The endosymbiotic theory hypothesises that mitochondria and chloroplasts evolved from bacterial cells. It can be assumed that whilst the organelles were evolving (in the early stages) that anaerobic respiration was the pathway that provided cellular life with energy. Some of these anerobic cells were able to use light energy, in an early form of photosynthesis, producing oxygen as a biproduct. This oxygen was then used by small bacteria allowing aerobic respiration to occur. The phagocitotic uptake of these small bacteria by protoeukaryotes allowed a mutalistic relationship to occur. The protoeukaryotes were able to respire aerobically using the bacterias oxidative phosphorylation pathways, and the bacteria were provided with a stable internal environment. These small aerobic bacteria evolved into what we now know as the mitochondria within cells. It is believed that chloroplasts evolved by a similar mechanism from early cyanobacteria .
- ↑ Becker et al (2009). The world of the cell. 7th ed. San francisco: Pearson Benjamin Cummings. 298,299.