Oxygen is a byproduct of photosynthesis and, correspondingly, carbon dioxide the byproduct of respiration.
Oxygen gives our cells the ability to break down food in order to get the energy we need to survive. Although other animals may use different organs to breathe with, they all get oxygen into their bodies through respiration.
Endosymbiosis is important because it is a theory that explains the origin of chloroplast and mitochondria. It is also a theory that explains how eukaryotic cells came to be.
