Cellular respiration is the process by which cells break down glucose and release 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 smaller molecules are broken down using oxygen to release energy stored in ATP. The overall equation is that glucose and oxygen are broken down to produce carbon dioxide, water, and energy. Fermentation is an alternative energy production process used by some cells that does not require oxygen but produces less energy than cellular respiration.