Microscopy lets us see cells and cell parts and even the location of molecules, but we have to dissect the cells if we want to ask what specific parts of the cell do. This scheme for fractionating cells by differential centrifugation may look familiar. Cells or tissues are homogenized or ground up in a buffered solution intended to resemble the physical chemical characteristics of cytoplasm. The homogenate is then centrifuged at low speed or centrifugal force or G-force. To sediment, we can also say pellet the largest structures in the cell, in this case the nuclei. The liquid above the nuclear pellet is again centrifuged at about to say 15,000 times G. These numbers are approximate and vary depending on the actual cell fractionation, but perhaps the next level of cell fractionation would occur at 15,000 times gravity and the pellet, the 15,000 G pellet, contains the next largest subcellular structures. In animal cells this would be mitochondria. In plant cells this would also perhaps include chloroplasts as well as mitochondria. You can follow the rest of the eukaryotic cell this section to the last supernatant on your own. The cytosol then is the last supernatant and it contains all of the soluble components or chemicals in the cytoplasm and no longer contains any of the particulates, any of the organelles or small structures in the cell. Again, this is a cell fractionation or a cellular dissection. Take a look at this electron micrograph of E. coli. Outline a cell fractionation protocol for these kinds of cells in which you've done differential centrifugation to get the different cell parts. Here are the basic parts, plasma membrane, cell wall, the cytoplasm, the interior of the cell containing the cytoplasm of course, which has mostly ribosomes, and the lighter region of the cell which actually contains the bacterial DNA which is not associated with a lot of proteins. It's basically a circular double helix which you may remember. So again, devise a cell fractionation protocol for E. coli. Once we've separated the parts of the cell, we can ask what do each of the parts do? The biochemical and molecular techniques we've developed over the last 75 to 100 years allow us to identify the particular chemical reactions that accomplish different cell functions. They allow us to associate specific reactions with specific dissected cell compartments and they allow us to design probes to visualize molecules and chemical reactions inside the cells and tissues using microscopic techniques.