In multi-pass membrane proteins, the interaction between multiple transmembrane domains determines their structure and function. G protein-coupled receptors are the largest family of membrane proteins. They contain seven transmembrane alpha-helices which act to transmit signals between the cell's extracellular and intracellular environments. In contrast, many channel-forming membrane proteins, such as porins, contain multiple beta-strands of the protein. These form hydrogen bonds to form a continuous cylindrical beta-sheet, creating a rigid ring-like structure called a beta-barrel. The amino acids in the strands alternate between polar and non-polar residues. Non-polar groups point towards the outside of the barrel and interact with the hydrophobic membrane. The polar side chains orient towards the inner hydrophilic opening, which forms a channel from the extracellular to intracellular space allowing the passage of small polar solutes.