Yes, the best understanding that we have now says that bacteria became mitochondria (and also chloroplasts, the agents of photosynthesis. Bacteria are capable of amazing things in terms of respiration. There are bacteria that use iron as an electron acceptor; meaning, that instead of "burning" (or oxidizing) carbon compounds, they use the energy in the carbon to "rust" iron. However, the bacteria that we were before we took up mitochondrion were almost certainly anaerobic, meaning that we turned sugar into alcohol or acetate.
The first aerobic bacteria were very probably small. Membranes are required for aerobic respiration. The surface area law (smaller objects have a greater surface area proportional to volume) meant that it was beneficial to these bacteria to be small. However, that meant that they were more likely to be eaten by larger bacteria.
The evidence that mitochondria (and chloroplasts) were once free-living organisms lies in the fact that they still have remnants of genomes. That's right, mitochondria have their own DNA! It's been about a billion years since they were on their own, so much of what their genome was supposed to do has been transferred over to our own genome, but the remnants of their genome are still there.
What is really interesting about mitochondrial genomes is that they have managed to evolve something that no free living organism ever has. Every free living organism on this planet, from bacteria to plants to you, follows the same genetic code. That is not true for mitochondria, since for a billion years, they have not been subject to the same selective pressures. For example, for every free-living organism, the sequence adenosine-uracil-adenosine signifies the amino acid "isoleucine" when making a protein; however, in human mitochondria, the same adenosine-uracil-adenosine sequence signifies "methionine." This is why I love evolutionary theory! It explains so much!
Well said! And very informative.
It is also interesting to note that the biochemistry inside the nucleus of the cell is quite different from that going on outside of the nucleus, but still inside the cell.
Fourth Grade Class Member Bob
zylphed, you are a veritible font of information. This is fascinating. Thanks for the detailed explanations. I have a strong background in biology (as a registered Med Tech (MT-ASCP), and RN-BSN), but it's been many years since the classroom; I have to read some of your posts with my lips moving. :)