Fat is considered the culprit of many ills. Doctors and scientists advise reducing fat intake or eliminating it from the diet altogether. Of course, those who are obese or have chronic diseases are better off heeding this advice. However, the rest of us would be foolish to give up fat. Let's find out more about them with the facts below.
1. Consumption of fats does not necessarily lead to their deposition in the body.
Many people think that consuming fat will definitely affect their figure in the form of deposits on the waist, hips and abdomen. If you eat more than your body requires, then yes, this problem can arise. For example, if you consume starchy carbohydrates in unlimited quantities, you can expect an increase in insulin levels, and then fat will be deposited. But if you eat evenly consuming fats and proteins, then this problem can be avoided. In everything you need to know when to stop.
2. No need to avoid nuts
Nuts contain useful forms fat - monone saturated fats, which help you feel full faster, but also increase healthy cholesterol. Nuts do not have any effect on weight gain, because you can’t eat a lot of them due to their satiety, and besides, they are poorly digested by the body. Consequently, the cell walls of nuts are not easily destroyed during chewing. This means that they pass through the body in transit and do not release all their fat.
3. There is no need to completely eliminate saturated fats from the body.
It has always been believed that saturated fats are the enemies of health, so they were advised to be excluded from the diet. But today it has become clear that moderate consumption of saturated fat does not cause any harm. And some of them even need to be included in a healthy eating program.
Virgin coconut oil is one of the healthy sources of saturated fat. It contains lauric acid, which is found nowhere else except in breast milk. It is a powerful immune stimulant. It is recommended to fry foods in coconut oil.
4. Just because a product label says “no trans fats” doesn’t mean they aren’t there.
Many manufacturers believe that if a product contains a very small amount of an ingredient, then there is no need to list it on the label. It happens that a product contains only 0.5 g of trans fat, but you will not find it among the ingredients on the packaging. After eating several servings of this product, you will not even know that you have eaten enough of this harmful ingredient.
5. Nutrients from vegetables without fat are less absorbed
Studies have shown that salad seasoned with fat or sauce with fats is much better absorbed by the body and receives more of what it needs. nutrients- carotenoids. If you constantly eat salads without fat, then carotenoids will not be absorbed by the body at all. They are responsible for the colors red, yellow, orange and green and are important in preventing many diseases. To ensure your body absorbs all the nutrients from vegetables, eat them with healthy fats.
6. Extra virgin olive oil is not suitable for frying.
Although it contains healthy monounsaturated fats, it high temperatures loses its properties. It is better to use it for dressing salads or marinating meat. Olive oil is very delicate and spoils quickly, so it should be stored in a dark glass container with a tightly closed lid to avoid oxidation and preserve all its beneficial properties.
7. Fats have many functions in the body.
Without fats, our body and our organism cannot live. Here are a few reasons for this:
The brain needs fats. About 60% of the dry weight of the human brain is fat. Healthy nerve cells contain fats - docosahexanoic acid;
Sexual hormones are formed with the help of fats;
Fatty acids are essential for healthy skin and hair;
Fats are involved in metabolism, functions immune system, help stabilize blood sugar.
Thank you
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 of 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 a huge number of 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 in 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 ( 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. Besides, great importance have signals within the cell itself. 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 certain processes in the 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 the 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.
Cleaved lipid fragments are individually absorbed by cells small intestine. In general, fat digestion is a very complex process that 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 bolus of food entering the duodenum, fats are contained in the form of large droplets. 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 emulsifying lipids into 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 consuming 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 insulin levels 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 unites bio chemical 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 these same cells, which enter the venous blood and are sent 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 all the necessary types of 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 moisturize 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 the 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, since it reflects not only the chemical structure of substances, but also partially determines 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 them 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 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 petroleum 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.
Most important sources 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. Distinctive feature of these substances is that the human body is not able 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. The best sources of these acids are seafood, soy 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.
Beta lipids
Beta lipids are sometimes called 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 a specialist.Lipids- very diverse in their own way chemical structure substances 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 animal or plant tissue is treated with one or more (usually sequentially) organic solvents, such as chloroform, benzene or petroleum ether, some of the material goes into solution. The components of such a soluble fraction (extract) are called lipids. The lipid fraction contains substances 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 share some common structural features that make them 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 makes biological sense (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 a structural component of cells, or in the form of 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 is subject to 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 their physicochemical properties. 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 a lot of mono- and poly Not saturated 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 acids. Note that human fat, which melts at 15°C (it is liquid at body temperature), contains 70% oleic acid.
Glycerides are capable of entering into all chemical reactions characteristic of esters. The most important reaction is the saponification reaction, which results in the formation of glycerol and fatty acids 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, under certain conditions, is bound to 100 g of fat; this 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.
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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
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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.
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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 the gray matter of the 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 compounds.
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 constitute a large and quite heterogeneous in chemical composition group of organic substances that are part of living cells, soluble in low-polar organic solvents (ether, benzene, chloroform, etc.) and insoluble in water. IN general view they are considered to be derivatives of fatty acids.
A peculiarity of the structure of lipids is the presence in their molecules of both polar (hydrophilic) and non-polar (hydrophobic) structural fragments, which gives lipids an affinity for both water and the non-aqueous phase. Lipids are biphilic substances, which allows them to carry out their functions at the interface.
10.1. Classification
Lipids are divided into simple(two-component), if the products of their hydrolysis are alcohols and carboxylic acids, and complex(multicomponent), when as a result of their hydrolysis, other substances are also formed, for example phosphoric acid and carbohydrates. Simple lipids include waxes, fats and oils, as well as ceramides; complex lipids include phospholipids, sphingolipids and glycolipids (Scheme 10.1).
Scheme 10.1.General classification of lipids
10.2. Structural components of lipids
All groups of lipids have two obligatory structural components - higher carboxylic acids and alcohols.
Higher fatty acids (HFAs). Many higher carboxylic acids were first isolated from fats, which is why they are called fatty. Biologically important fatty acids can be saturated(Table 10.1) and unsaturated(Table 10.2). Their general structural features:
They are monocarbon;
Include an even number of carbon atoms in the chain;
Have a cis configuration of double bonds (if present).
Table 10.1.Essential saturated fatty acid lipids
In natural acids, the number of carbon atoms ranges from 4 to 22, but acids with 16 or 18 carbon atoms are more common. Unsaturated acids contain one or more double bonds in the cis configuration. The double bond closest to the carboxyl group is usually located between the C-9 and C-10 atoms. If there are several double bonds, then they are separated from each other by the methylene group CH 2.
The IUPAC rules for DRCs allow the use of their trivial names (see Tables 10.1 and 10.2).
Currently, our own nomenclature of unsaturated liquid liquids is also used. In it, the terminal carbon atom, regardless of the length of the chain, is designated by the last letter Greek alphabetω (omega). The position of double bonds is counted not, as usual, from the carboxyl group, but from the methyl group. Thus, linolenic acid is designated as 18:3 ω-3 (omega-3).
Linoleic acid itself and unsaturated acids with a different number of carbon atoms, but with the arrangement of double bonds also at the third carbon atom, counting from the methyl group, constitute the omega-3 family of liquid fatty acids. Other types of acids form similar families of linoleic (omega-6) and oleic (omega-9) acids. For normal life For a person, the correct balance of lipids of three types of acids is of great importance: omega-3 (linseed oil, fish fat), omega-6 (sunflower, corn oils) and omega-9 ( olive oil) in the diet.
Of the saturated acids in the lipids of the human body, the most important are palmitic C16 and stearic C18 (see Table 10.1), and among the unsaturated ones - oleic C18:1, linoleic C18:2, linolenic and arachidonic C 20:4 (see Table 10.2).
It should be emphasized the role of polyunsaturated linoleic and linolenic acids as compounds irreplaceable for humans (“vitamin F”). They are not synthesized in the body and should be supplied with food in an amount of about 5 g per day. In nature, these acids are found mainly in vegetable oils. They contribute
Table 10 .2. Essential unsaturated fatty acid lipids
*Included for comparison. ** For cis isomers.
normalization lipid profile blood plasma. Linetol, a mixture ethyl ethers higher fatty unsaturated acids, used as a hypolipidemic herbal medicine. Alcohols. Lipids may include:
Higher monohydric alcohols;
Polyhydric alcohols;
Amino alcohols.
In natural lipids, the most common are saturated and less often unsaturated long-chain alcohols (C 16 or more), mainly with an even number of carbon atoms. As an example of higher alcohols, cetyl CH 3 (CH 2 ) 15 OH and melissil CH 3 (CH 2) 29 OH alcohols that are part of waxes.
Polyhydric alcohols in most natural lipids are represented by the trihydric alcohol glycerol. Other polyhydric alcohols are found, such as the dihydric alcohols ethylene glycol and 1,2 propanediol, as well as myoinositol (see 7.2.2).
The most important amino alcohols that are part of natural lipids are 2-aminoethanol (colamine), choline, and serine and sphingosine, which also belong to the α-amino acids.
Sphingosine is an unsaturated long-chain dihydric amino alcohol. The double bond in sphingosine has trance-configuration, and the asymmetric atoms C-2 and C-3 - D-configuration.
Alcohols in lipids are acylated with higher carboxylic acids at the corresponding hydroxyl groups or amino groups. In glycerol and sphingosine, one of the alcohol hydroxyls can be esterified with a substituted phosphoric acid.
10.3. Simple lipids
10.3.1. Waxes
Waxes are esters of higher fatty acids and higher monohydric alcohols.
Waxes form a protective lubricant on the skin of humans and animals and protect plants from drying out. They are used in the pharmaceutical and perfume industries in the production of creams and ointments. An example is palmitic acid cetyl ester(cetin) - main component spermaceti. Spermaceti is secreted from the fat contained in the cavities of the skull of sperm whales. Another example is Palmitic acid melissil ester- component of beeswax.
10.3.2. Fats and oils
Fats and oils are the most common group of lipids. Most of them belong to triacylglycerols - complete esters of glycerol and IVG, although mono- and diacylglycerols are also found and take part in metabolism.
Fats and oils (triacylglycerols) are esters of glycerol and higher fatty acids.
In the human body, triacylglycerols play the role of a structural component of cells or a reserve substance (“fat depot”). Their energy value is approximately twice that of proteins
or carbohydrates. However increased level triacylglycerols in the blood is one of the additional risk factors for the development of coronary heart disease.
Solid triacylglycerols are called fats, liquid triacylglycerols are called oils. Simple triacylglycerols contain residues of the same acids, while mixed ones contain residues of different ones.
Triacylglycerols of animal origin usually contain predominantly saturated acid residues. Such triacylglycerols are usually solids. On the contrary, vegetable oils contain mainly residues of unsaturated acids and have a liquid consistency.
Below are examples of neutral triacylglycerols and their systematic and (in parentheses) commonly used trivial names, based on the names of their constituent fatty acids.
10.3.3. Ceramides
Ceramides are N-acylated derivatives of the alcohol sphingosine.
Ceramides are present in small quantities in the tissues of plants and animals. Much more often they are part of complex lipids - sphingomyelins, cerebrosides, gangliosides, etc.
(see 10.4).
10.4. Complex lipids
Some complex lipids are difficult to classify unambiguously, since they contain groups that allow them to be classified simultaneously into different groups. According to general classification Lipids (see Diagram 10.1) Complex lipids are usually divided into three large groups: phospholipids, sphingolipids and glycolipids.
10.4.1. Phospholipids
The group of phospholipids includes substances that remove phosphoric acid during hydrolysis, for example glycerophospholipids and some sphingolipids (Scheme 10.2). In general, phospholipids are characterized by a fairly high content of unsaturated acids.
Scheme 10.2.Classification of phospholipids
Glycerophospholipids. These compounds are the main lipid components of cell membranes.
According to their chemical structure, glycerophospholipids are derivatives l -glycero-3-phosphate.
l-Glycero-3-phosphate contains an asymmetric carbon atom and, therefore, can exist in the form of two stereoisomers.
Natural glycerophospholipids have the same configuration, being derivatives of l-glycero-3-phosphate, formed during metabolism from dihydroxyacetone phosphate.
Phosphatides. Among glycerophospholipids, the most common are phosphatides - ester derivatives of l-phosphatidic acids.
Phosphatidic acids are derivatives l -glycero-3-phosphate, esterified with fatty acids at alcohol hydroxyl groups.
As a rule, in natural phosphatides, in position 1 of the glycerol chain there is a residue of a saturated acid, in position 2 - an unsaturated acid, and one of the hydroxyls of phosphoric acid is esterified with a polyhydric alcohol or amino alcohol (X is the residue of this alcohol). In the body (pH ~7.4), the remaining free hydroxyl of phosphoric acid and other ionic groups in phosphatides are ionized.
Examples of phosphatides are compounds containing phosphatidic acids esterified for phosphate hydroxyl with corresponding alcohols:
Phosphatidylserines, esterifying agent - serine;
Phosphatidylethanolamines, esterifying agent - 2-aminoethanol (in biochemical literature often, but not quite correctly, called ethanolamine);
Phosphatidylcholines, esterifying agent - choline.
These esterifying agents are related because ethanolamine and choline moieties can be metabolized from the serine moiety by decarboxylation and subsequent methylation with S-adenosylmethionine (SAM) (see 9.2.1).
A number of phosphatides, instead of an amino-containing esterifying agent, contain residues of polyhydric alcohols - glycerol, myoinositol, etc. The phosphatidylglycerols and phosphatidylinositols given below as examples belong to acidic glycerophospholipids, since their structures do not contain fragments of amino alcohols, which give phosphatidylethanolamines and related compounds a neutral character.
Plasmalogens. Less common than ester glycerophospholipids are lipids with an ether linkage, in particular plasmalogens. They contain an unsaturated residue
* For convenience, the way of writing the configuration formula of the myoinositol residue in phosphatidylinositols has been changed from that given above (see 7.2.2).
alcohol linked by an ether bond to the C-1 atom of glycero-3-phosphate, such as plasmalogens with an ethanolamine fragment - L-phosphatidal ethanolamines. Plasmalogens make up up to 10% of all CNS lipids.
10.4.2. Sphingolipids
Sphingolipids are structural analogues of glycerophospholipids in which sphingosine is used instead of glycerol. Another example of sphingolipids is the ceramides discussed above (see 10.3.3).
An important group of sphingolipids are sphingomyelins, first discovered in nervous tissue. In sphingomyelins, the hydroxyl group of C-1 ceramide is esterified, as a rule, with choline phosphate (less often with colamine phosphate), so they can also be classified as phospholipids.
10.4.3. Glycolipids
As the name suggests, compounds of this group include carbohydrate residues (usually D-galactose, less often D-glucose) and do not contain a phosphoric acid residue. Typical representatives of glycolipids - cerebrosides and gangliosides - are sphingosine-containing lipids (therefore they can be considered sphingolipids).
IN cerebrosides the ceramide residue is linked to D-galactose or D-glucose by a β-glycosidic bond. Cerebrosides (galactocerebrosides, glucocerebrosides) are part of the membranes of nerve cells.
Gangliosides- carbohydrate-rich complex lipids - were first isolated from the gray matter of the brain. Structurally, gangliosides are similar to cerebrosides, differing in that instead of a monosaccharide they contain a complex oligosaccharide, including at least one remainder V-acetylneuraminic acid (see Appendix 11-2).
10.5. Properties of lipids
and their structural components
A special feature of complex lipids is their biphilicity, caused by non-polar hydrophobic and highly polar ionized hydrophilic groups. In phosphatidylcholines, for example, the hydrocarbon radicals of fatty acids form two non-polar “tails”, and the carboxyl, phosphate and choline groups form the polar part.
At the interface, such compounds act as excellent emulsifiers. As part of cell membranes, lipid components provide high electrical resistance of the membrane, its impermeability to ions and polar molecules, and permeability to non-polar substances. In particular, most anesthetic drugs are highly lipid soluble, which allows them to penetrate the membranes of nerve cells.
Fatty acids are weak electrolytes( p K a~4.8). They're in small degree dissociated into aqueous solutions. At pH< p K a non-ionized form predominates, at pH > p Ka, i.e., under physiological conditions, the ionized form RCOO - predominates. Soluble salts of higher fatty acids are called soaps. Sodium salts of higher fatty acids are solid, potassium salts are liquid. As salts of weak acids and strong bases of soap are partially hydrolyzed in water, their solutions have an alkaline reaction.
Natural unsaturated fatty acids that have cis- double bond configuration, have a large supply of internal energy and, therefore, compared to trance-isomers are thermodynamically less stable. Their cis-trans -isomerization occurs easily when heated, especially in the presence of radical reaction initiators. IN laboratory conditions this transformation can be carried out by the action of nitrogen oxides formed during the decomposition of nitric acid when heated.
Higher fatty acids exhibit the general chemical properties of carboxylic acids. In particular, they easily form the corresponding functional derivatives. Fatty acids with double bonds exhibit the properties of unsaturated compounds - they add hydrogen, hydrogen halides and other reagents to the double bond.
10.5.1. Hydrolysis
Using the hydrolysis reaction, the structure of lipids is determined, and also obtained valuable products(soap). Hydrolysis is the first stage of utilization and metabolism of dietary fats in the body.
Hydrolysis of triacylglycerols is carried out either by exposure to superheated steam (in industry) or by heating with water in the presence of mineral acids or alkalis (saponification). In the body, lipid hydrolysis occurs under the action of lipase enzymes. Some examples of hydrolysis reactions are given below.
In plasmalogens, as in ordinary vinyl esters, the ether bond is cleaved in an acidic, but not in an alkaline, environment.
10.5.2. Addition reactions
Lipids containing unsaturated acid residues in their structure add hydrogen, halogens, hydrogen halides, and water through double bonds in an acidic environment. Iodine number is a measure of the unsaturation of triacylglycerols. It corresponds to the number of grams of iodine that can add to 100 g of a substance. The composition of natural fats and oils and their iodine numbers vary within fairly wide limits. As an example, we give the interaction of 1-oleoyl-distearoylglycerol with iodine (the iodine number of this triacylglycerol is 30).
Catalytic hydrogenation (hydrogenation) of unsaturated vegetable oils is an important industrial process. In this case, hydrogen saturates the double bonds and liquid oils turn into solid fats.
10.5.3. Oxidation reactions
Oxidative processes involving lipids and their structural components are quite diverse. In particular, the oxidation of unsaturated triacylglycerols by oxygen during storage (auto-oxidation, see 3.2.1), accompanied by hydrolysis, is part of the process known as rancidity of oil.
The primary products of the interaction of lipids with molecular oxygen are hydroperoxides formed as a result of a chain free radical process (see 3.2.1).
Lipid peroxidation - one of the most important oxidative processes in organism. It is the main cause of damage to cell membranes (for example, in radiation sickness).
Structural fragments of unsaturated higher fatty acids in phospholipids serve as targets for attack active forms of oxygen(AFC, see Appendix 03-1).
When attacked, in particular by the hydroxyl radical HO, the most active of ROS, the lipid molecule LH undergoes homolytic rupture S-N connections in the allylic position, as shown in the lipid peroxidation model (Scheme 10.3). The resulting allylic radical L" instantly reacts with molecular oxygen present in the oxidation environment to form the lipid peroxyl radical LOO". From this moment, a chain cascade of lipid peroxidation reactions begins, as continuing education allylic lipid radicals L", renewing this process.
Lipid peroxides LOOH are unstable compounds and can spontaneously or with the participation of metal ions of variable valence (see 3.2.1) decompose to form lipidoxyl radicals LO", capable of initiating further oxidation of the lipid substrate. Such an avalanche-like process of lipid peroxidation poses a danger of destruction of membrane structures cells.
The intermediately formed allylic radical has a mesomeric structure and can further undergo transformations in two directions (see diagram 10.3, paths A And b), leading to intermediate hydroperoxides. Hydroperoxides are unstable and even at ordinary temperatures decompose to form aldehydes, which are further oxidized into acids - the final products of the reaction. The result is general case two monocarboxylic and two dicarboxylic acids with shorter carbon chains.
Unsaturated acids and lipids with residues of unsaturated acids under mild conditions are oxidized with an aqueous solution of potassium permanganate, forming glycols, and in more rigid conditions (with the rupture of carbon-carbon bonds) - the corresponding acids.
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 of plants, as well as waxes).
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 body, 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 the 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. The 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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