Erratum

There is a mistake in the textbook regarding the permeability of the lipid bilayer. A pure phospholipid bilayer is only slightly permeable to water. Water molecules move across the cell membrane through water channels, which only admit water molecules. These channels are so narrow that particles larger than the water molecule cannot squeeze through. Because of the positive charge at the center of the channel, positively charged ions such as H3O+ (the hydrogen ion) are deflected. This prevents proton leakage through the channel.