In aerobic respiration, high energy electrons derived from food molecules (such as carbohydrates) are delivered to O₂ forming water; the energy released is used primarily to generate ATP, which in turn is use to synthesize molecules and other cellular functions.

In oxygenic photosynthesis electrons, derived from water, are energized by light, and that energy is used to generate ATP and carbohydrates.

In aerobic bacteria, electron transport chains are localized in the plasma membrane; in cyanobacteria, the photosystems are located on internal membrane systems. 

The same mechanisms (aerobic respiration and photosynthesis) occur within eukaryotic cells, not on the plasma membrane, but within distinct cytoplasmic organelles: mitochondria for aerobic respiration and chloroplasts for photosynthesis.

The exact details of the endosymbiotic process, however, remain unclear. 

In particular, the nature of the original host cell remains the subject of sometimes heated debate.   

In part, this is due to the large amount of gene transfer from the bacterial endosymbionts to the host cell nucleus. 

At the same time, it is not clear exactly where the eukaryotic nucleus came from.  Did the original endosymbiotic event involve more than two types of organisms? 

The original endosymbiotic events that produce modern eukaryotes occurred approximately 2 billion years ago, making unambiguous details somewhat difficult to come by.  

What does appear to be the case, is that all eukaryotes may well be descended from a single endosymbiotic event that gave rise to mitochondria. 

Plants appear to be descended from a second endosymbiotic event, which gave rise to chloroplasts. 

• What advantage would the host cell get from these symbioses? was there an advantage for the engulfed bacteria? 
• Are the mitochondria of humans and yeast homologous or analogous? What data would you need to answer this question? 
• What evidence would lead you to suggest that there were multiple symbiotic events that gave rise to the mitochondria of different eukaryotes?
• Are mitochondria and chloroplasts homologous or analogous?
• Why might one argue that the early eukaryotic ancestors of plants did not have cell walls?
• Do plants have mitochondria?