yeah you need to know it.
it is called cell fractionation, and it is somewhere in the biozone book.
the main principle behind it is that heavier/denser organelles, will sink to the bottom
main purpose is to get samples of individual types of organelles.
basically what happens is:
-cells put in some kind of blender, chopped up to break cell membrane, releasing all of the organelles.
-sample is then put into a test tube, and placed into a machine called a centrifuge.
-the thing then spins, applying a force to the solution. heavier organelles will fall to the bottom the easiest. (i.e. at a slow speed). once the heaviest organelles have been separated to the bottom of the test tube, forming what is called a pallet, the solution above it, the supernatant, is removed, placed in another test tube.
-the process is then repeated, at a higher speed, and the next heaviest organelles will form a pellet. Process continues until you have separated all of the organelles.
As heaviest organelles form a pellet the easiest, the order of collection is usually nuclei, mitochondria, ribosomes etc...
here is a pic which might make it clearer...
http://www.freewebs.com/ltaing/chpt7.3Cellfractionation.gif