Cells have many mitochondria because they are the site of aerobic respiration. They release energy in the form of ATP. They can also synthesise proteins.
In ultracentrifugation Cell tissues are homogenised in order to break open the cell and release its contents.
The resulting suspension is filtered in order to remove cell debris.
The suspension is kept ice cold because this reduces any potential enzyme activity.
An isotonic solution is used to prevent osmosis so that organelles are not damaged or burst by excess water enetering the cell.