But to find out how these organelles function it is useful to separate one type of organelle from all the rest. Watch how fractionation is carried out in this real-life experiment. Unlike a prison cell, a biological cell is designed to allow things in and out. In this part of the programmme cell specialist Paul Nurse describes how this unique interaction takes place. Watch this part of the programme to see how molecules are transported in and out by these ever efficient gatekeepers.
Osmosis is a rather complicated form of movement.
Don't miss the interview with AS Biology expert Mary Jones for some specialist knowldge on this third type of passive transport.