In Bt cotton, the Bt to xin present  in plant tissue as protoxin is converted into active toxin due to

The Bt toxin protein exists as inactive protoxins in Bacillus thuringiensis but once an insect ingests the inactive toxin, it is converted into an active form of toxin due to the alkaline pH of the gut which solubilise the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of the insect.