Cellular respiration is the process by which cells break down glucose to obtain energy. It occurs in two stages - the first in the cytoplasm where glucose is broken down without oxygen, and the second in the mitochondria where molecules are further broken down with oxygen to release energy. The overall equation is: C6H12O6 + O2 → CO2 + H2O + Energy. Fermentation allows some cells to obtain energy without oxygen through a less efficient process. Cellular respiration and photosynthesis use the same compounds but are reverse processes, with respiration producing CO2 and consuming O2 while photosynthesis does the opposite using solar energy.