In neutral solution, an amino acid molecule exists as a dipolar ion (zwitter ion) i.e., molecule containing positive and negative ionic groups. The charge on the ion changes with $\mathrm{pH}$. In an acid solution (low $\mathrm{pH}$ ) the amino acid picks up $\mathrm{H}^{+}$ions and becomes positively charged. In alkaline solution (high $\mathrm{pH}$ ) the amino acid donates $\mathrm{H}^{+}$ions to the medium and becomes negatively charged. The $\mathrm{pH}$ at which the amino acid is electrically neutral or the molecule exists as a zwitterion is called the isoelectric $\mathrm{pH}$.