Experimentation has shown that cells actively pump potassium ions (K+) in and pump sodium ions (Na+) out against concentration gradients. Since this goes against the natural process of diffusion, students can guess that it requires energy and it does. Cells expend ATP (ATP –>ADP + P + energy) in this process. The conclusion is that cells maintain water balance by pumping salt ions in or out; water follows. (A an animated depiction of this may be seen the summary video at the end of the next section.)