Cell membrane - a structural element of the cell, protecting it from the external environment. With the help of it, it interacts with the intercellular space and is part of the biological system. Its membrane has a special structure consisting of a lipid bilayer, integral and semi-integral proteins. The latter are large molecules that perform various functions. Most often, they are involved in the transport of special substances, the concentration of which on different sides of the membrane is carefully regulated.
General plan of the cell membrane structure
The plasma membrane is a collection of molecules of fats and complex proteins. Its phospholipids, with their hydrophilic residues, are located on opposite sides of the membrane, forming a lipid bilayer. But their hydrophobic areas, consisting of fatty acid residues, are turned inward. This allows you to create a fluid liquid-crystal structure that can constantly change shape and is in dynamic equilibrium.
This feature of the structure allows you to limit the cell from the intercellular space, because the membrane is normally impermeable to water and all substances dissolved in it. Some complex integral proteins, semi-integral and surface molecules are immersed in the thickness of the membrane. Through them, the cell interacts with the outside world, maintaining homeostasis and forming integral biological tissues.
Plasma membrane proteins
All protein molecules that are located on the surface or in the thickness of the plasma membrane are divided into types depending on the depth of their occurrence. There are integral proteins penetrating the lipid bilayer, semi-integral proteins that originate in the hydrophilic region of the membrane and go outside, as well as surface proteins located on the outer area of the membrane. Integral protein molecules permeate the plasmalemma in a special way and can be connected to the receptor apparatus. Many of these molecules permeate the entire membrane and are called transmembrane. The rest are anchored in the hydrophobic portion of the membrane and either exit to the inner or outer surface.
Cell ion channels
Most often, ion channels act as integral complex proteins. These structures are responsible for the active transport of certain substances into or out of the cell. They consist of several protein subunits and an active site. When exposed to a specific ligand on the active center, represented by a specific setamino acids, there is a change in the conformation of the ion channel. Such a process allows you to open or close the channel, thereby starting or stopping the active transport of substances.
Some ion channels are open most of the time, but when a signal is received from a receptor protein or when a specific ligand is attached, they can close, stopping the ion current. This principle of operation boils down to the fact that until a receptor or humoral signal is received to stop the active transport of a certain substance, it will be carried out. As soon as the signal is received, the transport should be stopped.
Most of the integral proteins that act as ion channels work to inhibit transport until a specific ligand is attached to the active site. Then the ion transport will be activated, which will allow the membrane to be recharged. This algorithm of operation of ion channels is typical for cells of excitable human tissues.
Types of embedded proteins
All membrane proteins (integral, semi-integral and surface) perform important functions. It is precisely because of their special role in the life of the cell that they have a certain type of integration into the phospholipid membrane. Some proteins, more often these are ion channels, must completely suppress the plasmalemma in order to realize their functions. Then they are called polytopic, that is, transmembrane. Others are localized by their anchor site in the hydrophobic site of the phospholipid bilayer, and the active site extends only to the internal or only to the externalsurface of the cell membrane. Then they are called monotopic. More often they are receptor molecules that receive a signal from the surface of the membrane and transmit it to a special "intermediary".
Renewal of integral proteins
All integral molecules completely penetrate the hydrophobic area and are fixed in it in such a way that their movement is allowed only along the membrane. However, the ingress of the protein into the cell, just like the spontaneous detachment of the protein molecule from the cytolemma, is impossible. There is a variant in which the integral proteins of the membrane enter the cytoplasm. It is associated with pinocytosis or phagocytosis, that is, when a cell captures a solid or liquid and surrounds it with a membrane. It is then pulled inside along with the proteins embedded in it.
Of course, this is not the most efficient way to exchange energy in the cell, because all the proteins that previously served as receptors or ion channels will be digested by the lysosome. This will require their new synthesis, for which a significant part of the energy reserves of macroergs will be spent. However, during the "exploitation" of the molecules of ion channels or receptors are often damaged, up to the detachment of sections of the molecule. This also requires their resynthesis. Therefore, phagocytosis, even if it occurs with the splitting of its own receptor molecules, is also a way of their constant renewal.
Hydrophobic interaction of integral proteins
As it wasdescribed above, integral membrane proteins are complex molecules that seem to be stuck in the cytoplasmic membrane. At the same time, they can freely swim in it, moving along the plasmalemma, but they cannot break away from it and enter the intercellular space. This is realized due to the peculiarities of the hydrophobic interaction of integral proteins with membrane phospholipids.
Active centers of integral proteins are located either on the inner or outer surface of the lipid bilayer. And that fragment of the macromolecule, which is responsible for tight fixation, is always located among the hydrophobic regions of phospholipids. Due to interaction with them, all transmembrane proteins always remain in the thickness of the cell membrane.
Functions of integral macromolecules
Any integral membrane protein has an anchor site located among the hydrophobic residues of phospholipids and an active center. Some molecules have only one active center and are located on the inner or outer surface of the membrane. There are also molecules with multiple active sites. All this depends on the functions performed by integral and peripheral proteins. Their first function is active transport.
Protein macromolecules, which are responsible for the passage of ions, consist of several subunits and regulate the ion current. Normally, the plasma membrane cannot pass hydrated ions, since it is a lipid by nature. The presence of ion channels, which are integral proteins, allows ions to penetrate into the cytoplasm and recharge the cell membrane. This is the main mechanism for the occurrence of the membrane potential of excitable tissue cells.
Receptor molecules
The second function of integral molecules is receptor function. One lipid bilayer of the membrane implements a protective function and completely limits the cell from the external environment. However, due to the presence of receptor molecules, which are represented by integral proteins, the cell can receive signals from the environment and interact with it. An example is the cardiomyocyte adrenal receptor, cell adhesion protein, insulin receptor. A particular example of a receptor protein is bacteriorhodopsin, a special membrane protein found in some bacteria that allows them to respond to light.
Intercellular interaction proteins
The third group of functions of integral proteins is the implementation of intercellular contacts. Thanks to them, one cell can join another, thus creating a chain of information transfer. Nexuses work according to this mechanism - gap junctions between cardiomyocytes, through which the heart rhythm is transmitted. The same principle of operation is observed in synapses, through which an impulse is transmitted in nerve tissues.
Through integral proteins, cells can also create a mechanical connection, which is important in the formation of an integral biological tissue. Also, integral proteins can play the role of membrane enzymes and participate in the transfer of energy, including nerve impulses.
