Fractionation

You will remember that plant and animal cells (eukaryotes) are different from bacteria cells because they have organelles. These are structures smaller than cells that help the cells stay organized. They have membranes around them and usually have a few specific jobs. By specializing different organelles eukaryotic cells are able to localize different activities to specific sites in the cell. This makes the cell more efficient because the cell by concentrating the proteins needed for specific activities, the cell can have fewer copies of each protein. This is like how you can just have a washing machine in the laundry room, which is the same location as the dirty clothes and the dryer, rather than needing a washing machine in every room just in case someone thought that that would be a good place to wash clothes. This characteristic of cells makes it both harder and easier to study specific cellular activities. It is harder because if you grind up a whole giant cell and there is only one washing machine, it will be hard to find that specific machine when it is surrounded by lots of other machines. It is easier if you know what room (organelle) your machine is in (the laundry room). Because you can collect just that specific organelle and have concentrated your activity. This works because the organelles are surrounded by membranes, so,  unlike if you were to smash up a house and everything got all mixed up, the organelles are all self contained.

There are many different ways of isolating specific organelles. One is differential centrifugation. This approach relies on the fact that all the organelles have different densities. Of you make all different densities of sucrose and stack them you end up with a gradient with very dense sugar on the bottom and light sugar on the top. If you then put mashed up cells at the top and spin it at high speeds for a long time, everything will sink to the point where it the the same density as the sugar around it. So you end up with heavy things like nuclei at the bottom, light things like membranes at the top, and all of the middle sized stuff in the middle. This way you can find which fraction has the thing you are interested in and get just an enrichment of that and not the rest of the other cell gunk. It takes some extra effort, and some extra mess, but you have more options of what you can do next. 

Like anything else in science, or life really, things get much more interesting if you add fluorescent dyes and black lights.