Re: Why we cannot cluster all the cells' function together?

Wissy,
	This is sort of a two-fold question.  I’ll address the first part 
first.  The short answer is that the various enzymes, cofactors and other 
cellular machinery specific to certain processes are generally only 
located where they are needed.  Using your example of the nucleus and 
mitochondria, then, one can say that the proteins and membranes required 
to concentrate protons on one side of the membrane and allow them to fall 
down slowly enough for the cell to make ATP from them is only found in 
the mitochondria.  Conversely, the proteins and chemicals necessary to 
create RNA and DNA from DNA templates are found only in the nucleus.  
These processes only occur in one organelle, and so the cell does not 
wish to waste energy by equipping each organelle with the machinery to 
carry out every process the cell can perform – that would be very 
wasteful.  For example, if you owned two factories – one that produced 
shoes and one that produced radios – would you want them both to have to 
be able to make each others’ product?  No, you would want the radio 
factory to be very, very efficient at making radios, and the shoe factory 
to be very, very efficient at making shoes.  
	The reason that eukaryotic cells (those with membrane-bound 
organelles) create so many compartments to house their cellular functions 
is actually quite simple, when you think about it.  Some cellular 
processes would simply interfere with other processes and decrease the 
efficiency of the cell.  For instance, the very low pH required for the 
lysosomes and mitochondria to perform their recycling and ATP producing 
functions is far too acidic for proteins to be made.  The cell, then, 
separates these low pH environments from the ribosome so that proteins 
can be made efficiently WHILE the mitochondria and lysosome and carrying 
out their functions.  There are some cells that do not separate 
functionality like this.  Prokaryotes (bacteria) have no internal 
membrane compartmentalization.  All of their cellular processes are 
performed in a common cytosol.  This is one of the major reasons that 
bacteria are limited in their complexity.  Without being able to carry 
out many specialized cellular processes simultaneously, their cells are 
only capable of carrying out processes that are compatible with one 
another.  That is why bacteria cannot carry out the same low-pH energy 
production methods we do (in general, anyway).  
Thanks for your question!
Billy.

Wikipedia has two particularly good articles that address your question.  
Organelles: http://en.wikipedia.org/wiki/Organelle

and Eukaryotes: http://en.wikipedia.org/wiki/Eukaryotes