Plasma membrane: hidden boundaries

The life of the cell becomes possible only because different enzymes and substances do not mix, and the cell makes up the whole. All this becomes possible only through a variety of membranes. And the cell as a whole is delimited from others by a special structure called "cytoplasmic membrane".

Is it visible in a light microscope? The answer is negative, yes, we see boundaries, but the membrane itself is too thin structure. Sometimes we do not see even the border of cells, for example, when we examine liver cells in a light microscope. Although why then, in other cases, do we see the boundaries of the cells, is it not a membrane?

In fact, these are the above-membrane layers of carbohydrates, which are located 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 wrinkle, which means that they are surrounded by a membrane that is characterized by selective permeability.

It was also found that the cell membrane is well permeable if substances soluble in lipids try to penetrate it. In the classical concept, the hydrophilic ends of the membrane molecules were considered to be exposed to the outside, and the hydrophobic ends to the inside. Electron microscopy has proved that the matter is much more complicated. In particular, in electronic photographs, it can be seen that the outer layers become dense, and not internal, that is, the lipid layers are located at the edges.

The plasma membrane is impenetrable to macromolecules due to its structure, therefore cytoplasm proteins are not able to leave the cell through it. Proteins, being in the cell, create an osmotic pressure, due to which the necessary amount of water gets inside the cell. However, this process is not infinite, because in the tissue fluid from the outside also there are other substances that counterbalance 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 many lipids in the membrane that have dielectric properties. Reluctantly revealed its properties of the plasma membrane.

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 - the sodium-potassium pump. The transfer of ions in this case is carried out by a special enzyme, which works on the energy of the cell, this process is expensive for it. The cage 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 electron microscopy data believed this. All hydrocarbon-containing parts of glycoprotein molecules protrude beyond the outer surface of the membrane and participate in the formation of the supercilipid 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 organism. And in the inner half of the membrane contains a lot of cholesterol.

Biochemical studies of the cell membrane have shown 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 be seen even with an electron microscope.

As you can see, the elementary membrane is not so simple, and in order to understand all the processes that are taking place, scientists had to construct and reject many hypotheses.