cells. 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 go to the liver.
- Liver cells. In liver cells, some of the transport forms of lipids will disintegrate, and new substances are synthesized from them. For example, cholesterol and phospholipid compounds are formed here, which are then excreted in bile and contribute to normal digestion.
- Cells of other organs. Some lipids travel with the blood to other organs and tissues. Depending on the cell type, lipids are converted into certain type connections. All cells, one way or another, synthesize lipids to form the cell wall ( lipid bilayer). In the adrenal glands and gonads, steroid hormones are synthesized from some lipids.
Resynthesis of lipids in the liver and other organs
Resynthesis is the process of formation of certain substances from simpler ones that were absorbed earlier. In the body, this process occurs during internal environment some cells. Resynthesis is necessary so that tissues and organs receive everything required types lipids, and not just those consumed with food. Resynthesized lipids are called endogenous. The body spends energy on their formation.At the first stage, lipid resynthesis occurs in the intestinal walls. Here, fatty acids ingested from food are converted into transport forms that are transported through the blood to the liver and other organs. Part of the resynthesized lipids will be delivered to the tissues; from the other part, substances necessary for life will be formed ( lipoproteins, bile, hormones, etc.), the excess is converted to adipose tissue and is put aside “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 through the transmission of nerve impulses. In this case, 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 thanks to the myelin sheath of nerve cells. Myelin, which prevents the chaotic propagation of impulses, consists of approximately 75% lipids. As in cell membranes, here they form a double layer ( bilayer), which is wrapped several times around the nerve cell.The myelin sheath in the nervous system contains the following lipids:
- phospholipids;
- cholesterol;
- galactolipids;
- glycolipids.
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 take part 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, 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 - estrogens ( estriol, estradiol, etc.).
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 so-called sebaceous glands, which release a certain amount of fat-rich secretion to the surface. This substance performs many useful functions.Lipids are important for hair and skin for the following reasons:
- a significant part of the hair substance consists of complex lipids;
- skin cells change rapidly, and lipids are important as an energy resource;
- secret ( secreted substance) sebaceous glands moisturizes the skin;
- Thanks to fats, the firmness, elasticity and smoothness of the skin is maintained;
- a small amount of lipids on the surface of the hair gives it a healthy shine;
- the 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.).
Classification of lipids
In biology and chemistry there are quite a lot various classifications 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 ones ( 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, as it reflects not only chemical structure substances, but also partially determines the chemical properties. Biology and medicine have their own additional classifications that use 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 largest amount of exogenous lipids enters the body with food, but there are other routes. For example, when using various cosmetics or medicines the body may 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. Some components of complex lipids cannot be synthesized by the body independently, which affects the course of certain biological processes.
Fatty acid
Fatty acids are a class of organic compounds that are a structural part of lipids. Depending on which fatty acids are included in 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 through food. Each acid is a structural component for specific cells, enzymes or compounds. Once absorbed, 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 Food Industry (margarine, etc.).
Saturated and unsaturated fatty acids
All fatty acids according to their chemical structure are divided into saturated and unsaturated. Saturated acids are less beneficial for the body, and some of them are even harmful. This is explained by the fact that there are no double bonds in the molecule of these substances. These are chemically stable compounds and are less easily absorbed by the body. Currently, the connection between some saturated fatty acids and the development of atherosclerosis has been proven.Unsaturated fatty acids are divided into two large groups:
- Monounsaturated. These acids have one double bond in their structure and are therefore more active. It is believed that eating them can lower cholesterol levels and prevent the development of atherosclerosis. The greatest amount of monounsaturated fatty acids is found in a number of plants ( avocado, olives, pistachios, hazelnuts) and, accordingly, in oils obtained from these plants.
- Polyunsaturated. Polyunsaturated fatty acids have several double bonds in their structure. A distinctive feature of these substances is that human body unable to synthesize them. In other words, if the body does not receive polyunsaturated fatty acids from food, over time this will inevitably lead to certain disorders. Best sources These acids are seafood, soybean and linseed oil, sesame seeds, poppy seeds, sprouted wheat, etc.
Phospholipids
Phospholipids are complex lipids containing a phosphoric acid residue. These substances, along with cholesterol, are the main components of cell membranes. These substances also take part 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 promote emulsification ( dissolution) other fats. Depending on which substance is more in bile, cholesterol or phospholipids, you can determine the risk of developing cholelithiasis.Glycerol and triglycerides
In terms of its chemical structure, 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. Most important function These substances are 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 work the muscles ( skeletal muscles, cardiac 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.
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