KSE. Topic 4.
1. What are the main elements and substances that make up a living cell?
Depending on the quantity of chemical elements included in the composition of the substances that form a living organism, it is customary to distinguish several groups of atoms. First group(about 98% of the cell's mass) are formed by four elements: hydrogen, oxygen, carbon and nitrogen. They are called macronutrients. These are the main components of all organic compounds. Along with two elements second group- sulfur and phosphorus, which are necessary components of biological polymer molecules (from the Greek polys - many; meros - part) - proteins and nucleic acids, they are often called bioelements.
In smaller quantities, the cell composition, in addition to the mentioned phosphorus and sulfur, includes 6 elements: potassium and sodium, calcium and magnesium, iron and chlorine. Each of them performs an important function in the cell. For example, Na, K and Cl ensure the permeability of cell membranes for various substances and the conduction of impulses along the nerve fiber. Ca and P are involved in the formation of the intercellular substance of bone tissue, determining bone strength. In addition, Ca is one of the factors on which normal blood clotting depends. Iron is part of hemoglobin, a protein in red blood cells that is involved in the transfer of oxygen from the lungs to the tissues. Finally, Mg in plant cells is included in chlorophyll - the pigment that determines photosynthesis, and in animals it is part of biological catalysts - enzymes involved in biochemical transformations.
All other elements - third group(zinc, copper, iodine, fluorine, etc.) are contained in the cell in very small quantities. Their total contribution to the cell mass is only 0.02%. That's why they are called microelements. However, this does not mean that the body needs them less than other elements. Microelements are also important for a living organism, but are included in its composition in smaller quantities. Zinc, for example, is part of the molecule of the pancreatic hormone - insulin, which is involved in the regulation of carbohydrate metabolism, and iodine is a necessary component of thyroxine - the thyroid hormone, which regulates the metabolic rate of the entire organism as a whole and its growth during development.
All of the listed chemical elements participate in the construction of the body in the form of ions or as part of certain compounds - molecules of inorganic and organic substances.
Inorganic substances included incell composition
Water. The most common inorganic compound in living organisms is water. Its content varies widely: in the cells of tooth enamel there is about 10% water, and in the cells of a developing embryo - more than 90%. On average, in a multicellular organism, water makes up about 80% of body weight.
The role of water in the cell is very important. Its functions are largely determined by its chemical nature. The dipole nature of the structure of molecules determines the ability of water to actively interact with various substances. Its molecules cause the breakdown of a number of water-soluble substances into cations and anions. As a result, the ions quickly enter into chemical reactions. Most chemical reactions involve interactions between water-soluble substances.
Thus, the polarity of molecules and the ability to form hydrogen bonds make water a good solvent for a huge number of inorganic and organic substances. In addition, as a solvent, water provides both the influx of substances into the cell and the removal of waste products from it, since most chemical compounds can penetrate the outer cell membrane only in dissolved form.
The purely chemical role of water is no less important. Under the influence of certain catalysts - enzymes - it enters into hydrolysis reactions, i.e. reactions in which OH - or H - water groups are added to the free valences of various molecules. As a result, new substances with new properties are formed.
Water is, to a certain extent, a heat regulator; Due to the good thermal conductivity and high heat capacity of water, when the ambient temperature changes, the temperature inside the cell remains unchanged or its fluctuations are significantly less than in the environment surrounding the cell.
Mineral salts. Most of the inorganic substances of the cell are in the form of salts - either dissociated into ions or in the solid state. Among the former, the cations K -, Na + Ca 2+ are of great importance, which provide such an important property of living organisms as irritability. In the tissues of multicellular animals, calcium is part of the intercellular “cement”, which determines the adhesion of cells to each other and their ordered arrangement in the tissues. The buffering properties of the cell depend on the concentration of salts inside the cell.
Buffer call the ability of a cell to maintain a slightly alkaline reaction of its contents at a constant level.
Insoluble mineral salts, for example calcium phosphate, are part of the intercellular substance of bone tissue, in the shells of mollusks, ensuring the strength of these formations.
Organic substances that make up the cell
Organic compounds make up on average 20-30% of the cell mass of a living organism. These include biological polymers - proteins, nucleic acids and carbohydrates, as well as fats and a number of small molecules - hormones, pigments, ATP and many others. Different types of cells contain different amounts of organic compounds. Complex carbohydrates - polysaccharides - predominate in plant cells; in animals there are more proteins and fats. However, each of the groups of organic substances in any type of cell performs similar functions.
Biological polymers - proteins. Among the organic substances of cells, proteins occupy first place both in quantity and in importance. In animals they account for about 50% of the dry mass of the cell. In the human body there are 5 million types of protein molecules that differ not only from each other, but also from the proteins of other organisms. Despite such diversity and complexity of structure, they are built from only 20 different amino acids. A combination of two amino acids into one molecule is called a dipeptide, three amino acids - a tripeptide, etc., and a compound consisting of 20 or more amino acid residues is called a polypeptide.
Carbohydrates, or saccharides - organic substances with the general formula C n (H 2 0) m. For most carbohydrates, the number of water molecules corresponds to the number of carbon atoms. That's why these substances were called carbohydrates.
In an animal cell, carbohydrates are found in quantities not exceeding 1-2, sometimes 5%. Plant cells are the richest in carbohydrates, where their content in some cases reaches 90% of dry weight (potato tubers, seeds, etc.). Carbohydrates are simple and complex.
Simple carbohydrates are called monosaccharides. Depending on the number of carbon atoms in the molecule, monosaccharides are called trioses - 3 atoms, tetroses - 4, pentoses - 5 or hexoses - 6 carbon atoms. Of the six-carbon monosaccharides - hexoses - the most important are glucose, fructose and galactose. Glucose is contained in the blood (0.08-0.12%). Pentoses - ribose and deoxyribose - are part of nucleic acids and ATP.
If two monosaccharides are combined in one molecule, the compound is called a disaccharide. Disaccharides include food sugar - sucrose, obtained from cane or sugar beets, which consists of one molecule of glucose and one molecule of fructose, and milk sugar, formed by molecules of glucose and galactose.
Complex carbohydrates formed from many monosaccharides are called polysaccharides. The monomers of polysaccharides such as starch, glycogen, and cellulose are glucose. Polysaccharides, as a rule, are branched polymers.
Fats (lipids) are compounds of high molecular weight fatty acids and trihydric alcohol glycerol. Fats do not dissolve in water, they are hydrophobic (from the Greek hydor - water and phobos - fear). Cells always contain other complex hydrophobic fat-like substances called lipoids.
The role of fats is also important as solvents of hydrophobic organic compounds necessary for the normal course of biochemical transformations in the body.
Biologically, polymers are nucleic acids. The importance of nucleic acids in a cell is very great. The peculiarities of their chemical structure provide the possibility of storing, transferring and inheriting to daughter cells information about the structure of protein molecules that are synthesized in each tissue at a certain stage of individual development.
Since most properties and characteristics are determined by proteins, it is clear that the stability of nucleic acids is the most important condition for the normal functioning of cells and entire organisms. Any changes in the structure of nucleic acids entail changes in the structure of cells or the activity of physiological processes in them, thus affecting viability.
The study of the structure of nucleic acids, which was first established by the American biologist J. Watson and the English physicist F. Crick, is extremely important for understanding the inheritance of traits in organisms and the patterns of functioning of both individual cells and cellular systems - tissues and organs.
There are two types of nucleic acids: DNA and RNA.
All living systems contain chemical elements and chemical compounds built from them, both organic and inorganic, in various proportions.
Based on their quantitative content in the cell, all chemical elements are divided into 3 groups: macro-, micro- and ultramicroelements.
Macroelements make up up to 99% of the cell's mass, of which up to 98% are 4 elements: oxygen, nitrogen, hydrogen and carbon. In smaller quantities, cells contain potassium, sodium, magnesium, calcium, sulfur, phosphorus, and iron.
Microelements are mainly metal ions (cobalt, copper, zinc, etc.) and halogens (iodine, bromine, etc.). They are contained in quantities from 0.001% to 0.000001%.
Ultramicroelements. Their concentration is below 0.000001%. These include gold, mercury, selenium, etc.
A chemical compound is a substance in which the atoms of one or more chemical elements are connected to each other through chemical bonds. Chemical compounds are inorganic and organic. Inorganic substances include water and mineral salts. Organic compounds are compounds of carbon with other elements.
The main organic compounds of the cell are proteins, fats, carbohydrates and nucleic acids.
Chemical elements and inorganic substances of the cell
The difference between living and inanimate nature is clearly manifested in their chemical composition. Thus, the earth’s crust is 90% oxygen, silicon, aluminum and sodium (O, Si, Al, Na), and in living organisms about 95% is carbon, hydrogen, oxygen and nitrogen (C, H, O, N) . In addition, this group of macroelements includes eight more chemical elements: Na - sodium, Cl - chlorine, S - sulfur, Fe - iron, Mg - magnesium, P - phosphorus, Ca - calcium, K - potassium, the content of which is calculated in tenths and hundredths of a percent. Microelements equally necessary for life are found in much smaller quantities: Cu - copper, Mn - manganese, Zn - zinc, Mo - molybdenum, Co - cobalt, F - fluorine, J - iodine, etc.
Only 27 elements (out of 105 that are known today) perform specific functions in organisms. And as we have already noted, only four - C, H, O, N - serve as the basis of living organisms. It is from them that organic substances (proteins, nucleic acids, carbohydrates, fats, etc.) mainly consist.
The first place among macroelements belongs to carbon. It is characterized by the ability to form almost all types of chemical bonds. Carbon, to a greater extent than other elements, is capable of forming large molecules. Its atoms can connect with each other, forming rings and chains. As a result, complex molecules of large sizes arise, characterized by enormous diversity (more than 10 million organic substances have been described to date). In addition, carbon atoms in the same chemical compound exhibit both oxidizing and reducing properties.
Carbon is the basis of all organic compounds. The high content of oxygen and hydrogen is associated with the presence of pronounced oxidizing and reducing properties. Thanks to only three elements - C, H, O - there is a whole set of carbohydrates (sugars), the generalized formula of which looks like CnH2nOn (where n is the number of atoms). To these three elements, N and S atoms are added in proteins, and N and P atoms in nucleic acids.
All other elements mentioned above also play a significant role in living organisms. Thus, Mg atoms are part of chlorophyll, and Fe atoms are part of hemoglobin. Iodine is contained in the thyroxine molecule (thyroid hormone), and Zn is contained in the insulin molecule (pancreatic hormone). The presence of Na and K ions is necessary for the conduction of nerve impulses and for transport through the cell membrane. Salts P and Ca are present in large quantities in bones and shells of mollusks, which ensures the high strength of these formations.
It should be noted that the largest part (up to 85%) of the chemical composition of living organisms is water. Since it is a universal solvent for many inorganic and organic substances, it turns out to be an ideal medium for carrying out various chemical reactions. Water is involved in various biochemical reactions (for example, during photosynthesis). It removes excess salts and waste products from the body. The high heat capacity and relatively high thermal conductivity inherent in water are essential for the thermoregulation of organisms (when sweat evaporates, for example, the skin cools).
All living systems on the planet contain chemical elements in varying volumes, as well as organic and inorganic chemical compounds.
Chemical structure of the cell
Depending on the amount of a particular chemical element contained in a cell, three groups of chemical elements are distinguished:
Macroelements;
Microelements;
Ultra microelements.
98% of a cell's mass is made up of four macroelements: hydrogen, carbon, nitrogen and oxygen. Macroelements such as sodium, magnesium, potassium, phosphorus, iron and sulfur are present in much smaller quantities in the cell.
Trace elements form halogen and metal ions (copper, cobalt, zinc, bromine, iodine). Microelements in a living cell are contained in very small quantities (0.00001%). Ultra microelements include mercury, gold, selenium (less than 0.000001% of cell mass).
Chemical compounds
Chemical compounds are substances that consist of joined atoms of two or more chemical elements. Chemical compounds are divided into two groups:
Organic chemical compounds (mineral salts and water);
Inorganic chemical compounds (combination of chemical elements with carbon).
The main organic chemical compounds include proteins, carbohydrates, fats and nucleic acids.
Concept and functions of proteins
Proteins are high-molecular organic substances consisting of alpha amino acid residues connected by peptide bonds. Proteins are a very important substance in human life.
Functions of proteins:
Protective function. When a virus enters the body, the protein begins increased synthesis in the body, thereby eliminating it.
Structural function. Due to the content of such a component as collagen in proteins, the process of scarring damage occurs in the human body.
Motor. Protein is directly involved in the process of muscle tissue contraction.
Transport. Protein atoms carry oxygen and nutrients to cells.
Energy. When protein breaks down, it releases the energy a person needs for life.
In addition, proteins also act as catalysts for all chemical reactions occurring in the body.
Carbohydrates, their role in life
Carbohydrates– organic poly- and monomers containing oxygen, hydrogen and carbon. The main function of carbohydrates is the energy function - the breakdown of 1g of carbohydrates releases 17 kJ of energy.
Carbohydrates in the form of cellulose form the walls of many plant species. Thanks to carbohydrates, living organisms also store nutrients, which are stored in the form of starch.
Fats
Organic chemicals also include fats. Fats are divided into two groups: complex and simple. Simple lipids consist of fatty acid residues and glycerol. Complex fats are the synthesis of simple lipids with carbohydrates and proteins.
Chemical cell contents. The cells of living beings differ significantly from their environment not only in the structure of the chemical compounds that make up them, but also in the set and content of chemical elements. Of the currently known chemical elements, about 90 have been found in living nature. Depending on the content of these elements in the organisms of living beings, they can be divided into three groups:
1) macronutrients, that is, elements contained in cells in significant quantities (from tens of percent to hundredths of a percent). This group includes oxygen, carbon, nitrogen, sodium, calcium, phosphorus, sulfur, potassium, chlorine. Together, these elements make up about 99% of the mass of cells, with 98% coming from the first four elements (hydrogen, oxygen, carbon and nitrogen).
2) microelements, which account for less than hundredths of a percent of the mass. These elements include iron, zinc, manganese, cobalt, copper, nickel, iodine, fluorine. In total they make up about 1% of the cell mass. Despite the fact that the content of these elements in the cell is small, they are necessary for its life. In the absence or low content of such elements, various diseases occur. A lack of iodine, for example, leads to thyroid disease in humans, and a lack of iron can cause anemia.
3) ultramicroelements, the content of which in the cell is extremely small (less than 10-12%). This group includes bromine, gold, selenium, silver, vanadium and many other elements. Most of these elements are also necessary for the normal functioning of organisms. For example, a deficiency of selenium leads to cancer, and a deficiency of boron causes disease in plants. Some elements of this group, like microelements, are part of enzymes.
Unlike living organisms, the most common elements in the earth's crust are oxygen, silicon, aluminum and sodium. Since the content of carbon, hydrogen and nitrogen in living matter is higher than in the earth’s crust, we can conclude that the molecules that contain these elements are necessary for the implementation of processes that ensure life.
The four most common elements in living matter have one thing in common: they easily form covalent bonds by pairing electrons. In order to form stable electronic bonds, the hydrogen atom on the outer electron shell lacks one electron, the oxygen atom - two, nitrogen - three and carbon - four electrons. These elements can easily react with each other, filling the outer electron shells. In addition, three elements: nitrogen, oxygen and carbon are capable of forming both single and double bonds, due to which the number of chemical compounds built from these elements significantly increases.
Carbon, hydrogen and oxygen have proven to be suitable for the formation of living matter also because they are the lightest among the elements that form covalent bonds. From a biological point of view, the ability of a carbon atom to form covalent bonds with four other carbon atoms at once is also very important. Thus, covalently bonded carbon atoms are capable of forming frameworks for a huge number of different organic molecules.
And other inorganic substances, their role in the life of cells. Most of the chemical compounds that make up a cell are characteristic only of living organisms. However, the cell contains a number of substances that are also found in inanimate nature. This is primarily water, which on average makes up about 80% of the cell mass (its content may vary depending on the type of cell and its age), as well as some salts.
Water is an extremely unusual substance in physical and chemical terms, which differs significantly in properties from other solvents. The first cells arose in the primordial ocean and, in the process of further development, learned to use these unique properties of water.
Compared to other liquids, water is characterized by an unusually high boiling point, melting point, specific heat capacity, as well as high heat of evaporation, fusion, thermal conductivity and surface tension. This is due to the fact that water molecules are more tightly bound to each other than molecules of other solvents.
The high heat capacity of water (the ability to absorb heat with a slight change in its own temperature) protects the cell from sudden temperature fluctuations, and the property of water, such as the high heat of evaporation, is used by living organisms to protect against overheating: evaporation of liquid by plants and animals is a protective reaction to rising temperatures . The presence of high thermal conductivity in water makes it possible to distribute heat evenly between individual parts of the body. Water is practically incompressible, due to which cells maintain their shape and are characterized by elasticity.
The unique properties of water are determined by the structural features of its molecule, which arise as a result of the specific arrangement of electrons in the oxygen and hydrogen atoms that make up the molecule. The oxygen atom, which has two electrons in its outer electron orbit, combines them with two electrons of hydrogen atoms (each hydrogen atom has one electron in its outer electron orbit). As a result, two covalent bonds are formed between an oxygen atom and two hydrogen atoms. However, the more negative oxygen atom tends to attract electrons to itself. As a result, each of the hydrogen atoms acquires a small positive charge, and the oxygen atom carries a negative charge. The negatively charged oxygen atom of one water molecule is attracted to the positively charged hydrogen atom of another molecule, resulting in the formation of a hydrogen bond. Thus, water molecules become connected to each other.
An important property of a hydrogen bond is its lower strength compared to (it is approximately 20 times weaker than a covalent bond). Therefore, hydrogen bonds are relatively easy to form and easy to break. However, even at 100° there is still quite a strong interaction between water molecules. The presence of hydrogen bonds between water molecules provides it with some structure, which explains its unusual properties such as high boiling, melting and high heat capacity.
Another characteristic property of a water molecule is its dipole. As mentioned above, the hydrogen atoms in a water molecule carry a small positive charge, and the oxygen atoms carry a negative charge. However, the H-O-H bond angle is 104.5°, so in a water molecule the negative charge is concentrated on one side and the positive charge on the other. The dipole of a water molecule characterizes its ability to navigate in an electric field. It is this property of water that determines its uniqueness as a solvent: if the molecules of substances contain charged groups of atoms, they enter into electrostatic interactions with water molecules, and these substances dissolve in it. Such substances are called hydrophilic. Cells contain a large number of hydrophilic compounds: these are salts, low molecular weight organic compounds, carbohydrates, and nucleic acids. However, there are a number of substances that contain almost no charged atoms and do not dissolve in water. These compounds include, in particular, lipids (fats). Such substances are called hydrophobic. Hydrophobic substances do not interact with water, but interact well with each other. Lipids, which are hydrophobic compounds, form two-dimensional structures (membranes) that are almost impermeable to water.
Because of its polarity, water dissolves more chemicals than any other solvent. It is in the aqueous environment of the cell, where various chemical substances are dissolved, that numerous chemical reactions take place, without which life activity is impossible. Water also dissolves reaction products and removes them from cells and multicellular organisms. Due to the movement of water in the organisms of animals and plants, various substances are exchanged between tissues.
One of the important properties of water as a chemical compound is that it participates in many chemical reactions occurring in the cell. These reactions are called hydrolysis reactions. In turn, water molecules are formed as a result of many reactions occurring in living organisms.
The mass of a hydrogen atom is very small; its only electron in a water molecule is held by an oxygen atom. As a result, the nucleus of a hydrogen atom (proton) is able to break away from a water molecule, resulting in the formation of a hydroxyl ion (OH -) and a proton (H +).
H2O<=>H + + OH -
This process is called water dissociation. Hydroxyl and hydrogen ions formed during the dissociation of water are also participants in many important reactions occurring in the body.
In addition to water, an important role in the life of the cell is played by the dissolved in it, which are represented by cations of potassium, sodium, magnesium, calcium and others, as well as anions of hydrochloric, sulfuric, carbonic and phosphoric acids.
Many cations are characterized by an uneven distribution between the cell and its environment: for example, in the cytoplasm of the cell the concentration of K+ is higher, and the concentration of Na + and Ca 2+ is lower than in the environment surrounding the cell. External to the cell can be either the natural environment (for example, the ocean) or body fluids (blood), which are close in ionic composition to sea water. The uneven distribution of cations between the cell and the environment is maintained during life; the cell spends a significant part of the energy generated in it on this. The uneven distribution of ions between the cell and the environment is necessary for the implementation of many processes important for life, in particular for the conduction of excitation through nerve and muscle cells and muscle contraction. After cell death, the concentration of cations outside and inside the cell quickly equalizes.
The anions of weak acids contained in the cell (HC0 3 -, HPO 4 2-) play an important role in maintaining a constant concentration of hydrogen ions (pH) inside the cell. Despite the fact that during the life of a cell both alkalis and acids are formed, normally the reaction in the cell is almost neutral. This is due to the fact that anions of weak acids can bind protons of acids and hydroxyl ions of alkalis, thus neutralizing the intracellular environment. In addition, anions of weak acids enter into chemical reactions carried out in the cell: in particular, anions of phosphoric acid are necessary for the synthesis of a compound so important for the cell as ATP.
Inorganic substances are found in living organisms not only in dissolved but also in solid states. For example, bones are formed mainly from calcium phosphate (magnesium phosphate is also present in smaller quantities), and shells are formed from calcium carbonate.
Organic substances of the cell. Biopolymers
Living organisms contain a huge variety of compounds that are practically never found in inanimate nature and are called organic compounds. The molecular frameworks of these compounds are built from carbon atoms. Among organic compounds, low molecular weight substances can be distinguished (organic acids, their esters, amino acids, free fatty acids, nitrogenous bases, etc.). However, the bulk of the dry matter of the cell is represented by high-molecular compounds, which are polymers. Polymers are compounds formed from low molecular weight repeating units (monomers) sequentially linked to each other by a covalent bond and forming a long chain, which can be either straight or branched. Among polymers, homopolymers are distinguished, consisting of identical monomers. If we designate a monomer with some symbol, for example the letter X, then the structure of the homopolymer can be conventionally represented as follows: -Х-Х-…-Х-Х. Heteropolymers contain monomers of different structures. If the monomers that make up the heteropolymer are designated as X and Y, then the structure of the heteropolymer can be represented, for example, in the form XXUUKHU...XXUUKHY. Biopolymers (that is, polymers found in nature) include proteins, nucleic acids, and carbohydrates.
Squirrels
Protein structure. Among the organic compounds present in the cell, the main ones are proteins: they account for at least 50% of the dry matter. All proteins contain carbon, hydrogen, oxygen, and nitrogen. In addition, almost all of them contain sulfur. Some proteins also contain phosphorus, iron, magnesium, zinc, copper, and manganese. Thus, iron is part of the hemoglobin protein found in the red blood cells of many animals, and magnesium is found in the pigment chlorophyll, necessary for photosynthesis.
A characteristic feature of proteins is their large molecular weight: it ranges from several thousand to hundreds of thousands and even millions of kilodaltons. The monomer, that is, the structural unit of any protein, is amino acids, which are characterized by a similar, but not exactly the same structure.
As can be seen from the presented formula, the amino acid molecule consists of two parts. The part circled is the same for all amino acids. It contains an amino group (-NH 2) attached to a carbon atom, followed by a carboxyl group (-COOH). The second part of the amino acid molecule, depicted in the formula as the Latin letter R, is called the side chain, or radical. It has a different structure for different amino acids. Proteins contain 20 different amino acids as structural elements (monomers), so proteins can contain 20 structurally different side chains. Side radicals can be negatively or positively charged, contain aromatic rings and heterocyclic structures, hydrophobic groups, hydroxyl (-OH) groups or sulfur atoms.
In protein molecules, successive amino acid molecules are linked to each other covalently, forming long, unbranched polymer chains. The amino acids in the chain are arranged in such a way that the amino group of one amino acid interacts with the carboxyl group of another. When these two groups interact, a water molecule is released and a peptide bond is formed. The resulting compound is called a peptide. If a peptide consists of two amino acids, it is called a dipeptide; if it consists of three, it is called a tripeptide. Protein molecules can contain hundreds and even thousands of amino acid residues. Thus, proteins are polypeptides. It should be noted that protein molecules are not randomly constructed polymers of varying lengths - each protein molecule is characterized by a specific sequence of amino acids, which is determined by the structure of the gene encoding the protein.
The sequence of amino acid residues in a protein molecule determines its primary structure, that is, its formula. Just as the alphabet, which contains 33 letters, allows you to create a huge number of words, with the help of 20 amino acids you can create an almost unlimited number of proteins, differing both in the number of amino acids they contain and in their sequence. The total number of different proteins found in all types of living organisms is on the order of 10 10 -10 12 . The most important task of modern biology is to determine the primary structure of proteins, as well as to establish the relationship between the primary structure and the functional activity of proteins. Since the sequence of amino acids is determined by the structure of the gene, the primary structure of proteins is currently determined by determining the sequence of nucleotides in the corresponding gene, using genetic engineering methods.
A protein molecule in its native (undamaged) state has a characteristic spatial structure, or conformation. It is determined by how the polypeptide chain of a protein folds in solution. Most often, individual sections of the polypeptide chain fold into a helix (α-helix) or form zigzag structures located antiparallel - the so-called folded layer, or β-structure. The formation of the α-helix and β-structure leads to the formation of the secondary structure of the protein. In this case, the side chains of amino acids are located on the outside of the helix or zigzag structure. The helical structure is stabilized by hydrogen bonds that form between NH groups located on one turn and CO groups located on the other turn of the helix. These hydrogen bonds are parallel to the axis of the helix.
The folded layer type structure is also stabilized by hydrogen bonds that form between parallel layers. Although hydrogen bonds are weaker than covalent bonds, their presence in significant quantities makes structures such as an α-helix or β-sheet layer quite strong.
Helical regions and structures such as the folded layer undergo further packaging, resulting in the formation of the tertiary structure of the protein. At this stage, soluble proteins usually form a globular coil-like structure, in which charged amino acid residues are on the surface and hydrophobic amino acid residues are inside the coil. In this case, amino acid residues that are located far from each other in the polypeptide chain often come together. Each protein is characterized by its own packaging method, which is set at the level of the primary structure of the protein, that is, it depends on the order of amino acids in the polypeptide chain.
Many proteins consist of several polypeptide chains of the same or different structures. When such chains combine, a complex protein is formed, which is characterized by a quaternary structure. Such proteins are called oligomers, and the individual polypeptide chains included in the oligomer are called monomers.
Most protein molecules are capable of maintaining their biological activity, that is, the ability to perform their inherent function only in a narrow range of temperatures and acidity of the environment. When the temperature increases or the acidity changes to extreme values, changes occur in the structure of proteins, which are called denaturation. An example of denaturation is the coagulation of egg whites that occurs during boiling. During denaturation, covalent bonds are not broken, but the quaternary, tertiary and secondary structure characteristic of a given protein is destroyed, as a result of which, in a denatured state, the polypeptide chains of proteins form random and disordered coils and loops.
Functions of proteins. Proteins are characterized by a significant diversity of functions. The largest and most biologically important group of proteins are enzyme proteins, which are catalysts that accelerate the occurrence of various chemical reactions.
The second largest group of proteins is represented by proteins that are the structural elements of the cell. These, for example, include the fibrillar protein collagen, the main structural protein that is part of connective and bone tissue. Other types of proteins are components of the contractile and motor systems. These are, for example, actin and myosin, two main elements of the contractile system of muscles. The cytoskeleton of the cell is formed from structural proteins, which are bundles of fibrillar proteins that connect various organelles of the cell with each other and with the plasma membrane of the cell.
Some proteins perform a transport function; they are able to bind and transport various substances through the bloodstream. The most famous of these proteins is hemoglobin, which is found in the red blood cells of vertebrates and, by binding with oxygen, carries it from the lungs to the tissues. Serum lipoproteins transport complex lipids in the bloodstream, and serum albumin transports free fatty acids.
Transport proteins also include proteins embedded in biological membranes and carrying out the transfer of various substances across these membranes. Under normal conditions, the cell membrane is weakly permeable to substances such as K +, Na +, Ca 2+, since the pores formed by channel proteins are closed. However, some influences, such as electrical impulses or biologically active substances that bind to the channels, open the pore, as a result of which the ion that can penetrate through this channel moves from one side of the membrane to the other in the direction of decreasing concentration. The movement of ions in the opposite direction is carried out with the expenditure of energy by other membrane transport proteins called ion pumps.
In specialized cells of plants and animals, the synthesis of special regulators or hormones is carried out, some of which (but not all) are proteins that regulate various physiological processes. The most famous of these is perhaps insulin, a hormone produced in the pancreas that regulates glucose levels in the body's cells. When there is not enough insulin in the body, a disease known as diabetes mellitus occurs.
In addition, proteins are capable of performing a protective function. When viruses, bacteria, foreign proteins or other polymers enter the body of animals or humans, the body synthesizes special protective proteins called antibodies or immunoglobulins. These proteins bind to foreign polymers. The binding of antibodies to proteins of viruses or bacteria suppresses their functional activity and stops the development of infection. Antibodies have a unique property: they are able to distinguish foreign proteins from the body’s own proteins. This mechanism of protecting the body from pathogens is called immunity. Immunity to infectious diseases can be created by injecting very small amounts of certain biopolymers that are part of the microorganisms or viruses that cause the disease. In this case, antibodies are formed, which are subsequently able to protect the body if it becomes infected with this microorganism or virus. To provide protection, many living things secrete proteins called toxins, which in most cases are strong poisons.
With a lack of nutrition in animals, the breakdown of proteins into their constituent amino acids sharply increases; the latter, after appropriate transformations, can be used as a source of energy (energy function of proteins).
Some bacteria and all plants are capable of synthesizing all 20 amino acids that make up proteins. However, in the process of evolution, animals have lost the ability to synthesize 10 particularly complex amino acids, which they must obtain from plant and animal foods. These amino acids are called essential. They are part of plant and animal proteins obtained from food, which are broken down into amino acids in the digestive tract. In cells, these amino acids are used to build their own proteins characteristic of a given organism. The lack of essential amino acids in food causes severe metabolic disorders.
And their role in the process of life. At the temperature and acidity of the environment that is characteristic of a cell, the rate of most chemical reactions is low. However, in reality, reactions in a cell occur at a very high speed. This is achieved due to the presence in the cell of special catalysts - enzymes, which significantly increase the rate of chemical reactions. Enzymes are the largest and most specialized class of proteins. It is enzymes that ensure the occurrence of numerous reactions in the cell that make up cellular metabolism. Currently, more than a thousand enzymes are known. Their catalytic efficiency is unusually high: they are capable of accelerating reactions millions of times.
The catalytic activity of an enzyme is determined not by its entire molecule, but by a certain part of the enzyme molecule, which is called its active center. It is known that chemical catalysis is most often carried out due to the formation of a complex of the substance (substrate) converted during the reaction with the catalyst. And during the enzymatic reaction, the substrate interacts with the enzyme, and the binding of the substrate occurs precisely in the active center. Enzymes are characterized by a spatial correspondence between the substrate and the active center; they fit together “like a key to a lock.” Thus, enzymes are characterized by substrate specificity, so each enzyme ensures the occurrence of one or more reactions of the same type.
The binding of a substrate to an enzyme (formation of an enzyme-substrate complex) is accompanied by a redistribution of electron energy surrounding the substance (substrate) converted during the reaction due to interaction with the amino acids of the enzyme, which participate in the formation of the active center. As a result, individual bonds between atoms in the substrate molecule are weakened and destroyed much more easily than in solution. In other cases (reactions in which a bond is formed), two substrate molecules are brought so close together in the active center of the enzyme that a bond is easily formed between them. When an enzyme is denatured, its catalytic activity disappears because the structure of the active center is disrupted.
Many enzymes contain so-called cofactors - low molecular weight organic or inorganic compounds capable of carrying out certain types of reactions. Cofactors include, for example, the dinucleotide NAD (nicotinamide adenine dinucleotide), which ensures the dehydrogenation of various substrates. Its functions will be discussed in detail in the “Energy Exchange” section. A large number of enzymes are also known, which contain metals (iron, copper, cobalt, manganese), which are also involved in the transformation of substrates in the process of the catalytic act.
Nucleic acids
Another important class of biopolymers are nucleic acids, which are genetic carriers and also take part in the process of protein synthesis. Two types of nucleic acids have been found in living nature, namely: deoxyribonucleic acid(abbreviated DNA) and ribonucleic acid(RNA). DNA and RNA are found in all prokaryotes and eukaryotes, with the exception of viruses, some of which contain only RNA, while others contain only DNA. DNA and RNA are made up of monomers called mononucleotides. The mononucleotides that make up DNA and RNA have a similar, but not the same structure. Mononucleotides consist of three main components: 1) nitrogenous base, 2) pentose sugars and 3) phosphoric acid.
Mononucleotides that make up DNA contain the five-carbon sugar deoxyribose and one of four nitrogenous bases: adenine, guanine, cytosine And thymine(abbreviated as A, G, C and T).
Mononucleotides that make up RNA contain a five-carbon saccharibose, as well as one of four bases: adenine, guanine, cytosine And uracil(abbreviated as A, G, C and U).
Deoxyribonucleic acid (DNA). DNA is the carrier of genetic information and is concentrated in the cell mainly in the nucleus, where it is the main component of chromosomes (in eukaryotes, DNA is also found in mitochondria and chloroplasts). DNA is a polymer consisting of covalently linked mononucleotides, which include deoxyribose and four nitrogenous bases (adenine, guanine, cytosine and thymine). The number of mononucleotides that make up DNA is very large: in prokaryotic cells containing a single chromosome, all DNA is present in the form of one macromolecule with a molecular weight of more than 2 * 10 9 .
The structure of the DNA molecule was deciphered by Watson and Crick in 1953. The DNA molecule consists of two strands located parallel to each other and forming a right-handed helix. The width of the helix is about 2 nm, while the length can reach hundreds of thousands of nanometers. Mononucleotides that are part of one chain are connected sequentially due to the formation of covalent bonds between the deoxyribose of one and the phosphoric acid of another mononucleotide. The nitrogenous bases, which are located on one side of the resulting backbone of one DNA strand, form hydrogen bonds with the nitrogenous bases of the second strand. Thus, in a helical double-stranded DNA molecule, the nitrogenous bases are located inside the helix. The structure of the helix is such that the polynucleotide chains included in its composition can be separated only after unwinding of the helix.
The DNA molecule is structured in such a way that the number of nitrogenous bases of one type (adenine and guanine) included in its composition is equal to the number of nitrogenous bases of another type (thymine and cytosine), that is, A+G=T+C. This is due to the size of the nitrogenous bases: the length of the structure formed during the formation of a hydrogen bond between adenine-thymine and guanine-cytosine pairs is approximately 11 A. The sizes of these pairs correspond to the size of the inner part of the DNA helix. The pair A-G would be too large and C-T too small to form a spiral. Thus, the nitrogenous base located in one strand of DNA determines the base located at the same place in the other strand. The strict correspondence of nucleotides located parallel to each other in the paired chains of a DNA molecule is called complementarity. It is thanks to this property of the DNA molecule that accurate reproduction (replication) of genetic information is possible. In a cell, DNA replication (self-duplication) occurs as a result of the breaking of hydrogen bonds between the nitrogenous bases of neighboring DNA chains and the subsequent synthesis of two new (daughter) DNA molecules using the parent chains as a template. Such reactions were called matrix synthesis reactions.
Ribonucleic acid. RNA is a polymer consisting of covalently linked mononucleotides, which include ribose and four nitrogenous bases (adenine, guanine, cytosine and uracil). There are three different types of ribonucleic acids in cells: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). All three types of RNA molecules are single-stranded. And they all have a significantly lower molecular weight than DNA molecules. In most cells, the RNA content is many times (5 to 10) times higher than the DNA content. All three types of RNA are necessary to ensure protein synthesis in the cell.
Messenger RNA. Messenger RNA is synthesized in the nucleus during the transcription process, during which template synthesis of an RNA molecule is provided on one of the DNA strands. An mRNA molecule consists of approximately 300-30,000 nucleotides and is a structure complementary to a specific section of a single-stranded DNA molecule (gene). After synthesis, mRNA moves into the cytoplasm, where it attaches to ribosomes and is used as a template that determines the sequence of amino acids in the growing polypeptide chain. Thus, the sequence of nucleotides in the DNA chain, and then the mRNA synthesized using it as a template, determines the sequence of amino acids in the synthesized protein. Each of the thousands of proteins synthesized by a cell is encoded by a specific mRNA.
Transfer RNA. The function of tRNA is to transport certain amino acids to the newly synthesized polypeptide chain during protein synthesis carried out on ribosomes. The molecular weight of tRNA is small: the molecules contain from 75 to 90 mononucleotides.
Ribosomal RNA. Ribosomal RNA is part of ribosomes, the organelles that carry out protein synthesis. rRNA molecules consist of 3-5 thousand mononucleotides.
Carbohydrates
Carbohydrates, or saccharides, are compounds with the general formula (CH 2 O) n, which are aldehyde alcohols or keto alcohols. Carbohydrates are divided into mono-, di- and polysaccharides.
Monosaccharides, or simple sugars, most often consist of a thread (pentose) or six (hexose) carbon atoms and have co-(generally the formulas (CH 2 O) 5 and (CH 2 O) 6.
The most common simple sugar is the six-carbon sugar glucose, which is the parent monomer from which many polysaccharides are built. Glucose is also the main source of energy in the cell. Pentoses (ribose and deoxyribose) are part of nucleic acids and ATP.
A disaccharide molecule combines two simple sugars. The most famous representatives of disaccharides are sucrose, or table sugar, the molecule of which consists of glucose and fructose molecules.
Polysaccharide molecules are long chains built from many monosaccharide units, and the chains can be either linear or branched. Most polysaccharides contain repeating units of the same type or two alternating types as monomers, so they cannot serve as informational biopolymers.
Living nature contains a huge amount of carbohydrates. This is primarily due to the widespread occurrence of two polysaccharides: starch and cellulose. Starch is found in large quantities in plants. It is the form of polysaccharide in which fuel is stored. Cellulose is the main component of extracellular fibrous and lignified plant tissues. In the digestive tract of animals there are no enzymes capable of breaking down cellulose into monomers. However, these enzymes are present in bacteria that live in the digestive tract of some animals, allowing them to use cellulose as food
Polysaccharides are part of the hard walls of plant and bacterial cells; they are also a component of the softer membranes of animal cells. Thus, carbohydrates perform two main functions in the cell: energy and construction.
Lipids
Lipids are water-insoluble organic compounds found in cells. These substances can be extracted (dissolved) with non-polar solvents such as chloroform, benzene or ether. Several classes of lipids are known, but the most important function in the body is apparently performed by phospholipids, which are esters of the trihydric alcohol glycerol and phosphoric acid. When a phospholipid molecule is formed, two hydroxyl groups of glycerol interact with high-molecular fatty acids containing 16-18 carbon atoms, and one hydroxyl group interacts with phosphoric acid. All phospholipid molecules contain a polar head and a nonpolar tail formed by two fatty acid molecules. At the oil-water interface, phospholipid molecules are oriented so that their polar heads are immersed in water and their hydrophobic tails are immersed in oil. Phospholipids spread over the surface of water in the form of a monolayer, in which the fatty acid tails are oriented towards the relatively hydrophobic air, and the charged heads are directed towards the aquatic environment.
Phospholipid molecules are capable of forming two-dimensional structures, which are called bilayers: the bilayer is formed from two monolayers of phospholipids, oriented relative to each other so that the hydrophobic tails of the phospholipids are located inside the bilayer, and the polar heads are directed outward. This bilayer is characterized by very high electrical resistance. It is bilayers consisting of phospholipids that are the most important component of biological membranes. Biological membranes are natural films 5-7 nm thick, formed by a phospholipid bilayer containing protein molecules. Thus, lipids perform a construction function in the cell.
In addition, lipids are an important source of energy. with the complete conversion of 1 g of lipids into water and carbon dioxide in a cell, approximately 2 times more energy is released than with the same conversion of carbohydrates. Fat accumulated in subcutaneous tissue is a good heat-insulating material. In addition, lipids are a source of water, which is released in significant quantities during their oxidation. This is why many animals that store fat (for example, camels during desert crossings, bears, marmots, and gophers during hibernation) can go without water for a long time.
Some substances related to lipids have high biological activity: these are a number of vitamins, for example vitamins A and B, as well as some hormones (steroid). An important function in the body of animals is performed by cholesterol, which is a component of cell membranes: improper metabolism of cholesterol in humans leads to the occurrence of atherosclerosis, a disease in which cholesterol is deposited in the form of plaques on the walls of blood vessels, narrowing their lumen. This leads to disruption of the blood supply to organs and is the cause of severe cardiovascular diseases such as stroke or myocardial infarction.
Cell- the elementary unit of life on Earth. It has all the characteristics of a living organism: it grows, reproduces, exchanges substances and energy with the environment, and reacts to external stimuli. The beginning of biological evolution is associated with the appearance of cellular life forms on Earth. Unicellular organisms are cells that exist separately from each other. The body of all multicellular organisms - animals and plants - is built from a greater or lesser number of cells, which are a kind of blocks that make up a complex organism. Regardless of whether a cell is an integral living system - a separate organism or constitutes only a part of it, it is endowed with a set of characteristics and properties common to all cells.
Chemical composition of the cell
About 60 elements of Mendeleev's periodic table, which are also found in inanimate nature, were found in cells. This is one of the proofs of the commonality of living and inanimate nature. In living organisms, the most abundant are hydrogen, oxygen, carbon and nitrogen, which make up about 98% of the mass of cells. This is due to the peculiar chemical properties of hydrogen, oxygen, carbon and nitrogen, as a result of which they turned out to be most suitable for the formation of molecules that perform biological functions. These four elements are capable of forming very strong covalent bonds by pairing electrons belonging to two atoms. Covalently bonded carbon atoms can form the frameworks of countless different organic molecules. Since carbon atoms readily form covalent bonds with oxygen, hydrogen, nitrogen, and sulfur, organic molecules achieve exceptional complexity and structural diversity.
In addition to the four main elements, the cell contains noticeable amounts (10th and 100th fractions of a percent) of iron, potassium, sodium, calcium, magnesium, chlorine, phosphorus and sulfur. All other elements (zinc, copper, iodine, fluorine, cobalt, manganese, etc.) are found in the cell in very small quantities and are therefore called trace elements.
Chemical elements are part of inorganic and organic compounds. Inorganic compounds include water, mineral salts, carbon dioxide, acids and bases. Organic compounds are proteins, nucleic acids, carbohydrates, fats (lipids) and lipoids. In addition to oxygen, hydrogen, carbon and nitrogen, they may contain other elements. Some proteins contain sulfur. Phosphorus is a component of nucleic acids. The hemoglobin molecule includes iron, magnesium is involved in the construction of the chlorophyll molecule. Microelements, despite their extremely low content in living organisms, play an important role in life processes. Iodine is part of the thyroid hormone - thyroxine, cobalt is part of vitamin B 12, the hormone of the islet part of the pancreas - insulin - contains zinc. In some fish, copper takes the place of iron in the oxygen-carrying pigment molecules.
Inorganic substances
Water
H 2 O is the most common compound in living organisms. Its content in different cells varies quite widely: from 10% in tooth enamel to 98% in the body of a jellyfish, but on average it makes up about 80% of body weight. The extremely important role of water in supporting life processes is due to its physicochemical properties. The polarity of molecules and the ability to form hydrogen bonds make water a good solvent for a huge number of substances. Most chemical reactions occurring in a cell can only occur in an aqueous solution. Water is also involved in many chemical transformations.
The total number of hydrogen bonds between water molecules varies depending on t °. At t ° When ice melts, approximately 15% of hydrogen bonds are destroyed, at t° 40°C - half. Upon transition to the gaseous state, all hydrogen bonds are destroyed. This explains the high specific heat capacity of water. When the temperature of the external environment changes, water absorbs or releases heat due to the rupture or new formation of hydrogen bonds. In this way, fluctuations in temperature inside the cell turn out to be smaller than in the environment. The high heat of evaporation underlies the efficient mechanism of heat transfer in plants and animals.
Water as a solvent takes part in the phenomena of osmosis, which plays an important role in the life of the body's cells. Osmosis is the penetration of solvent molecules through a semi-permeable membrane into a solution of a substance. Semi-permeable membranes are those that allow solvent molecules to pass through, but do not allow solute molecules (or ions) to pass through. Therefore, osmosis is the one-way diffusion of water molecules in the direction of the solution.
Mineral salts
Most of the inorganic substances in cells are in the form of salts in a dissociated or solid state. The concentration of cations and anions in the cell and in its environment is not the same. The cell contains quite a lot of K and a lot of Na. In the extracellular environment, for example in blood plasma, in sea water, on the contrary, there is a lot of sodium and little potassium. Cell irritability depends on the ratio of concentrations of Na +, K +, Ca 2+, Mg 2+ ions. In the tissues of multicellular animals, K is part of the multicellular substance that ensures the cohesion of cells and their ordered arrangement. The osmotic pressure in the cell and its buffering properties largely depend on the concentration of salts. Buffering is the ability of a cell to maintain the slightly alkaline reaction of its contents at a constant level. Buffering inside the cell is provided mainly by H 2 PO 4 and HPO 4 2- ions. In extracellular fluids and in the blood, the role of a buffer is played by H 2 CO 3 and HCO 3 -. Anions bind H ions and hydroxide ions (OH -), due to which the reaction inside the cell of extracellular fluids remains virtually unchanged. Insoluble mineral salts (for example, Ca phosphate) provide strength to the bone tissue of vertebrates and mollusk shells.
Organic cell matter
Squirrels
Among the organic substances of the cell, proteins are in first place both in quantity (10 - 12% of the total mass of the cell) and in importance. Proteins are high-molecular polymers (with a molecular weight from 6000 to 1 million and above), the monomers of which are amino acids. Living organisms use 20 amino acids, although there are many more. The composition of any amino acid includes an amino group (-NH 2), which has basic properties, and a carboxyl group (-COOH), which has acidic properties. Two amino acids are combined into one molecule by establishing an HN-CO bond, releasing a water molecule. The bond between the amino group of one amino acid and the carboxyl group of another is called a peptide bond. Proteins are polypeptides containing tens and hundreds of amino acids. Molecules of various proteins differ from each other in molecular weight, number, composition of amino acids and the sequence of their location in the polypeptide chain. It is therefore clear that proteins are extremely diverse; their number in all types of living organisms is estimated at 10 10 - 10 12.
A chain of amino acid units connected covalently by peptide bonds in a specific sequence is called the primary structure of the protein. In cells, proteins look like spirally twisted fibers or balls (globules). This is explained by the fact that in natural protein the polypeptide chain is laid out in a strictly defined way, depending on the chemical structure of its constituent amino acids.
First, the polypeptide chain folds into a spiral. Attraction occurs between atoms of neighboring turns and hydrogen bonds are formed, in particular, between NH and CO groups located on adjacent turns. A chain of amino acids, twisted in the form of a spiral, forms the secondary structure of the protein. As a result of further folding of the helix, a configuration specific to each protein arises, called the tertiary structure. The tertiary structure is due to the action of cohesive forces between hydrophobic radicals present in some amino acids and covalent bonds between the SH groups of the amino acid cysteine (S-S bonds). The number of amino acids with hydrophobic radicals and cysteine, as well as the order of their arrangement in the polypeptide chain, are specific to each protein. Consequently, the features of the tertiary structure of a protein are determined by its primary structure. The protein exhibits biological activity only in the form of a tertiary structure. Therefore, replacing even one amino acid in a polypeptide chain can lead to a change in the configuration of the protein and to a decrease or loss of its biological activity.
In some cases, protein molecules combine with each other and can only perform their function in the form of complexes. Thus, hemoglobin is a complex of four molecules and only in this form is it capable of attaching and transporting oxygen. Such aggregates represent the quaternary structure of the protein. Based on their composition, proteins are divided into two main classes - simple and complex. Simple proteins consist only of amino acids, nucleic acids (nucleotides), lipids (lipoproteins), Me (metalloproteins), P (phosphoproteins).
The functions of proteins in a cell are extremely diverse. One of the most important is the construction function: proteins are involved in the formation of all cell membranes and cell organelles, as well as intracellular structures. The enzymatic (catalytic) role of proteins is extremely important. Enzymes accelerate chemical reactions occurring in the cell by 10 and 100 million times. Motor function is provided by special contractile proteins. These proteins are involved in all types of movements that cells and organisms are capable of: the flickering of cilia and the beating of flagella in protozoa, muscle contraction in animals, the movement of leaves in plants, etc. The transport function of proteins is to attach chemical elements (for example, hemoglobin adds O) or biologically active substances (hormones) and transfer them to the tissues and organs of the body. The protective function is expressed in the form of the production of special proteins, called antibodies, in response to the penetration of foreign proteins or cells into the body. Antibodies bind and neutralize foreign substances. Proteins play an important role as sources of energy. With complete splitting 1g. proteins are released 17.6 kJ (~4.2 kcal).
Carbohydrates
Carbohydrates, or saccharides, are organic substances with the general formula (CH 2 O) n. Most carbohydrates have twice the number of H atoms as the number of O atoms, as in water molecules. That's why these substances were called carbohydrates. In a living cell, carbohydrates are found in quantities not exceeding 1-2, sometimes 5% (in the liver, in the muscles). Plant cells are the richest in carbohydrates, where their content in some cases reaches 90% of the dry matter mass (seeds, potato tubers, etc.).
Carbohydrates are simple and complex. Simple carbohydrates are called monosaccharides. Depending on the number of carbohydrate atoms in the molecule, monosaccharides are called trioses, tetroses, pentoses or hexoses. Of the six carbon monosaccharides - hexoses - the most important are glucose, fructose and galactose. Glucose is contained in the blood (0.1-0.12%). The pentoses ribose and deoxyribose are found in nucleic acids and ATP. If two monosaccharides are combined in one molecule, the compound is called a disaccharide. Table sugar, obtained from cane or sugar beets, consists of one molecule of glucose and one molecule of fructose, milk sugar - of glucose and galactose.
Complex carbohydrates formed from many monosaccharides are called polysaccharides. The monomer of polysaccharides such as starch, glycogen, cellulose is glucose. Carbohydrates perform two main functions: construction and energy. Cellulose forms the walls of plant cells. The complex polysaccharide chitin serves as the main structural component of the exoskeleton of arthropods. Chitin also performs a construction function in fungi. Carbohydrates play the role of the main source of energy in the cell. During the oxidation of 1 g of carbohydrates, 17.6 kJ (~4.2 kcal) is released. Starch in plants and glycogen in animals are deposited in cells and serve as an energy reserve.
Nucleic acids
The importance of nucleic acids in a cell is very great. The peculiarities of their chemical structure provide the possibility of storing, transferring and inheriting to daughter cells information about the structure of protein molecules that are synthesized in each tissue at a certain stage of individual development. Since most of the properties and characteristics of cells are determined by proteins, it is clear that the stability of nucleic acids is the most important condition for the normal functioning of cells and entire organisms. Any changes in the structure of cells or the activity of physiological processes in them, thus affecting life. The study of the structure of nucleic acids is extremely important for understanding the inheritance of traits in organisms and the patterns of functioning of both individual cells and cellular systems - tissues and organs.
There are 2 types of nucleic acids – DNA and RNA. DNA is a polymer consisting of two nucleotide helices arranged to form a double helix. Monomers of DNA molecules are nucleotides consisting of a nitrogenous base (adenine, thymine, guanine or cytosine), a carbohydrate (deoxyribose) and a phosphoric acid residue. The nitrogenous bases in the DNA molecule are connected to each other by an unequal number of H-bonds and are arranged in pairs: adenine (A) is always against thymine (T), guanine (G) against cytosine (C).
Nucleotides are connected to each other not randomly, but selectively. The ability for selective interaction of adenine with thymine and guanine with cytosine is called complementarity. The complementary interaction of certain nucleotides is explained by the peculiarities of the spatial arrangement of atoms in their molecules, which allow them to come closer and form H-bonds. In a polynucleotide chain, neighboring nucleotides are linked to each other through a sugar (deoxyribose) and a phosphoric acid residue. RNA, like DNA, is a polymer whose monomers are nucleotides. The nitrogenous bases of three nucleotides are the same as those that make up DNA (A, G, C); the fourth - uracil (U) - is present in the RNA molecule instead of thymine. RNA nucleotides differ from DNA nucleotides in the structure of the carbohydrate they contain (ribose instead of deoxyribose).
In a chain of RNA, nucleotides are joined by forming covalent bonds between the ribose of one nucleotide and the phosphoric acid residue of another. The structure differs between two-stranded RNA. Double-stranded RNAs are the custodians of genetic information in a number of viruses, i.e. They perform the functions of chromosomes. Single-stranded RNA transfers information about the structure of proteins from the chromosome to the place of their synthesis and participates in protein synthesis.
There are several types of single-stranded RNA. Their names are determined by their function or location in the cell. Most of the RNA in the cytoplasm (up to 80-90%) is ribosomal RNA (rRNA), contained in ribosomes. rRNA molecules are relatively small and consist of an average of 10 nucleotides. Another type of RNA (mRNA) that carries information about the sequence of amino acids in proteins that must be synthesized to ribosomes. The size of these RNAs depends on the length of the DNA region from which they were synthesized. Transfer RNAs perform several functions. They deliver amino acids to the site of protein synthesis, “recognize” (by the principle of complementarity) the triplet and RNA corresponding to the transferred amino acid, and carry out the precise orientation of the amino acid on the ribosome.
Fats and lipids
Fats are compounds of high-molecular fatty acids and trihydric alcohol glycerol. Fats do not dissolve in water - they are hydrophobic. There are always other complex hydrophobic fat-like substances called lipoids in the cell. One of the main functions of fats is energy. During the breakdown of 1 g of fats into CO 2 and H 2 O, a large amount of energy is released - 38.9 kJ (~ 9.3 kcal). The fat content in the cell ranges from 5-15% of the dry matter mass. In living tissue cells, the amount of fat increases to 90%. The main function of fats in the animal (and partly plant) world is storage.
When 1 g of fat is completely oxidized (to carbon dioxide and water), about 9 kcal of energy is released. (1 kcal = 1000 cal; calorie (cal) is an extra-system unit of the amount of work and energy, equal to the amount of heat required to heat 1 ml of water by 1 °C at standard atmospheric pressure 101.325 kPa; 1 kcal = 4.19 kJ) . When 1 g of proteins or carbohydrates is oxidized (in the body), only about 4 kcal/g is released. In a variety of aquatic organisms - from single-celled diatoms to basking sharks - fat will "float", reducing average body density. The density of animal fats is about 0.91-0.95 g/cm³. The density of vertebrate bone tissue is close to 1.7-1.8 g/cm³, and the average density of most other tissues is close to 1 g/cm³. It is clear that you need quite a lot of fat to “balance” a heavy skeleton.
Fats and lipids also perform a construction function: they are part of cell membranes. Due to poor thermal conductivity, fat is capable of a protective function. In some animals (seals, whales) it is deposited in the subcutaneous adipose tissue, forming a layer up to 1 m thick. The formation of some lipoids precedes the synthesis of a number of hormones. Consequently, these substances also have the function of regulating metabolic processes.
