This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
At isoelectric pH protein: 
Select and explain. 
A. Have Net charge 'O', B. Are positively charged, C. Are negatively charged, D. Don't migrate
A: Have Net charge 'O'
Answer: A: Have Net charge 'O' 
Explanation: At physiological pH (7.4), all amino acids have both a negatively charged carboxyl ion (C00-)and a positively charged ammonium ion (-NH3). The amino acids are therefore dipolar ion (Zwitter ions)