In the kidneys, water and salt move in and out of the nephron through the membranes of the cells that make up the tube. The cell membrane must prevent important chemicals from leaking out and undesirable chemicals from leaking in, as well as allow movement of material such as oxygen and waste products in and out of the cell. How is this accomplished? We'll describe two different ways by which chemicals move through the membrane: passive diffusion and active transport.

Active Transport

Active transport across membranes can move molecules regardless of the concentration gradient (see passive diffusion below). It requires energy (hence the term "active"), which is provided by ATP molecules. One well-studied active transport mechanism, shown in this animation, is the sodium-potassium pump, which pumps potassium ions into the cell while pumping sodium ions out. In the animation, the purple tube that spans through the lipid bilayer is the pump.

Sodium ion	Phosphate	ADP	Potassium ion

To start the pumping process, a sodium ion attaches to a protein. Next, an ATP molecule splits (yellow explosion), yielding a phosphate group and an ADP molecule. The sodium is released outside the cell, and a potassium ion enters the pump to be released inside the cell after the phosphate group is released.

Passive Diffusion

Passive diffusion is the process by which molecules space themselves apart into the surrounding area. For example, if you were in a gym with a large group of people running in place, you wouldn't huddle in a corner; instead, you would all spread out. Molecules thus move from a more concentrated area to a less concentrated one down the concentration gradient.

Water

A cell membrane is selectively permeable; some molecules such as oxygen and carbon dioxide, can pass through unimpeded. Water, represented by a green molecule in the animation, is also one such molecule. The animation shows water passing through the lipid bilayer from the inside (top) to the outside (bottom) of the cell.