Membrane permeability characterizes the permeability of molecules through cell membranes. All cells are demarcated from intercellular space by biomembranes and simultaneously contain cell organelles that are themselves surrounded by membranes. The permeability of membranes is necessary for the smooth flow of biochemical reactions.
What is membrane permeability?
Membrane permeability characterizes the permeability of molecules through cell membranes. Membrane permeability is defined as the permeability of biomembranes to fluids and solutes. However, cell membranes are not permeable to all substances. Therefore, they are also called semipermeable membranes (semipermeable membranes). Biomembranes consist of two phospholipid layers, which are permeable to gases such as oxygen or carbon dioxide, as well as lipid-soluble nonpolar substances. These substances can pass through the membranes via normal diffusion. Polar and hydrophilic molecules are not allowed to pass through. They can only be transported through the membrane by passive or active transport processes. Membranes protect the intracellular space and the space within cell organelles. They ensure the maintenance of specific chemical and physical conditions for important biochemical reactions without interference from outside. The permeability of membranes ensures the selective transport of vital substances from the extracellular space into the cell and the expulsion of metabolic products from the cell. The same is true for individual cell organelles.
Function and task
Membranes are imperative for the undisturbed progression of vital biochemical reactions within cells and cell organelles. Membrane permeability is equally vital for supplying cells with important nutrients such as proteins, carbohydrates or fats. Minerals, vitamins and other active substances must also be able to pass through the membrane. At the same time, metabolic products are produced that must be disposed of from the cell. However, membranes are only permeable to lipophilic molecules and small gas molecules such as oxygen or carbon dioxide. Polar hydrophilic or even large molecules can only pass through the membrane via transport processes. There are passive and active ways of membrane transport for this purpose. Passive transport works without supplying energy in the direction of a potential or concentration gradient. Smaller lipophilic molecules or gas molecules are subject to normal diffusion. For larger molecules, normal diffusion is no longer possible. Here, certain transport proteins or channel proteins can facilitate transport. The transport proteins span the membrane like a tunnel. Smaller polar molecules can be passed through this tunnel via the action of polar amino acids. This also allows the transport of small charged ions through the tunnel. Another passive transport possibility results from the action of carrier proteins that are specialized for certain molecules. Thus, when the molecule docks, they change their conformation and thus transport it across the membrane. Active membrane transport requires the supply of energy. The corresponding molecule is transported against a concentration gradient or electrical gradient. Energy supplying processes result from the hydrolysis of ATP, the build-up of a charge gradient in the form of an electric field or the increase of entropy by building up a concentration gradient. For substances that cannot penetrate the membrane at all, endocytosis or exocytosis is available. In endocytosis, a droplet of fluid is incorporated through the invagination of the biomembrane and transported into the cell. This creates a so-called endosome, which transports important substances into the cytoplasm. During exocytosis, waste products in the cytoplasm are carried outward by membrane-enveloped transport vesicles.
Diseases and disorders
Disorders of membrane permeability can lead to various disease states. The changes affect the permeability of various ions. Membrane permeability disorders are also often the result of cardiovascular disease. In this case, the electrolyte balance of the body can be affected. However, many hereditary causes also cause membrane permeability disorders.Various proteins are involved in the assembly of the membrane and are responsible for the correct function of the double lipid layer. Genetic alterations of certain proteins are responsible, among other things, for changes in membrane permeability. One example is the disease myotonia congenita Thomsen. This disease is a genetically determined disorder of muscle function. In this case, a gene is mutated that codes for the chloride channels of muscle fiber membranes. The permeability of the chloride ions is reduced. This results in easier muscle fiber depolarization than in healthy individuals. The tendency for muscle contraction is increased, which is perceived as stiffness. For example, a closed fist can only be opened with a certain delay. Also the eyes can only be opened after 30 seconds after closing, which is called eyelid-lag. Furthermore, there are autoimmune diseases that are specifically directed against biomembranes. In this context, the so-called antiphospholipid syndrome (APS) is known. In this disease, the body’s immune system is directed against proteins that are bound to the phospholipids of the membrane. As a result, the blood becomes more coagulable. The probability of heart attacks, strokes and pulmonary embolisms is increased. Membrane permeability disorders are also found in the so-called mitochondriopathies. In the mitochondria, energy is obtained from the combustion of carbohydrates, fats and proteins. Mitochondria are cell organelles that are also surrounded by a membrane. Within these energy power plants, free radicals are produced to a high degree. If these are not captured, damage to the membranes occurs. This severely limits the function of the mitochondria. However, there are many reasons for the reduced effectiveness of radical scavengers.
