Consider a transmembrane protein that forms a hydrophilic pore across the plasma membrane of a eukaryotic cell. When this protein is activated by binding a specific ligand on its extracellular side it allows Na⁺ to enter the cell. The protein is made of five similar transmembrane subunits, each containing a membrane-spanning α helix with hydrophilic amino acid side chains on one surface of the helix and hydrophobic amino acid side chains on the opposite surface. Considering the function of the protein as a channel for Na⁺ ions to enter the cell, propose a possible arrangement of the five membrane-spanning α helices in the membrane.