Zwitterionic Form in Amino Acid

In my laboratory workbook, it has been mentioned that the zwitterionic form of the amino acid is the dominant form of the amino acid at pI and no movement of the molecules takes place under an external electric field. I am a bit confused by this statement.


It is clearly not possible for all the molecues to exist in the zwitterionic form. Some of them must exist in any of the other charged forms, even at pI. This should lead to these few charged molecules, moving in an electric field.


My question is:


* Is it true that all molecules are not zwitterions at pI?
* When we say no migration of ions takes place at pI, do we actually mean no "NET" migration of ions occurs i.e. whatever number of ions move towards one side, causes an equal number of ions to move towards the other?


