Passive mass transport is the diffusion of substrates across a biomembrane. This diffusion occurs along the concentration gradient and does not require energy. The diffusion process may be impaired in the intestines of HIV patients, for example.
What is passive mass transfer?
Passive solute transport is the diffusion of substrates across the biomembrane of cells in the human body. Cells or cell formations are separated from each other in the body by a biomembrane. This flexible separation layer, through its specialized structures, allows the transport of specific molecules and information into and out of the cell interior. There are two basic modes of transport of substances into and out of the membrane. Membranes have selective permeability. They allow some substances to diffuse while providing a barrier to others. The mass transfer mode of active transport allows membranes to open selectively to molecules for which they are not actually permeable due to their charge, concentration or size. Active transport always takes place with the expenditure of energy. A distinction must be made between this and the mass transport type of passive transport. In this form of mass movement through a cell membrane, no energy is required. Passive transport can be equated with diffusion processes that take place along the concentration gradient and establishes a concentration balance between the two sides of the membrane.
Function and task
Within a cell or cellular compartment, there exists chemically and charge-wise a certain milieu that is required for the cell to perform its function. This milieu is maintained solely by biomembrane properties and selective permeability. Passive and active solute transport provide the cell or cell compartment with exactly the right amount of the substances needed to maintain a conducive milieu. There are two different types of passive transport. Simple diffusion involves lipid-soluble molecules and occurs at an extremely slow rate. They diffuse freely across the cell membrane. This form of passive transport is the one with the least effort. The second type of passive diffusion is facilitated diffusion, which again can be divided into two subtypes. One of these subtypes is carrier-mediated facilitated diffusion. In this form of passive mass transfer, the membrane accepts the substrate with the help of a so-called carrier. The carrier is a protein for labeling the substance to which the substrate binds. Since simple diffusion occurs at low velocity, the carrier helps transport the substance across the biomembrane. The number of all carrier molecules is limited. For this reason, transport through a carrier molecule is subject to saturation kinetics. Passive mass transport through carrier molecules may also be subject to competitive inhibition. When a carrier molecule binds to its substrate, it changes conformation and rearranges itself accordingly. As a result, the substrate molecule is carried across the biomembrane and is only released on the opposite side. Some carriers can only carry one molecule at a time and thus have a uniport. Other carriers have binding sites for two different molecular substrates and only change conformation when both binding sites are occupied. Thus, the two molecules are either symported in the same direction or antiported in opposite directions. Thus, there is no dependence on the electrical gradient. The second type of facilitated diffusion is through pores and channels. This form of transport involves amino acids in particular. For example, during ion transport, the substrate of the amino acid is taken up into the cell membrane through pores. The channels are formed by proteins. Special binding sites are present on these protein-containing channels. Thus, facilitated diffusion through pores and channels is a selective mass transport that can be influenced electrically and chemically. Almost all channels are opened only in response to specific signals. For example, a ligand-gated channel responds only to a messenger substance such as a hormone. Some channels are voltage-gated and open for diffusion with a change in membrane potential. After concentration equilibration, the channels close again.
Diseases and ailments
When membrane permeability, and therefore passive solute transport, is disturbed, the permeability of various ions is no longer ideally regulated. Such membrane permeability disorders often develop from cardiovascular disease and sometimes affect electrolyte balance. Sometimes membrane permeability disorders are also hereditary. Various proteins build up the biomembrane and give it a selectively permeable double lipid layer. If the proteins involved in this process are altered, membrane permeability also changes. This phenomenon is present, for example, in myotonia congenita Thomsen. This genetic disorder of muscle function mutates a gene responsible for coding the individual chloride channels in the muscle fiber membranes. Due to the mutation, permeability to chloride ions is reduced, causing muscle stiffness. Autoimmune diseases can also target the biomembrane, such as antiphospholipid syndrome. As part of the disease, the immune system attacks the phospholipid-bound proteins of the membrane. The resulting increased tendency to blood clotting also increases the risk of heart attacks and strokes. Mitochondriopathies also alter membrane permeability. The mitochondria are the body’s own energy power plants that throw off free radicals during energy production. In healthy individuals, these substances are scavenged. This process fails in mitochondiopathy patients, damaging the membranes and greatly reducing the mitochondria‘s energy-generating capacity. The passive and active transport of substances through the membranes of the small intestine is specifically affected by disorders such as HIV enteropathy. This phenomenon particularly affects HIV patients with chronic diarrhea and may be associated with decreased activity of interstinal enzymes.
