Porin proteins are formed from a beta barrel, inserted into the membrane. This allows the formation of a pore through which molecules can diffuse. These are generally only found in Gram-negative bacteria, as it is easy for H+ to pass through (the pore is not highly selective), and so cannot be used in membranes requiring the maintenance of a proton gradient. Aromatic amino acids are usually found on the section of the protein at the peripheries of the membrane - this provides additional surface area to interact with the membrane (due to the larger side chains), helping to hold the protein in place and preventing the porin from moving vertically.
α-helical membrane proteins are formed from α helices, spanning the membrane (transmembrane). Due to the smaller diameter of the channel formed, these membrane proteins are much more specific, preventing the loss of proton gradients. This demonstrates their increased selectivity, allowing this class of channel to be used in more circumstances. α-helical membrane proteins may have extracellular and cytosolic domains, allowing for gating by a ligand (if a gate is present).