Proteins with transporting functions can be roughly classified into 3 categories: ATP-powered pumps, ion channels, and transporters. Pumps utilize the energy released by ATP hydrolysis to power the movement of the substrates across the membrane, against their electrochemical gradient. Channels at the open state can transfer the substrates (ions or water) down their electrochemical gradient, at an extremely high efficiency (up to 108 s-1). Transporters facilitate the movement of a specific substrate either against or following their concentration gradient, at a lower speed (about 102 -104 s-1); as generally believed, conformational change of the transporter protein is involved in the transfer process.
According to the Human Genome Organization (HUGO) Gene Nomenclature Committee, all human transporters can be grouped into the solute-carrier (SLC) superfamily. Currently, there are 55 SLC families in the superfamily, with a total of at least 362 putatively functional protein-coding genes (Hediger et al, 2004; He et al., 2009). At least 20-25% amino-acid sequence identity is shared by members belonging to the same SLC family. No homology is shared between different SLC families. While the HUGO nomenclature system by definition only includes human genes, the nomenclature system has been informally extended to include rodent species through the use of lower cases letters (e.g., Slc1a1 denotes the rodent ortholog of the human SLC1A1 gene). And it's worthwhile to mention that pumps, channels and aquaporins are not included in SLC superfamily.