Final answer:
Na+ does not fit through highly-selective K+ channels because the channels are tailored to the dehydrated size of K+ ions. Smaller Na+ ions are excluded due to stronger interactions with surrounding water molecules, which prevents them from binding effectively to the channel's amino acids.
Step-by-step explanation:
Despite Na+ being smaller than K+, it doesn't fit through the highly-selective K+ channel (KcsA) due to the specificity of ion channels based on the diameter of the pore and the need for the ion to shed its surrounding water molecules. The highly-selective K+ channels are tailored to accommodate the specific size of K+ ions once they are dehydrated. Smaller Na+ ions do not interact as efficiently with the amino acids lining the K+ channel because the interactions between the water molecules and the Na+ are stronger than those with the channel's amino acids, causing a size exclusion effect. This specificity is critical to the function of cells, as it allows for the regulation of ion flow, impacting processes like electrical signalling and cellular homeostasis.