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.
Whitening toothpastes won't bleach your teeth to a bright white -- but they will remove surface stains. For example, if drinking tea has given your teeth mild brown staining, a whitening toothpaste may help to get rid of some of the tea residue and improve the look of your teeth.