About 2.5 billion years ago, oxygen began slowly to accumulate in the atmosphere, as a result of the photosynthetic activity of the cyanobacteria. Those prokaryotes that were able to use oxygen in ATP production gained a strong advantage, and so they began to prosper and increase. Some of these cells may have evolved into modern forms of aerobic bacteria. Other cells may have become symbionts with larger cells and evolved into mitochondria.

The electron transport chain has three sites along it that pump protons
from the matrix. NADH donates its electrons to the chain at a point where the
energy input is sufficient to drive all three proton pumping sites. FADH is less
energetic than NADH and its electrons are donated at a point that drives two
proton pumping sites. It is also possible for the NADH produced in glycolysis to
enter the mitochondrial matrix and donate electrons to the electron transport
chain. Depending on the system, NADH from glycolysis may be able to drive two or
three proton pumping sites. For eukaryotes

Cell organelles are subunits within a cell that have their own specific function within that cell. They are known as cell organelles because they play similar roles that organs play in a body. There are many different types of cell organelles, each designed to do something specific, yet having the capability of working with the rest of the cell organelles in any given cell.