Cell fractionation is a combination of various methods used to isolate (fractionate) cell structures based on size and density. This technique takes cells apart and separates major organelles and other subcellular components from one another. There are two phases of cell fractionation: homogenization and centrifugation.
Homogenization is the process of breaking open the cells, usually by a bender. Cells are broken apart by chemicals, enzymes, or sound waves. Some scientists even force the cells through small spaces at high pressure to break them apart. The resulting mixture of disrupted cells, called homogenate, is then centrifuged to allow the isolation of the cell organelles. This procedure of differential centrifugation spins test tubes containing homogenate at a series of increasing speeds. For each increment of speed, the supernatant (liquid) is poured into another tube and centrifuged at a higher speed for a longer time. Each speed causes a fraction of the cell components to settle to the bottom of the tube, forming a pellet. At lower speeds, the pellet consists of larger components, and higher speeds yield a pellet with smaller components. Therefore at the end of the process, the mitochondria, the nucleus, the chloroplast and all other components have been isolated.
Cell fractionation enables researchers to prepare specific cell components in bulk and identify their functions, a task not usually possible with in tact cells. Scientists use this tool to increase their knowledge of organelle functions. To be able to do so they isolate organelles into pure groups, such as isolating the mitochondria or the nucleus. This method has already resulted in the knowledge several cell organelles’ functions.
For example, by centrifugation a specific cell fraction was determined to have enzymes that function in cellular respiration. This unknown cell fraction was rich in mitochondrias. Therefore there researchers obtained evidence that helped determine mitochondrias were the site of cellular respiration.
Reference: Campbell, Neil A. 2010. Campbell Biology. 9th ed. San Francisco, CA: Benjamin-Cummings.