Biochemistry

The cytoplasmic membrane provides a constant composition of intracellular contents. In its structure, the membrane represents a viscous lipid phase (lipid layer) with proteins immersed in it. The lipid layer consists mainly of phospholipids, cholesterol, glycolipids and is a double layer of molecules. In this case, the long residues of fatty acids of one and the other layer of lipid molecules are facing each other and form a liquid hydrophobic phase, and the hydrophilic groups of these lipids (choline, phosphoric acid, ethanolamine, etc.) are located outside. The structure of the membrane determines its main property - selective permeability, i.e., regulation of the intake of necessary nutrients into the cell and removal of metabolic products from it.This selectivity ensures the constancy of the internal environment of the cell, maintains the desired osmotic pressure, pH value, etc. Proteins that make up the membrane are located on the periphery (peripheral) or penetrate its entire thickness (integral) The functions of membrane proteins are diverse. Some of them are enzymes that act as catalysts for many important reactions, while others transport various substances (fatty acids, cholesterol) through the membrane. A special group of proteins forms “ pores ” in the membrane for the transfer of ions (hydrogen, sodium, potassium, etc.). Superficially located proteins and hydrophilic groups of lipids are associated with carbohydrates and form sites that can "recognize" other cells or substances. Such sites are called receptors. Connecting with specific receptors, substances (for example, hormones) transmit their signals inside the cell. Membranes are elastic and have the ability to spontaneously restore their integrity in case of damage. Cytoplasm. The inner space of the cell is filled with cytoplasm, in which the organelles of the cell are located. The cytoplasm is penetrated by numerous channels, which are called the endoplasmic reticulum (reticulum). The endoplasmic reticulum is a continuation of the nuclear membrane. It is a network of membranes forming tubules and vesicles; through the endoplasmic reticulum, various substances are transported from the cell to the external environment and vice versa; here the processes of synthesis and decomposition of chemicals occur. There are two types of reticulum - smooth and rough. The “ roughness ” of the latter is due to the numerous small particles of a spherical shape located on its surface - ribosomes. Ribosomes are small, dense granules of small sizes. They consist of two parts (subunits) of a rounded shape, the connection of which can be figuratively represented as a mushroom or figure eight. They are scattered throughout the cell. Some of them are associated with the endoplasmic reticulum, others are in 26