The diameter of the KcsA ${\mathit{K}}^{\mathbf{+}}$channel is $\mathbf{~}\mathbf{6}\mathit{A}\mathbf{°}$. Why can't ${\mathit{H}}_{\mathbf{2}}\mathit{O}$(diameter $\mathbf{2}\mathbf{.}\mathbf{75}\mathit{A}\mathbf{°}$) pass through this channel?

Trans-membrane proteins known as $K^{+}$channel passively transportpotassium ions from the cytoplasm to the extracellular environment.Streptomyces lividons has a $K^{+}$channel called KcsA. The integral membrane protein KcsA has 158 residues.

KcsA${\text{K}}^{\text{+}}$channel has a diameter of $6A°$, whereas${\text{H}}_{\text{2}}\text{O}$is 2.758. KcsA${\text{K}}^{\text{+}}$channels have a large central pore area, allowing hydrated K to pass through.

The pore’s top portion is filtered selectively, causing a transiting ion to obstruct the water hydration route into the KcsA ${\text{K}}^{\text{+}}$channel. By causing ahydrogen-bond modification, the ${\text{K}}^{\text{+}}$ion within the channel prevents water molecules from accessing.

Hence, the ion within the channel preventswater molecules from entering by altering the hydrogen bond.