Active solute transport is a form of transport of substrates across a biomembrane. Active transport occurs against a concentration or charge gradient and occurs under energy consumption. In mitochondriopathies, this process is impaired.
What is active solute transport?
Active solute transport is a mode of transport of substrates across a biomembrane. In the human body, phospholipid and bilayer biomembranes separate individual cell compartments. Based on their membrane components, the different biomembranes assume active roles in selective mass transport. As a separating layer between several compartments, the biomembrane is intrinsically impermeable to the majority of all molecules. Only lipophilic, smaller and hydrophobic molecules diffuse freely through the lipid bilayer. This type of tuned membrane permeability is also known as selective permeability. Diffusible molecules include, for example, gas, alcohol and urea molecules. Ions and other biologically active substances are mostly hydrophilic and are stopped by the barrier of the biomembrane. In order for ions, water and larger particles such as sugars to diffuse, the biomembrane has transport proteins. They are actively involved in the transport of substances. Transport through a biomembrane is also called membrane transport or membrane flux, if the membrane itself is displaced in the process. Biomembranes and their selective permeability maintain a specific cellular environment inside the cell that promotes internal functional processes. A cell and its compartments communicate with their environment and engage in selective mass and particle exchange. Mechanisms such as active solute transport allow selective passage of membranes on this basis. Active solute transport must be distinguished from passive solute transport and membrane-displacing solute transport.
Function and task
Transport of substances across a biomembrane occurs actively or passively. In passive transport, molecules pass through the membrane without energy consumption in the direction of a specific concentration or potential gradient. Thus, passive transport is a special form of diffusion. Thus, even larger molecules reach the other side of the membrane with the help of membrane transport proteins. Active transport, on the other hand, is a transport process that takes place with the consumption of energy against the gradient of a biosystem. Different molecules can thus be selectively transported across the membrane against the chemical concentration gradient or the electrical potential gradient. This plays a role especially for charged particles. In addition to charge aspects, concentration aspects are also relevant for the energy balance of these. The reduction of entropy in a closed system leads to the amplification of the concentration gradient. This relationship plays as important a role in the energy balance as charge transport against the electric field or the resting membrane potential. Although we are concerned with charge or energy balance in the system, the particle concentration and its change must be considered separately because of the selectively permeable biomembrane. Energy for active transport is provided on the one hand as chemical binding energy, for example in the form of hydrolysis of ATP. On the other hand, the breakdown of the charge gradient can serve as a driving force and thus generate electrical energy. The third possibility of energy provision results from an increase in the entropy present in the respective communicating system and thus from the decomposition of a concentration gradient elsewhere. A transport against the electric gradient is called electrogenic. Depending on the source of the energy and the type of work, a distinction is made between primary, secondary and tertiary active transport. Group translocation is a special form of active transport. Primary active transport occurs when ATP is consumed and inorganic ions and protons are transported out of the cell through a biomembrane by transport ATPases. An ion is thus pumped, with the help of an ion pump, for example, from the lower concentrated to the higher concentrated side. The sodium–potassium pump is the most important application of this process in the human body. It pumps out positively charged sodium ions under ATP consumption and simultaneously pumps in positively charged potassium ions into a cell.Thus, the resting potential of neurons remains constant and action potentials can be generated and transmitted. In secondary active transport, particles are transported along the electrochemical gradient. The potential energy of the gradient serves as a drive to transport a second substrate in the same direction against the electrical gradient or concentration gradient. This active transport plays a role specifically in sodium–glucose symport in the small intestine. If the second substrate is transported in the opposite direction, secondary active transport may also be present, for example, in sodium-calcium antiport using sodium-calcium exchangers. Tertiary active transport uses a concentration gradient established by secondary active transport based on primary active transport. This type of transport plays a role mainly for di- and tripeptide transport in the small intestine, which is done by peptide transporter 1. Group translocation transports monosaccharides or sugar alcohols as a special form of active transport, chemically modifying the transport substances by phosphorylation. The phosphoenolpyruvic acid phosphotransferase system is the most important example of this type of transport.
Diseases and disorders
Energy metabolism as well as specific transporter enzymes and transporter proteins play a role in active metabolic transport. If the transporter proteins or enzymes in question, due to mutations or errors in the transcription of the genetic material, are not present in their originally physiologically planned form, then active metabolic transport is only possible with difficulty or, in extreme cases, not at all. Some diseases of the small intestine, for example, are associated with this phenomenon. Diseases with disturbed ATP supply can also have devastating effects on active substance transport and cause functional disorders of various organs. Only in a few cases of such diseases is only a single organ affected. In most cases, energy metabolism disorders are multi-organ diseases that often have a genetic basis. In all mitochondriopathies, for example, the enzyme system involved in energy production by oxidative phosphorylation is affected. These disorders include, in particular, the disruption of ATP synthase. This enzyme is one of the most important transmembrane proteins and thus appears, for example, in the proton pump as a transport enzyme. The main task of the enzyme is to catalyze the synthase of ATP. To provide energy, ATP synthase cross-links energetically favored proton transport with ATP formation along the proton gradient. Thus, ATP synthase is one of the most important energy converters in the human body and can convert one form of energy into other forms of energy. Mitochondriopathies are malfunctions of mitochondrial metabolic processes and result in reduced body performance due to reduced ATP synthesis.
