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.
For a more controlled whitening process, bleaching trays can be custom made to fit the teeth, and a whitening gel is applied to the teeth for a number of hours each day. Another option is over-the-counter whitening strips that can be applied directly to teeth.