Question 24:
The pH at which an amino acid has no net electric charge and thus does not move in an electric field is called:
The pH where an amino acid carries zero net charge and remains stationary in an electric field is its isoelectric point. The correct term is option (B).
Correct Answer
(B) Isoelectric
At the isoelectric point (pI), the amino acid exists as a zwitterion with balanced positive (NH₃⁺) and negative (COO⁻) charges, calculated as pI = (pKa₁ + pKa₂)/2 for neutral side chains.
Key Concept
Below pI, the amino acid is cationic (migrates to cathode); above pI, anionic (to anode). This property enables separation via electrophoresis or isoelectric focusing.
Option Analysis
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(A) Isomer: Incorrect; molecules with same formula but different arrangements, unrelated to charge neutrality.
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(B) Isoelectric: Correct; specific pH of zero net charge, critical for protein solubility minimum.
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(C) Isoprene: Incorrect; C₅H₈ hydrocarbon unit in terpenes and rubber biosynthesis.
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(D) Isozyme: Incorrect; enzyme isoforms with identical function but different structures.
| Option | Term | Definition | Status |
|---|---|---|---|
| A | Isomer | Structural/ stereo variants | Wrong |
| B | Isoelectric | Zero net charge pH | Correct |
| C | Isoprene | Terpene building block | Wrong |
| D | Isozyme | Enzyme variants | Wrong |
Exam Preparation Tips
For glycine: pKa₁(COOH)=2.34, pKa₂(NH₃⁺)=9.60 → pI=5.97. Acidic amino acids have lower pI; basic have higher. Essential for NEET biochemistry.
