At the end of the 19th century, a branch of biology was formed, called biochemistry. She studies the chemical composition of a living cell. The main task of science is the knowledge of the characteristics of metabolism and energy that regulate the vital activity of plant and animal cells.
The concept of the chemical composition of cells
As a result of careful research, the scientists studied the chemical organization of cells and found that living things have more than 85 chemical elements in their composition. Moreover, some of them are mandatory for almost all organisms, while others are specific and are found in specific biological species. And the third group of chemical elements is present in the cells of microorganisms, plants and animals in fairly small quantities. Chemical elements in the composition of cells are most often in the form of cations and anions, from which mineral salts and water are formed, and carbon-containing organic compounds are synthesized: carbohydrates, proteins, lipids.
Organogenic elements
In biochemistry, these include carbon, hydrogen, oxygen and nitrogen. Their combination in the cell is from 88 to 97% of the other chemical elements that are in it. Carbon is especially important. All organic substances in a cell are composed of molecules containing carbon atoms. They are able to connect with each other, forming chains (branched and unbranched), as well as cycles. This ability of carbon atoms underlies the amazing variety of organic substances that make up the cytoplasm and cellular organelles.
For example, the inner contents of a cell consists of soluble oligosaccharides, hydrophilic proteins, lipids, various types of ribonucleic acid: transport RNA, ribosomal RNA and messenger RNA, as well as free monomers - nucleotides. The cell nucleus has a similar chemical composition . It also contains deoxyribonucleic acid molecules that make up chromosomes. All of the above compounds incorporate nitrogen, carbon, oxygen, and hydrogen atoms. This is a proof of their especially important value, since the chemical organization of cells depends on the content of organogenic elements that make up the cellular structures: hyaloplasm and organelles.
Macronutrients and their meanings
Chemical elements, which are also very often found in the cells of various species of organisms, are called macroelements in biochemistry. Their content in the cell is 1.2% - 1.9%. Cell macrocells include: phosphorus, potassium, chlorine, sulfur, magnesium, calcium, iron and sodium. All of them perform important functions and are part of various cellular organelles. So, a ferrous ion is present in the blood protein - hemoglobin, which transports oxygen (in this case it is called oxyhemoglobin), carbon dioxide (carbohemoglobin) or carbon monoxide (carboxyhemoglobin).
Sodium ions provide the most important type of intercellular transport: the so-called sodium-potassium pump. They are also part of the interstitial fluid and blood plasma. Magnesium ions are present in chlorophyll molecules (photopigment of higher plants) and participate in the process of photosynthesis, since they form reaction centers that capture photons of light energy.
Calcium ions provide the conduction of nerve impulses along the fibers, and are also the main component of osteocytes - bone cells. Calcium compounds are widespread in the world of invertebrates, in which shells are composed of calcium carbonate.
Chlorine ions take part in the recharging of cell membranes and provide the occurrence of electrical impulses that underlie nerve excitation.
Sulfur atoms are part of the native proteins and determine their tertiary structure, "linking" the polypeptide chain, resulting in the formation of a globular protein molecule.
Potassium ions are involved in the transport of substances through cell membranes. Phosphorus atoms are part of such an important energy-consuming substance as adenosine triphosphoric acid, and they are also an important component of deoxyribonucleic and ribonucleic acid molecules, which are the main substances of cellular heredity.
The functions of trace elements in cellular metabolism
About 50 chemical elements that make up less than 0.1% in the cells are called trace elements. These include zinc, molybdenum, iodine, copper, cobalt, fluorine. With a low content, they perform very important functions, as they are part of many biologically active substances.
For example, zinc atoms are located in the molecules of insulin (a pancreatic hormone that regulates blood glucose), iodine is an integral part of the thyroid hormones - thyroxine and triiodothyronine, which control the level of metabolism in the body. Copper, along with iron ions, is involved in hematopoiesis (the formation of red blood cells, platelets and white blood cells in the red bone marrow of vertebrates). Copper ions are part of the hemocyanin pigment present in the blood of invertebrates, such as mollusks. Therefore, their hemolymph color is blue.
Even lower in the cell are chemical elements such as lead, gold, bromine, silver. They are called micronutrients and are part of plant and animal cells. For example, gold ions were detected in corn kernels by chemical analysis. Bromine atoms in large numbers are part of the thallus cells of brown and red algae, such as sargassum, kelp, fucus.
All the above examples and facts explain how the chemical composition, function and structure of the cell are interconnected. The table below shows the content of various chemical elements in the cells of living organisms.
General characteristics of organic substances
The chemical properties of cells of various groups of organisms in a certain way depend on carbon atoms, the proportion of which is more than 50% of the cell mass. Almost all the dry matter of the cell is represented by carbohydrates, proteins, nucleic acids and lipids, which have a complex structure and a large molecular weight. Such molecules are called macromolecules (polymers) and consist of simpler elements - monomers. Protein substances play an extremely important role and perform many functions, which will be discussed below.
The role of proteins in the cell
A biochemical analysis of the compounds that make up a living cell confirms its high content of organic substances such as proteins. There is a logical explanation for this fact: proteins perform various functions and participate in all manifestations of cellular activity.
For example, the protective function of proteins is the formation of antibodies - immunoglobulins produced by lymphocytes. Protective proteins such as thrombin, fibrin and thromboblastin provide blood coagulation and prevent its loss in injuries and injuries. The composition of the cell includes complex proteins of cell membranes that have the ability to recognize foreign compounds - antigens. They change their configuration and inform the cell of a potential hazard (signaling function).
Some proteins have a regulatory function and are hormones, for example, oxytocin, produced by the hypothalamus, is reserved by the pituitary gland. Coming from it into the blood, oxytocin acts on the muscle walls of the uterus, causing its contraction. The vasopressin protein also performs a regulatory function by controlling blood pressure.
In muscle cells are actin and myosin, capable of contracting, which causes the motor function of muscle tissue. Trophic function is also characteristic of proteins , for example, albumin is used by the embryo as a nutrient for its development. Blood proteins of various organisms, such as hemoglobin and hemocyanin, carry oxygen molecules - they perform a transport function. If more energy-intensive substances, such as carbohydrates and lipids, are fully used, the cell begins to break down proteins. One gram of this substance gives 17, 2 kJ of energy. One of the most important functions of proteins is catalytic (protein-enzymes accelerate chemical reactions that occur in cytoplasm compartments). Based on the foregoing, we were convinced that proteins perform many very important functions and are necessarily part of the animal cell.
Protein biosynthesis
Consider the process of protein synthesis in a cell that occurs in the cytoplasm using organelles such as ribosomes. Thanks to the activity of special enzymes, with the participation of calcium ions, ribosomes are combined into polysomes. The main functions of ribosomes in a cell are the synthesis of protein molecules, which begins with the process of transcription. As a result, mRNA molecules are synthesized, to which polysomes are attached. Then the second process begins - broadcasting. Transport RNAs bind to twenty different types of amino acids and bring them to the polysomes, and since the functions of the ribosomes in the cell are the synthesis of polypeptides, these organelles form complexes with tRNAs, and amino acid molecules bind to each other by peptide bonds, forming a protein macromolecule.
The role of water in metabolic processes
Cytological studies confirmed the fact that the cell, the structure and composition of which we are studying, on average consists of 70% water, and in many animals that lead an aquatic way of life (for example, intestinal), its content reaches 97–98%. With this in mind, the chemical organization of cells includes hydrophilic (capable of dissolving) and hydrophobic (water-repellent) substances. Being a universal polar solvent, water plays an exceptional role and directly affects not only functions, but also the cell structure itself. The table below shows the water content in the cells of various types of living organisms.
The function of carbohydrates in the cell
As we found out earlier, carbohydrates also belong to important organic substances - polymers. These include polysaccharides, oligosaccharides and monosaccharides. Carbohydrates are part of more complex complexes - glycolipids and glycoproteins, from which cell membranes and supmembrane structures, for example glycocalyx, are built.
In addition to carbon, the composition of carbohydrates includes oxygen and hydrogen atoms, and some polysaccharides also contain nitrogen, sulfur and phosphorus. There are a lot of carbohydrates in plant cells: potato tubers contain up to 90% starch, up to 70% carbohydrate in seeds and fruits, and they occur in animal cells in the form of compounds such as glycogen, chitin and trehalose.
Simple sugars (monosaccharides) have the general formula CnH2nOn and are divided into tetrose, triose, pentose and hexose. The last two are most common in the cells of living organisms, for example, ribose and deoxyribose are part of nucleic acids, and glucose and fructose are involved in assimilation and dissimilation reactions. Oligosaccharides are often found in plant cells: sucrose is stored in the cells of sugar beets and sugarcane, maltose is found in sprouted grains of rye and barley.
Disaccharides have a sweet taste and dissolve well in water. Polysaccharides, being biopolymers, are represented mainly by starch, cellulose, glycogen and laminarin. The structural forms of polysaccharides include chitin. The main function of carbohydrates in the cell is energy. As a result of hydrolysis and energy exchange reactions, polysaccharides are broken down to glucose, and then it is oxidized to carbon dioxide and water. As a result, one gram of glucose releases 17.6 kJ of energy, and starch and glycogen stores are essentially a reservoir of cellular energy.
Glycogen is deposited mainly in muscle tissue and liver cells, plant starch - in tubers, bulbs, root crops, seeds, and in arthropods, for example spiders, insects and crustaceans, the trehalose oligosaccharide plays a major role in energy supply.
Carbohydrates differ from lipids and proteins in their ability to oxygen-free breakdown. This is extremely important for organisms living in conditions of oxygen deficiency or lack, for example, for anaerobic bacteria and helminths - parasites of humans and animals.
There is another function of carbohydrates in the cell - building (structural). It lies in the fact that these substances are the supporting structures of cells. For example, cellulose is part of the cell walls of plants, chitin forms the external skeleton of many invertebrates and is found in fungal cells, olisaccharides, together with lipid and protein molecules form a glycocalyx - supramembrane complex. It provides adhesion - the adhesion of animal cells to each other, leading to the formation of tissues.
Lipids: structure and functions
These organic substances, which are hydrophobic (insoluble in water), can be extracted, that is, extracted from cells using non-polar solvents such as acetone or chloroform. The functions of lipids in a cell depend on which of the three groups they belong to: fats, waxes or steroids. Fats are most widely distributed in all types of cells.
Animals accumulate them in the subcutaneous fat, the nerve tissue contains fat in the form of myelin sheaths of nerves. It also accumulates in the kidneys, liver, and in insects in the fat body. Liquid fats - oils - are found in the seeds of many plants: cedar, peanuts, sunflowers, olives. The lipid content in the cells ranges from 5 to 90% (in adipose tissue).
Steroids and waxes differ from fats in that they do not have fatty acid residues in their molecules. So, steroids are hormones of the adrenal cortex that affect puberty and are components of testosterone. They are also part of vitamins (for example, vitamin D).
The main functions of lipids in the cell are energy, building and protective. The first is due to the fact that 1 gram of fat during breakdown gives 38.9 kJ of energy - much more than other organic substances - proteins and carbohydrates. In addition, when oxidizing 1 g of fat, almost 1.1 g is released. water. That is why some animals, having a supply of fat in their body, can be without water for a long time. For example, gophers can be in hibernation for more than two months without needing water, and a camel does not drink water when crossing the desert for 10-12 days.
The construction function of lipids is that they are an integral part of cell membranes, as well as part of the nerves. The protective function of lipids is that a layer of fat under the skin around the kidneys and other internal organs protects them from mechanical injuries. A specific thermal insulation function is inherent in animals that have been in the water for a long time: whales, seals, and fur seals. A thick subcutaneous fat layer, for example, in a blue whale is 0.5 m, it protects the animal from hypothermia.
The value of oxygen in cell metabolism
Aerobic organisms, which include the vast majority of animals, plants and humans, use atmospheric oxygen for energy exchange reactions, leading to the breakdown of organic substances and the release of a certain amount of energy accumulated in the form of adenosine triphosphoric acid molecules.
So, with the complete oxidation of one mole of glucose occurring on mitochondrial cysts, 2800 kJ of energy is released, of which 1596 kJ (55%) is stored in the form of ATP molecules containing macroergic bonds. Thus, the main function of oxygen in a cell is the implementation of aerobic respiration, which is based on a group of enzymatic reactions of the so-called respiratory chain that occur in cellular organelles - mitochondria. In prokaryotic organisms - phototrophic bacteria and cyanobacteria - the oxidation of nutrients occurs under the influence of oxygen, which diffuses into the cells on the internal outgrowths of the plasma membranes.
We studied the chemical organization of cells, as well as examined the processes of protein biosynthesis and the function of oxygen in cellular energy metabolism.