The initial step in purifying subcellular structures is to release the cell’s contents by rupturing the plasma membrane and the cell wall, if present. To do this, the cells are suspended in a solution of appropriate pH and salt content, usually isotonic sucrose (0.25 M) or a combination of salts similar in composition to those in the cell’s interior. Many cells can then be broken by stirring the cell suspension in a high-speed blender or by exposing it to ultrahigh-frequency sound (sonication). Alternatively, plasma membranes can be sheared by special pressurized tissue homogenizers in which cells are forced through a very narrow space between a plunger and the wall of the vessel; the pressure of being forced between the vessel wall and the plunger ruptures the cell.

Recall that water flows into cells when they are placed in a hypotonic solution; that is, one with a lower concentration of ions and small molecules than is found inside the cell. This osmotic flow can be used to cause cells to swell, weakening the plasma membrane and facilitating its rupture. Generally the cell solution is best kept at 0 °C to preserve enzymes and other constituents after their release from the stabilizing forces of the cell.

Disrupting the cell produces a mix of suspended cellular components, the homogenate, from which the desired organelles can be retrieved. Because rat liver contains an abundance of a single cell type, this tissue has been used in many classic studies of cell organelles. However, the same isolation principles apply to virtually all cells and tissues, and modifications of these cell-fractionation techniques can be used to separate and purify any desired components.