1 concentration gradient The difference in the concentration of a chemical across a membrane. 2  cytoplasm The living substance inside a cell (not including the nucleus). 3  diffusion The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.
Not all stains can be lightened by over-the-counter or professional bleaching, and your dentist will be able to give you guidance. If you have sensitive teeth, gum disease or teeth with worn enamel, your dentist may discourage tooth whitening.
Before starting a bleaching treatment with a tooth whitening gel, see your dentist. He can make a custom tray to fit neatly with the contours of your mouth and explain how to properly use a gel. If you use an over-the-counter product, look for one with the American Dental Association Seal of Acceptance.