The vital activity of a cell becomes possible only because different enzymes and substances do not mix, and the cell constitutes integrity. All this becomes possible only thanks to a variety of membranes. And the cell as a whole is delimited from the others by a special structure called the “cytoplasmic membrane”.
Is it visible in a light microscope? The answer is no, yes, we see the boundaries, but the membrane itself is too thin a structure. Sometimes we don’t even see cell boundaries, for example, when we examine liver cells in a light microscope. Although why, then, in other cases, we see the boundaries of the cells, is it a membrane?
In fact, these are the supmembrane layers of carbohydrates that are between the cells. They absorb the dye, so with a successful cut, you might think that this is the plasma membrane.
In experiments, it was found that cells that were immersed in solutions with different osmotic pressures swell or shrink, which means they are surrounded by a membrane that is characterized by selective permeability.
It was also revealed that the cell membrane is well permeable if substances soluble in lipids try to penetrate into it. In the classical concept, the hydrophilic ends of the membrane molecules were considered to be turned outward, and hydrophobic - inward. Electron microscopy has proven that the matter is much more complicated. In particular, in electronic photos it is seen that the outer layers become dense, and not the inner, that is, the lipid layers are located at the edges.
Due to its structure, the plasma membrane is impervious to macromolecules, therefore, cytoplasmic proteins are not able to leave the cell through it. Proteins, being in the cell, create osmotic pressure, so that the right amount of water gets inside the cell. However, this process is not endless, because there are other substances in the tissue fluid outside that balance the osmotic pressure.
In order for the potential difference to remain stable, the plasma membrane must have dielectric properties. This also led scientists to the idea that there are a lot of lipids in the membrane, which also have dielectric properties. Reluctantly, the plasma membrane revealed its properties.
Its structure and functions are related, for example, the ability to maintain an unusual difference in the concentrations of potassium and sodium ions is associated with a special mechanism in the membrane - a sodium-potassium pump. In this case, the transfer of ions is carried out by a special enzyme that works on the energy of the cell, this process is expensive for it. The cell has to "pay" for the balance. Also require "investment" and the transfer of glucose, fatty acids, amino acids.
An interesting property of the cell membrane is also its asymmetry, that is, its inner and outer surfaces are not the same, although initially the researchers based on the data of electron microscopy thought so. All carbohydrate-containing parts of the glycoprotein molecules protrude beyond the outer surface of the membrane and participate in the formation of the supra-lipid layer. The outer surface of the cell also contains special molecules called receptors, they act with certain molecules of the external environment. So the activity of the cell is regulated, it can be stimulated or suppressed depending on the needs of the body. And in the inner half of the membrane contains a lot of cholesterol.
Biochemical studies of the cell membrane have proved that the proteins of the inner and outer membranes are not identical, and the various phospholipids in these two surfaces are also very diverse. Some of these features can even be seen with an electron microscope.
As you can see, the elementary membrane is not so simple, but in order to understand all the processes taking place, scientists had to build and reject many hypotheses.