Among the properties of the plasma membrane, its selective permeability is one of the main ones. Thanks to it, the division of the liquid media of a 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 characteristics
Selective permeability is a unique property of the phospholipid bilayer of the membrane with ion channels built into its thickness. This quality is characteristic of any cell, as well as membrane organelles: lysosomes, mitochondria, nucleus, Golgi complex, reticulum. The selectivity of the membrane is based on its structure, which includes hydrophobic regions of phospholipids.
After educationfompholipid bilayer with hydrophobic areas facing each other, the permeability of water through the plasmalemma is limited. It can get inside and outside the cell only through transmembrane channels, transport through which is carried out according to the laws of osmosis by diffusion. Selective permeability for water molecules is regulated by osmotic pressure. In the case of an increase in the concentration of s alts 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.
Transportation
The cell membrane separates two compartments - the intercellular space with the cytoplasm (or the cavity of the organelle and the cytoplasm). And between each compartment there must be a constant exchange of substances. The plasmalemma is characterized by active and passive transport.
Active proceeds with energy costs and allows you to transport substances from an area of lower concentration to a larger one. Passive transport is the free penetration of lipophilic substances into the cell through the plasmalemma, as well as the transfer of ions through special channels from an area of high concentration to an area with a lower content of the same type of ions.
Transmembrane receptors
The selective permeability of the membrane for ions is regulated by specialized ion channels built into the plasmalemma. For each ion, they are different and regulate fast active transport or passive slow transport of hydrated ions. Ion channels for potassium alwaysopen, and potassium exchange is carried out depending on the membrane potential.
Sodium is characterized by the presence of both slow and fast channels. Slow ones work on a similar principle as potassium ones, and the operation of fast channels is an example of active transport that occurs with energy costs. It takes place in the case of action potential generation, when a high-intensity intracellular influx of sodium ions is provided by short-term activation of fast channels, followed by membrane recharge.
The selective permeability of the plasmalemma is important for the transport of protein molecules, amino acids, vitamins and important cofactors of cell enzyme systems. These molecules are polar and hydrophilic, and therefore unable to penetrate the hydrophobic lipid bilayer. For their transport, there are special channels 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, its own specific carrier is built into the thickness of the membrane. This is the most rigorous and most highly specific way of organizing the selective permeability of the cell - a guarantee that no substance unnecessary in this phase of its development will 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 newthe 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 ones, such molecules are able to integrate into the lipid bilayer of the membrane and swim in it for a long time, after which they penetrate the cytoplasm and contact the intracellular or nuclear receptor.
An example is the penetration of steroid hormones. They freely pass through 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 that can overcome the lipid bilayer, dissolving in it, have an intracellular receptor. Hydrophilic molecules repel polarized regions of the membrane and therefore must either attach to a transmembrane transporter or attach to the surface receptor molecules of the membrane to transmit signals or enter the cell.
