Endosymbiotic origin of Cholorplast and Mitochondrian???

[Keith Robison]

jsmith at mondrian.CSUFresno.EDU (Jeff Smith) writes:

>  I have a question about the possiblility that the modern Chloroplast and 
>Mitochondrian originated from the phagocytosis of a prokaryotic cell or the
>symbiotic relationship between a prokaryotic cell and a eukaryotic cell. (and
>sorry about the misspelling of Chloroplast in the title!).  

>  The main thing I have trouble understanding is the possiblity of the
>phagocytosis of the prokaryotic cell.  Since we know that the prokaryotic cell
>is membrane bound and if call that the 1st membrane, then when the phagocytosis
>occurs it would create a second membrane, correct?  So, how does this membrane
>(the now double membrane) get passed on geneticly to future decendents?

>  Does my question make sense?  If so, can anyone help me clear this up?  Any
>help would be appreciated.


A good question -- just think about it.  How could the double membrane
be lost?  It would require a fusion of the two membranes into one (which
generally won't happen spontaneously) or a simple degeneration of
the membrane if it wasn't fed more membrane-stuff.  So, as long as the
membrane is fed, it will be maintained _epigenetically_ (no direct genetic
coding).  This is probably true for all membranes in the cell (i.e. there
is no gene which tells the cell to have a membrane, but since the cell
starts with a membrane and makes membrane components it maintains one).

Some plastids (such as Euglena & diatoms) are encased in a triple membrane!
Same story as other membranes, just one more iteration.

Keith Robison
Harvard University
Department of Cellular and Developmental Biology
Department of Genetics / HHMI

krobison at nucleus.harvard.edu