Vessels containing a cell culture usually suspended in growth medium. Bioreactor sizes can range from small (5-10 mL) to very large (thousands of liters) and can implement a variety of agitation methods to keep the cell suspension from settling to the bottom. Common agitation methods include an impeller or vertical wheel to stir or manually move the liquid media, physically rotating the bioreactor to keep cells in a free-fall (microgravity), or using a shaker to cause the liquid to move in a bag. In perfusion bioreactors, cells are attached to the inside walls of the bioreactor and culture media is pumped through channels to ensure cells receive proper nutrients.