See image — Biomolecules Chemistry Question

💡 Solution & Explanation

The isoelectric point (pI) of an amino acid or protein is the pH at which the molecule carries no net electrical charge. At this specific pH, several conditions are simultaneously true: Step 1 - Concept: At the isoelectric point, the molecule exists predominantly as a zwitterion (dipolar ion), where the positive and negative charges cancel each other out. Step 2 - Option (a): At the isoelectric point, the concentration of cationic form (fully protonated, net positive) equals the concentration of anionic form (fully deprotonated, net negative). This is true because the net charge is zero, meaning positive species and negative species are present in equal amounts, balancing each other. Step 3 - Option (b): The net charge on the molecule is zero at the isoelectric point by definition. The zwitterion has equal positive and negative charges internally, resulting in no net charge. This is true. Step 4 - Option (c): At the isoelectric point, the zwitterionic (dipolar ion) form is at its maximum concentration. The equilibrium strongly favors the zwitterion form at this pH, as it is the most stable form. This is true. Step 5 - Since all three statements (a), (b), and (c) are correct descriptions of what occurs at the isoelectric point, option (d) 'All of the above' is the correct and most complete answer. Therefore, the correct answer is D.