In membrane transport, substances pass through a biological membrane or are actively transported through membranes. Unlike active transport, diffusion is the simplest membrane transport pathway and requires no additional provision of energy. Disorders of membrane transport are associated with a variety of different diseases.
What is membrane transport?
Membrane transport is when substances pass through or are actively transported across a biological membrane. Biomembranes enclose area such as the cytoplasm of cells, generating a controlled region with a relatively independent environment to the outside world. The specific cellular milieu inside cells can only be established and maintained because of the shielding from the outside world. The bilayer of a biomembrane consists of phospholipids and is by itself permeable only to gases and small, in most cases uncharged molecules. For hydrophilic polar ions and other bioactive substances, the lipid bilayer corresponds to a barrier that requires additional mechanisms of transport to overcome. Membrane transport corresponds to the passage of substances through a biomembrane. Two different principles play a role in this. The first principle is diffusion or free permeation, the second is selective mass transport. In addition to simple diffusion, functional principles such as passive transport by channel proteins or carrier proteins and active transport are part of transmembrane transport. Membrane translocating transport in turn includes endocytosis, exocytosis, and transcytosis. Since membrane parts themselves are displaced during membrane-displacing transport, this is sometimes also referred to as membrane flux. Membrane transport supports cellular functions and cell communication with the environment. Selective mass transfer is enabled by transport mechanisms.
Function and task
The lipid bilayer or bimolecular lipid layer of a biomembrane corresponds to a barrier between aqueous compartments in the form of the extraplasmic and cytoplasmic space. Between the compartments, only small molecules can diffuse through a biomembrane, such as acetic acid in addition to water. For larger molecules, the diffusion rate is relatively low. The permeability of membranes for small molecules is also called semi-permeability and forms the basis of osmosis. According to current assumptions, any biomembrane is a fluid structure with transient irregularities within the lipid bilayer. Molecules with hydrophobic property dissolve through the hydrophobic membrane region due to their partition coefficient. Even larger particles such as steroid hormones can pass through membranes by diffusion. Specific molecules, on the other hand, make use of specific membrane transport. The transport pathway is linked to integral membrane transport proteins known as translocators. Specific transport is substrate specific and saturable. Translocators in this transport pathway include carriers that are loaded with the substrate and can induce a conformational change in the membrane to introduce their cargo. Because of the relatively high rates of transport, a permanent transport channel exists in each membrane. The integral membrane proteins with roles in membrane transport usually correspond to oligomeric structures. In specific transport, catalyzed diffusion without additional energy consumption or active transport under energy consumption is present. Catalyzed diffusion and active transport offer the possibility of transporting only a single particle unidirectionally, two particles together in the same direction or in opposite directions. Catalyzed diffusion by membrane transport proteins follows only the concentration balance along the present concentration gradient of substances between the two compartments of the cell. Active transport always occurs against the concentration gradient. Pores of the outer biomembrane serve the non-specific passage of hydrophilic particles. The actual transport channel of a biomembrane consists of β-sheets. Membrane transport is irreplaceable for all body functions and tissues of the body, such as the nervous system and its voltage-gated ion channels.
Diseases and disorders
Disruption of membrane transport systems can cause severe cellular damage and even organ failure.Within the intestine or kidneys, disorders of membrane transport result in, for example, resorption and secretion disorders. Mitochondriopathies, for example, lead to membrane transport disorders. In this case, the enzyme system that enables energy production by means of oxidative phosphorylation is affected. Disorders of ATP synthase are particularly noteworthy in this context. This enzyme is one of the most important transmembrane proteins, which, for example, assumes the function of a transport enzyme within the proton pump. In the healthy body, the enzyme catalyzes the provision of ATP and enables energy-favored proton transport along the proton gradient under ATP formation. ATP synthase is therefore one of the most important energy converters in the human organism, transforming one form of energy into another. Mitochondriopathies are now malfunctions of mitochondrial metabolic processes that cause a reduced supply of ATP synthesis and thus reduce the body’s performance. In addition, all transporter proteins and enzymes may ultimately be affected by mutations or transcriptional defects. Mutations in the genetic material of transporter proteins cause the affected proteins to be present in a modified form, making active mass transport more difficult. This phenomenon is relevant, for example, to some diseases of the small intestine. In turn, disturbances in membrane flux can be associated with a wide variety of diseases. For example, endocytosis is often impeded in tumors. Infections or neurogenerative diseases can also cause disturbances in this respect. Neuropathies with impaired walking ability and reduced nerve conduction velocity as well as sensory disturbances are an example of neurodegenerative complaints due to impaired membrane flow. Also, mutation-related Huntington’s disease neurogenically disrupts membrane flux. In addition, neurotransmitter exocytosis may be impaired due to toxins. Impaired exocytosis also underlies metabolic diseases such as cystic fibrosis. Disorders of pinocytosis are also now associated with diseases such as Alzheimer’s disease. Disorders of membrane transport can not only have many different causes, but ultimately lead equally to many different symptoms and a wide variety of diseases.
