Semipermeability refers to biomembranes that are selectively permeable to certain substances and cannot be passed by other substances. Semipermeability is the basis of osmosis and characterizes the cells of all living things. Disturbances in semipermeability have devastating consequences for electrolyte and water balance in cellular compartments.
What is semipermeability?
Semipermeability refers to biomembranes that are selectively permeable to certain substances and cannot be passed by other substances. Semipermeability literally means “semipermeability.” The term stands for a property of physical or substantial interfaces. Semipermeable surfaces allow certain particles to pass while blocking others from passing. In medical technology and biology, semipermeability plays a role primarily in the context of membranes. Semipermeable membranes possess selective permeability and allow certain particles to pass through the membrane in a specific direction. The corresponding membranes represent a separation system that allows certain substances to pass to the other side of the membrane without specific transport systems. Membranes surround cells in which a specific milieu must be maintained for the sake of survival. Without the semipermeability of membranes, the maintenance of the specific cellular milieu would be unthinkable. Furthermore, in biology, semipermeability is the basis for processes such as osmosis, osmoregulation, and turgor.
Function and task
The term membrane transport is used to summarize all substance passage through biomembranes. Two fundamentally different mechanisms characterize membrane transport: in addition to free permeation in the sense of diffusion, specific transport exists. Membranes consist of a lipid bilayer, which by itself represents a barrier between the aqueous compartments of the cell. The extraplasmic and cytoplasmic spaces are separated in this way. Different environments may prevail in the compartments. In certain biological systems, a cell membrane is permeable to smaller molecules thanks to its fluidity. This permeability exists in the biological system, for example, for water, which moves along the membrane in the direction of the higher concentration according to the existing concentration gradient. This principle is a basic building block of many organisms and thus also a basis of the human organism. Semipermeable membranes are permeable especially to solvents. Solutes are often retained by the membrane in order to maintain the cellular environment behind the separating layer. Thus, semipermeable membranes allow molecules up to a given molecular mass or size to pass through, while those above the given molecular mass or size are prevented from passing. Meanwhile, scientists consider transient irregularities within the lipid bilayers of membranes to be the primary cause of semipermeability. As the basis of osmosis, semipermeability is an important building block of all living organisms. The term osmosis is used to describe the directed flow of molecular particles through selectively permeable or semipermeable membranes. In order to achieve a regulated water balance, the cells of all living organisms depend on osmosis and thus semipermeability. Semipermeability is also critical for osmoregulation. This refers to the ability to regulate concentrations of osmotically active substances in metabolism. This ability serves to prevent osmotic stress and also helps living organisms to derive certain benefits from their osmotic potential. In addition, semipermeability forms the basis of the turgor pressure of plants. This pressure corresponds to a hydrostatic pressure in cells that enables physiological processes such as gas exchange or various transport processes.
Diseases and ailments
Systemic inflammatory reactions such as sepsis can show effects on permeability. In this context, the mediator substance histamine is released. After release, vascular permeability increases, among other effects. Many other inflammatory reactions exist with effects on membrane permeability of different tissues. One of them is pancreatitis, in which the semipermeability of the pancreatic duct system is affected by disturbances.The membrane permeability of the cells decreases in this case. This phenomenon can be recognized, for example, by the passage of X-ray contrast medium in the context of diagnostic imaging. Other membrane permeability disorders occur in the context of cardiovascular diseases. All membrane permeability disorders in most cases entail an imbalance in the electrolyte balance. Apart from the described correlations, membrane permeability disorders may also have a hereditary basis. For example, a hereditary mutation of membrane proteins can significantly alter the permeability of a cell membrane, including in diseases such as myotonia congenita Thomsen. In this disease, chloride channels within muscles altered by genetic mutation impair membrane passage for chloride ions. Without the passage of these ions, muscles cannot function at their full potential. Ultimately, all membrane permeability disorders show significant effects on the whole organism. For example, if a semipermeable membrane is suddenly no longer permeable to solvents, the water balance in the compartments of the cell becomes unbalanced. If a semipermeable membrane is in turn too permeable, the specific milieu of the cell compartments also changes in this case. In both cases, the affected cell may be doomed to die as the intended working environment of its compartments becomes unbalanced. Autoimmune diseases can also affect membrane permeability. Antiphospholipid syndrome, for example, specifically targets biomembranes and alters their physiological permeability. In plants, some membrane permeability or semipermeability disorders associated with parasitic organisms are also observed. Certain parasites secrete wilt toxins in the sense of marasmines. These substances condition semipermeability disorders to cause an increase in permeability in the plasma of the host cell to gain unimpeded access.
