Among the properties of a plasma membrane, its selective permeability is one of the main ones. Thanks to it, the separation of the liquid media of the multicellular organism into compartments is formed, in each of which its own composition of electrolytes and organic substances is formed. Any organelle or cell framed by a plasma membrane strictly separates the environment of the body and regulates the transport of substances in two directions.
Definition and characterization
Selective permeability is a unique property of a phospholipid bilayer of a membrane with ion channels embedded in its thickness. This quality is characteristic of any cell, as well as of membrane organelles: lysosomes, mitochondria, nucleus, Golgi complex, reticulum. The selectivity of the membrane is based on its structure, which includes hydrophobic sites of phospholipids.
After the formation of a fomolipid bilayer with hydrophobic sections facing each other, the permeability of water through the plasma membrane is limited. Inside the cell and out, it can only get through transmembrane channels, the transport of which is carried out according to the laws of osmosis by diffusion. The selective permeability to water molecules is controlled by osmotic pressure. In the case of an increase in the concentration of salts inside the cell, water penetrates through the channels into the cytoplasm, and in the case of an increase in extracellular osmotic pressure, it rushes into the intercellular space.
Transport
The cell membrane delimits two compartments - the intercellular space with the cytoplasm (or organelle cavity and cytoplasm). And between each compartment there should be a constant exchange of substances. Plasmaolemma is characterized by active and passive transport.
Active flows with energy consumption and allows you to transfer substances from a region of lower concentration to a larger one. Passive transport - the free penetration of lipophilic substances into the cell through the plasma membrane, as well as the transfer of ions through special channels from a region of high concentration to a region with a lower content of ions of the same type.
Transmembrane receptors
The selective permeability of the membrane for ions is regulated by specialized ion channels built into the plasma membrane. For each ion, they are different and regulate fast active transport or passive slow transport of hydrated ions. Ion channels for potassium are always open, and potassium exchange is carried out depending on the membrane potential.
Sodium is characterized by the presence of both slow and fast channels. Slow work on the same principle as potassium, and the work of fast channels is an example of active transport that runs with energy costs. It takes place in the case of the generation of an action potential when, by short activation of fast channels, a high-intensity intracellular influx of sodium ions is ensured with subsequent recharging of the membrane.
The selective permeability of the plasma membrane is important for the transport of protein molecules, amino acids, vitamins, and important cofactors of the enzyme systems of the cell. These molecules are polar and hydrophilic, and therefore not able to penetrate the hydrophobic lipid bilayer. For their transport, special channels are present in the thickness of the membrane, which are complex glycoproteins.
Transmembrane transfer
Attachment of specialized ligands to receptors activates the passage of a substance into the cell. For each type of such molecules in the thickness of the membrane has its own specific carrier. This most rigorous and highly specific way of organizing selective cell permeability is a guarantee that the substance unnecessary in this phase of its development does not penetrate into the cytoplasm.
The structure of the transmembrane specific carrier is encoded in the genetic material of the nucleus. And the process of assembling a new channel for the transport of substances is regulated by the cell itself. This means that at each stage of its differentiation, it is able to initiate or stop the flow of certain substances into its cytoplasm, depending on external conditions.
Intracellular receptors
Cell and membrane organelles have selective permeability due to intracellular receptors. They are designed to receive signals from lipophilic substances. Unlike hydrophobic, such molecules are able to integrate into the lipid bilayer membrane and swim for a long time in it, after which they penetrate the cytoplasm and come into contact with the intracellular or nuclear receptor.
An example is the penetration of steroid hormones. They freely pass the cytolemma and, after contact with a specific receptor, activate or suppress a certain link in the metabolic chains. The possibility of free passage of lipophilic substances through the plasma membrane is also an example of selective permeability.
All lipophilic substances capable of overcoming the lipid bilayer, dissolving in it, have an intracellular receptor. Hydrophilic molecules repel from the polarized portions of the membrane, and therefore, to transmit signals or enter the cells, they must either attach to the transmembrane carrier or attach to the surface receptor molecules of the membrane.