Arrangements of α-sheet blocks. For each block only one sheet is shown. a) is to be compared with the potassium channel of Fig 1j; The two adjacent blocks of α-sheet are of opposite polarity such that two rows of adjacent δ-atoms arise with potential for forming a cation channel. b) shows two blocks of adjacent α-sheet with the same polarity; they are liable to simply form a larger block of α-sheet. c) shows two α-sheet blocks with their sheets at right angles; the single row of δ-atoms seen has the potential to form a water channel as in aquaporin in Fig 1k, or a proton channel. Note the characteristic zigzag shape of the α-strands in this view. Membranes made of mixtures of α- and β-sheet blocks are also illustrated in Fig 2b.