Various integral proteins have multiple membrane-spanning   -helices. Bacteriorhodopsin, a protein found in a photosynthetic bacterium captures the energy from light and uses it to pump protons diagonally the bacterial membrane. Like a numerous other integral proteins, like as the G protein-coupled receptors &  the polypeptide   chain of bacteriorhodopsin loops backwards, forwards diagonally the lipid bilayer seven times. Each of the seven transmembrane -helices is associated to the next through a short hydrophilic region of the polypeptide chain   which is exposed   either   on the cytosolic   or extracellular   side of the membrane. Another multiple membrane spanning proteins have from two to up to 14 transmembrane helices. For instance, the anion exchange band 3 protein of

figure: Integral membrane proteins. (a) One membrane-spanning region (e.g. glycophorin); (b) multiple membrane-spanning regions (e.g. bacteriorhodopsin).

The erythrocyte   plasma  membrane  which  transports  HCO ^- and Cl^- across  the membrane loops forwards and backwards across the lipid bilayer up to 14 times.