Final answer:
The specificity of the voltage-gated K+ channel for K+ ions is due to the amino acid sequence and the size of the pore, which work together to select for the K+ ions.
Step-by-step explanation:
The voltage-gated K+ channel's specificity for K+ ions is a result of two main factors: the amino acid sequence in the pore and the size of the pore. The amino acids within the structure of the channel protein are sensitive to charge and will open in response to changes in the transmembrane voltage, contributing to the availability of the channel to the selected ion, in this case, K+ ions. However, the specificity comes from both the size of the pore, which matches the diameter of the dehydrated K+ ion, and the arrangement of the amino acids within the channel that participate in size exclusion, allowing only K+ ions to pass and excluding other ions based on their size.
The sequence of amino acids in the pore often provides the key interactions with the ion, and this sequence is consistent with the size of the ion once it has shed its surrounding water molecules. Moreover, the size of the pore acts as a physical constraint, preventing smaller or larger ions from passing through efficiently, which also contributes to the specificity of the channel towards K+ ions. Therefore, both the sequence of amino acids in the pore and the size of the pore are correct in determining the channel's specificity, making option 5 the correct answer: B and C are correct.