Figure 1. Direction of net chloride movement in two cells, one with high intracellular chloride (a) and one with low chloride (b). (a) When the intracellular chloride concentration is higher, the electrical force on the chloride ions dominates and chloride ions move out of the cell. (b) When the intracellular chloride concentration is low, the chemical driving force dominates and chloride moves into the cell.

Figure 2. Schematic illustration of a chloride secretory epithelium such as the intestinal crypt cells. Chloride is transported across the basolateral membrane, into the cell, through the ion-coupled chloride cotransporter ( Na⁺/ K⁺/2 Cl^– cotransporter). This raises the intracellular chloride concentration so that, when the apical membrane chloride channel opens, chloride flows out of the cell into the lumen. This results in vectorial movement of chloride from the blood to the intestinal lumen.

Figure 3. Schematic illustration of the transmembrane topology of CFTR: NBF, nucleotide-binding fold; R, regulatory domain, the location of multiple phosphorylation sites. NH₂ indicates the N-terminus of the protein,  COOH indicates the C-terminus of the protein.

Figure 4. Structure of the bacterial homologue of the ClC channel. (a) Schematic illustrating the transmembrane topology. helices shown as cylinders. Note that several of the helices do not traverse the entire membrane but form reentrant loops. Protein regions participating in forming the ion binding site are in red. Red sphere represents a chloride ion. The protein is formed as an inverted tandem repeat, the N-terminal half is green and the C-terminal half is blue. (b) Illustration of the three-dimensional ClC channel protein structure. Colour scheme as in (a). (c) Top view of the homodimer that contains two of the subunits shown in (b), one in red and the other in blue. helices are shown as coils. Chloride ions are shown as green balls. Reproduced with permission from Dutzler et al. 2002.

Figure 5. Barttin is an auxiliary subunit for the kidney ClC-Ka and ClC-Kb channels. (a) Illustrates the proposed transmembrane topology of barttin. (b) Illustrates the polarized location of transporters and channels in the  Cl^– absorptive epithelium of the thick ascending limb of the loop of Henle. Note the location of barttin and the ClC-Kb channels in the basolateral membrane. (c) Illustrates the location of channels and transporters in the  K⁺ secretory epithelium of the inner ear. Note the location of barttin and the ClC channels in the basolateral membrane. (b, c) Reproduced with permission from Estevez et al. 2001.

Figure 6. Overview of GABA_A receptor structure. (a) Illustration of top view of channel showing five subunits assembled around central channel axis. The channel is surrounded by an inner ring formed by the five M2 membrane-spanning segments (blue). The outer helix ring (red) is formed by the M1, M3 and M4 helices from each subunit. (b) GABA or glycine receptor subunit transmembrane topology. (c) 4 Å resolution structure of the homologous acetylcholine receptor. Extracellular domain is shown in green. Membrane-spanning segments are coloured as in (a). Limits of membrane indicated by black dashed lines. (a, c) Reproduced with permission from Miyazawa et al. 2003.