ranging between 150 and 200 Da. However, the water pores completely block ions including protons, essential to conserve the membrane's electrochemical potential difference. Water molecules traverse through the pore of the channel in single file. The presence of water channels increases membrane permeability to water. These are also essential for the water transport system in plants and tolerance to drought and salt stresses. Structure Aquaporin proteins are composed of a bundle of six transmembrane α-helices. They are embedded in the cell membrane. The amino and carboxyl ends face the inside of the cell. The amino and carboxyl halves resemble each other, apparently repeating a pattern of nucleotides. Some researchers believe that this was created by the doubling of a formerly half-sized gene. Between the helices are five regions (A – E) that loop into or out of the cell membrane, two of them hydrophobic (B, E), with an asparagine–proline–alanine ("NPA motif") pattern. They create a distinctive hourglass shape, making the water channel narrow in the middle and wider at each end. Another and even narrower place in the channel is the "ar/R selectivity filter", a cluster of amino acids enabling the aquaporin to selectively let through or block the passage of different molecules. Aquaporins form four-part clusters (tetramers) in the cell membrane, with each of the four monomers acting as a water channel. Different aquaporins have different sized water channels, the smallest types allowing nothing but water through. X-ray profiles show that aquaporins have two conical entrances. This hourglass shape could be the result of a natural selection process toward optimal permeability. It has been shown that conical entrances with suitable opening angle can indeed provide a large increase of the hydrodynamic channel permeability. NPA motif Aquaporin channels appear in simulations to allow only water to pass, as the molecules effectively queue up in single file. Guided by the aquaporin's local electric field, the oxygen in each water molecule faces forwards as it enters, turning around half way along and leaving with the oxygen facing backwards. Why this rotation occurs is not entirely clear yet. Some researchers identified an electrostatic field generated by the two aquaporin half-helices HB and HE as the reason. Others suggested that hydrogen bonds between asparagine amino acids in the two NPA regions and the oxygen in the water cause the rotation. It is still unclear whether the rotation of water molecules has any biological significance.