yes savalika is right, human and other mammalians RBC do not require nucleus and other cell organelles after maturation...when it is immature it contains all type of organelles because during development it synthesis all type ot materials which are going to be required till 120 days journey,,, when these materials get finished , RBC also gets destroyed by spleen....and yes why organelles are not required at maturation , so that RBC can store more and more Hb in it to transport oxygen