Biochemistry

increase in osmotic pressure, reverse processes are observed in the regulation of water metabolism. Among other hormones involved in the regulation of water metabolism, it is necessary to note thyroxine - a thyroid hormone, parathyrin - a parathyroid hormone, androgens and estrogens - sex hormones. They stimulate the secretion of water by the kidneys. An important role in the hydration and dehydration of tissues is played by minerals. Sodium ions increase the hydration of tissues and retain water in the body. Potassium and calcium ions, on the contrary, dehydrate tissues and promote the removal of water from the body. The flow of water into the body is regulated by a feeling of thirst, which occurs as a result of reflex excitation of certain parts of the cerebral cortex when the osmotic pressure of the blood plasma changes: All water introduced into the body is more or less rapidly absorbed and enters the bloodstream. 1.3 CELL STRUCTURE Cell is one of the forms of organization of living matter that underlies the structure and development of plants and animals. The sizes, shape and structure of the cells that make up the organs and tissues are different. They depend on the stage of development and function of the cell, their species, etc. In general, the diameter of the cells is from 1 micron to several centimeters. However, some of them are large, for example, nerve cells with long processes reaching 1 m. The most typical for the cells are spherical, oval, cylindrical, cubic. The number of cells in the body and even in its individual organs can be huge, for example, in the cortex of the cerebral hemispheres of the human brain contains 14-15 billion nerve cells, and in the blood - up to 25 billion red blood cells. In their structure, the cells of plants, animals and humans, like atoms, are similar to each other. Each of them contains in the middle a dense formation - the nucleus, which floats in the "semi-liquid" cytoplasm. The cell is surrounded by a cell membrane. A cell consists of many elements, the totality of which has a certain value not only for itself, but for the whole organism as a whole. If the cell structure is somehow violated, then its functions will change, it will lose its properties as an organized unit and die. Cell contents are a very complex system of diverse components. The cell structure diagram obtained using an electron microscope is presented in Figure 1.1. 24