I am really happy to be the first to answer. When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products. That is what I managed to find for you. And if you need to further develop your topic, you may actually do that with the help of the writers I like much. Google by Marvelous essay service.