When a hydrophilic sol like gelatin is subjected to electric field, the sol particles move

At pH < isoelectric pH, -NH₂ group is protonated to $-\stackrel{+}{\mathrm{N}}{\mathrm{H}}_3$ and the sol is positively charged. At pH > isoelectric pH, -COOH is deprotonated to —COO^- and the sol is negatively charged.    At isoelectric pH both groups are equally ionised, the sol particles carry no net charge.