A molecule with both an acidic and a basic group (positive and negative charge). Example: glycine.

From the German der Zwitter, hybrid or hermaphrodite.

A zwitterion is a dipolar ion that is capable of carrying both a positive and negative charge simultaneously.

The best examples of this are amino acids. Amino acids have a basic NH2 group and an acidic COOH group. What usually happens is that the H from the COOH group (carboxylic acid end) migrates to the NH2 (ammonia) group; this causes what is known as a "dipole moment."

This writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/

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