The membranes of two organelles, mitochondria and chloroplasts, are not part of the endomembrane system. Both of these organelles are specialized to harness energy for the cell. Interestingly, they are both semi-autonomous organelles that grow and multiply independently of the other membrane-bound compartments, and they contain their own genomes. As we discuss in Chapter 27, the similarities between the DNA of these organelles and the DNA of certain bacteria have led scientists to conclude that these organelles originated as bacteria that were captured by a eukaryotic cell and, over time, evolved to their current function.