When fractionating cells, use a cold (reduce enzyme activity), isotonic (maintains water potential outside organelles to prevent them from shrivelling or bursting), buffer (prevent damage to organelles from pH) solution.
To homogenise the cells, blend them in a blender with the cold and isotonic buffer solution, then filter to remove the large debris.
Differential ultracentrifugation is used because the organelles have different densities so form a pellet at different speeds. This can be used to separate organelles.
