Centrifugation

Centrifugationis a process that involves the use of the centrifugal force for the separation of mixtures, used in industry and in laboratory settings. Inchemistryandbiology, centrifugation increases the effective gravitational force on a mixture in a test tube, to rapidly and completely bring the precipitate ("pellet") to the bottom of the tube. The remainingsolutionis called the "supernate," "supernatant," or supernatant liquid.The supernatant liquid is then separated from the precipitate by decantation or withdrawal with a Pasteur pipette.The equipment used for centrifugation is called acentrifuge, and the vessel that spins the samples is called arotor. Generally, a motor causes the rotor to spin around a fixed axis, applying a force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration is used to separate substances of greater and lesser density.

There are many different kinds of centrifuges, including those for very specialized purposes. In the chemical and food industries, special centrifuges can process a continuous stream of particle-laden liquid.

English military engineer Laval (1707-1751) invented a whirling arm apparatus to determine drag, and Antonin Prandl invented the first centrifuge in order to separate cream from milk to make it easier to churn butter.

Differential Centrifugation

If you had sufficient time and a vibration-free environment, you could patiently wait and the force of gravity would bring most suspended particles to the bottom of a centrifuge tube. The smallest particles would probably stay in suspension due to brownian motion, and most macromolecules would be uniformly distributed because they would be in solution rather than suspension. I don't know about you, but I don't have the kind of patience needed in order to rely solely on gravity for separation of solid from liquid components. Besides, for practical purposes the pellet you obtained would be way too easily disrupted for effective separation of solid material from supernatant. Gravity would not be a terribly effective way of separating suspended materials based on size or other characteristics.

Density gradient centrifugation using tubes is the most widely employed technique for separating cells and cell organelles and for isolating cellular macromolecules. However, although it is one of the cell biologists most valuable tools, it is not without disadvantages, as the amount of material that can be fractionated in a single tube is so small.

When large quantities of sample must be fractionated (to isolate sparse organelles such as lysosomes or peroxisomes), a very large number of tubes and gradients is needed. Much larger quantities of sample may be fractionated using zonal rotors.Azonal rotor consists of a large cylindrical chamber subdivided into a number of sector-shaped compartments by vertical septa (or vanes) that radiate from the axial core to the rotor wall. The entire chamber is used during centrifugation and is loaded with a single density gradient, each sector-shaped compartment serving as a large centrifuge tube.The large chamber capacity of these rotors (typically 1 and 2 liters) eliminates the need for multiple runs and multiple density gradients.