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.
If you have a mouthful of dental work, you may want to skip at-home whiteners. Bleaching won't whiten porcelain veneers, crowns, or white (porcelain or composite) fillings, so you could wind up with teeth that look mismatched in color, says Kantor.