ABSTRACT
Controlled transport of solutes across cell membranes is a prerequisite for maintaining viability and functionality of the cell. The process is controlled by proteins that are embedded to the cell membrane and capable of mediating uxes of small molecules and/or ions either by generating small selective pores in the membrane (ion channels) or by carrying out a specic translocation processes (transporters). When a channel protein opens, thousands to millions of ions pass the membrane through a pore, down the electrochemical gradient of the individual ion. In contrast, transport proteins do not form pores. Rather they possess a binding site for the transported solute that is only accessible to one side of the membrane at any given time. For that reason, transport proteins-or carriersfacilitate the movement of only one or very few molecules per transport cycle. The transported substance can either be an ion or a small molecule and the movement can either occur with or against its electrochemical gradient. When the transport occurs against the gradient, the process is energetically coupled, either directly through the hydrolysis of ATP by the transport protein itself or indirectly by the use of transmembrane ion gradient. Not surprisingly, a vast amount of different transport proteins are found in both prokaryotic and eukaryotic organisms, transporting everything from nutrients, amino acids, and metabolites to ions, drugs, proteins, toxins, and transmitter molecules. It is generally assumed that about 10% of all human genes are transporter-related where the major classes in humans encompass ATP-driven ion pumps (e.g., the ubiquitously expressed Na-K ATPase), ATP binding cassette (ABC) transporters (e.g., the cystic brosis transmembrane conductance regulator and the multidrug resistance transporter p-glycoprotein), cytochrome B-like proteins, aquaporins (water transporters), and the solute carrier superfamily (SLC) (https://www. bioparadigms.org).
