A scientist carried out an experiment to separate the organelles in an animal cell by mass.  

The scientist mixed the cells with a buffer solution which had the same water potential as the  cells. The cells were broken open with a blender to release the organelles.  

The extracted mixture was filtered and then spun in a centrifuge at a high speed to separate the  heaviest type of organelle. These sank to the bottom, forming solid pellet 1.  

The liquid above pellet 1 was poured into a clean centrifuge tube and spun in the centrifuge at a  higher speed to separate the next heaviest type of organelle. These organelles sank to the bottom, forming solid pellet 2. 

This procedure was repeated twice more to obtain pellet 3 and pellet 4, each containing a single  type of organelle. 

 What is the main function of the type of organelle extracted in pellet 2?