ABSTRACT
The simple experiment many schoolchildren carry out to demonstrate the effect of gravity on a suspension of particles remains as instructive today as it always has. The problems of contamination and poor recoveries in differential centrifugation fractions are compounded by the heterogeneity in the size of any one type of biological particle. In most cases, however, to increase the purity of differential centrifugation fractions, one or more of the density gradient techniques that are discussed in the chapter must be used. When centrifugation begins, the particles sediment through the gradient towards the bottom of the centrifuge tube as with differential centrifugation. As with differential centrifugation, the speed at which the particles sediment depends principally on their size and to a smaller extent on their density and shape. In rate-zonal centrifugation all the particles essentially start from approximately the same place, i.e. a narrow band on top of the gradient.
