Cell membrane: structure and function
The main structural unit of a living organism is a cell, which is a differentiated site of the cytoplasm, surrounded by a cell membrane. Due to the fact that the cell performs many important functions, such as reproduction, nutrition, movement, the shell must be plastic and dense.
History of the discovery and study of the cell membrane
In 1925, Grendel and Gorder conducted a successful experiment to identify the "shadows" of red blood cells, or empty shells. Despite several blunders, scientists discovered the lipid bilayer. Their works were continued by Danielle, Dawson in 1935, Robertson in 1960. As a result of years of work and the accumulation of arguments in 1972, Singer and Nicholson created a liquid-mosaic model of the structure of the membrane. Further experiments and studies have confirmed the works of scientists.
What is a cell membrane? This word was used more than a hundred years ago, translated from Latin, it means "film", "skin".So designate the border of the cell, which is a natural barrier between the inner contents and the external environment. The structure of the cell membrane implies semi-permeability, thanks to which moisture and nutrients and decomposition products can freely pass through it. This shell can be called the main structural component of the organization of the cell.
Consider the basic functions of the cell membrane.
1. Separates the inner contents of the cell and the components of the external environment.
2. Contributes to maintaining a constant chemical composition of the cell.
3. Regulates the correct metabolism.
4. Provides interconnection between cells.
5. Recognizes signals.
6. Protection function.
The outer cell membrane, also called the plasma membrane, is an ultramicroscopic film, the thickness of which ranges from five to seven nanometers. It consists mainly of protein compounds, phospholides, water. The film is elastic, easily absorbs water, and rapidly restores its integrity after damage.
Differs in a universal structure.This membrane occupies a border position, participates in the process of selective permeability, the elimination of decomposition products, synthesizes them. The relationship with the "neighbors" and reliable protection of internal content from damage makes it an important component in such a matter as the structure of the cell. The cell membrane of animal organisms is sometimes covered with a very thin layer - glycocalix, which contains proteins and polysaccharides. Plant cells outside the membrane are protected by a cell wall that serves as a support and shape support. The main component of its composition is fiber (cellulose) - a polysaccharide that is not soluble in water.
Thus, the outer cell membrane performs the function of restoring, protecting and interacting with other cells.
The structure of the cell membrane
The thickness of this movable shell varies from six to ten nanometers. The cell membrane of the cell has a special composition, the basis of which is the lipid bilayer. Hydrophobic tails, inert to water, are placed on the inside, while hydrophilic heads that interact with water are turned out.Each lipid represents a phospholipid, which is the result of the interaction of substances such as glycerin and sphingosine. The lipid skeleton is closely surrounded by proteins, which are located in a non-continuous layer. Some of them are immersed in the lipid layer, others pass through it. As a result, water-permeable areas are formed. The functions performed by these proteins are different. Some of them are enzymes, the rest are transport proteins that transfer various substances from the external environment to the cytoplasm and back.
The cell membrane is penetrated through and closely connected by integral proteins, and the peripheral connection is less strong. These proteins perform an important function, which is to maintain the structure of the membrane, receive and convert signals from the environment, transport substances, catalyze reactions that occur on the membranes.
The basis of the cell membrane is a bimolecular layer. Due to its continuity, the cell has barrier and mechanical properties. At different stages of life this bilayer can be broken.As a result, structural defects of through hydrophilic pores are formed. In this case, all functions of such a component as the cell membrane can change. The core may suffer from external influences.
The cell membrane of a cell has interesting features. Due to the fluidity of this shell is not a rigid structure, and the bulk of proteins and lipids, which are part of its composition, moves freely on the plane of the membrane.
In general, the cell membrane is asymmetric, so the composition of the protein and lipid layers is different. Plasma mambranes in animal cells from their outer side have a glycoprotein layer that performs receptor and signaling functions, and also plays an important role in the process of cell integration into tissue. The cell membrane is polar, that is, on the outside the charge is positive, and on the inside it is negative. In addition to all of the above, the cell wall has selective insight.This means that, in addition to water, only a certain group of molecules and ions of dissolved substances is passed into the cell.The concentration of a substance such as sodium in most cells is much lower than in the external environment. A different ratio is characteristic of potassium ions: their number in the cell is much higher than in the environment. In this regard, sodium ions are inherent in the desire to penetrate into the cell membrane, and potassium ions tend to free themselves outside. Under these circumstances, the membrane activates a special system that performs a “pumping” role, leveling the concentration of substances: sodium ions are pumped to the cell surface, and potassium ions are pumped inwards. This feature is one of the most important functions of the cell membrane.
A similar desire of sodium and potassium ions to move inward from the surface plays a large role in the matter of transporting sugar and amino acids into the cell. In the process of active removal of sodium ions from the cell membrane creates the conditions for new intakes of glucose and amino acids inside. On the contrary, in the process of the transfer of potassium ions into the cell, the number of transporters of decomposition products from inside the cell to the external environment is replenished.
How does the cell feed through the cell membrane?
Many cells absorb substances through processes such as phagocytosis and pinocytosis. In the first embodiment, a flexible outer membrane creates a small depression in which the captured particle is located. Then the diameter of the recess becomes larger until the surrounded particle enters the cellular cytoplasm. Through phagocytosis, some protozoa are fed, for example, amoebae, as well as blood cells - leukocytes and phagocytes. Similarly, cells absorb fluid that contains essential nutrients. This is called pinocytosis.
The outer membrane is closely connected with the endoplasmic reticulum of the cell.
In many types of basic tissue components, protrusions, folds, and microvilli are located on the membrane surface. Plant cells on the outside of this shell are covered with another, thick and distinctly visible in the microscope. The fiber from which they are made helps to form a support for tissues of plant origin, such as wood. Animal cells also have a number of external structures that are on top of the cell membrane.They are purely protective in nature, an example of this is chitin contained in integumentary insect cells.
In addition to the cell, there is an intracellular membrane. Its function is to divide the cell into several specialized closed compartments - compartments or organelles, where a certain environment should be maintained.
Thus, it is impossible to overestimate the role of such a component of the basic unit of a living organism as the cell membrane. The structure and functions suggest a significant expansion of the total surface area of the cell, improvement of metabolic processes. The structure of this molecular structure includes proteins and lipids. Separating the cell from the external environment, the membrane ensures its integrity. With its help, the intercellular bonds are maintained at a sufficiently strong level, forming tissues. In this regard, it can be concluded that one of the most important roles in the cell is played by the cell membrane. The structure and functions performed by it are radically different in different cells, depending on their purpose. Through these features, a variety of physiological activity of cell membranes and their roles in the existence of cells and tissues is achieved.