When plants are used to make paper, it is usually necessary to use a special chemical process to break down the lignin found inside the cell walls of the plant. Generally, this is done with the Kraft process. If the fibers used to make the paper are recycled, it is not necessary to undergo this process because the lignin has already been removed. If the lignin is not removed from the pulp, the resulting paper will turn yellow when it is exposed to light and air.