A chloroplast derived from a secondary endosymbiotic event is often surrounded by four membranes. Trace the origins of these membranes from innermost to outermo

Question

A chloroplast derived from a secondary endosymbiotic event is often surrounded by four membranes. Trace the origins of these membranes from innermost to outermost.

 

First membrane:

 

Second membrane:

 

Third membrane:

 

Fourth membrane:

 

Why is the endosymbiotic origin of the mitochondrion considered to have predated that of the chloroplast?

Answer 1

The endosymbiotic theory is one that is widely accepted as a reasonable explanation of why chloroplasts and mitochondria have double plasma membranes. It is postulated that this came about with Mitochondria first and then much later with chloroplasts in terms of evolution. The idea is that both mitochondria and chloroplasts were at one-time free-living uni-cellular organelles that became engulfed (phagocytosed) by other prokaryotic organisms and were not digested but became inherent as a dependent functional part of the engulfing organism. This is supported by the fact that mitochondria and chloroplasts have their own circular rather than linear DNA which divides independently of the "host" cell and in a manner much like binary fission rather than mitosis. The plasma membrane is of course a bilayer; so, the inner most layer of the chloroplast membrane is the innermost plasma membrane. Superior to that we have the outer layer of the inner membrane. There is a second outer bilayer that is initially derived from the original engulfing cell.