C6H12O6 + 6 O2 6 CO2 + 6 H2O + 36 ATP
Cell respiration is the process of liberating energy from glucose. The energy liberated is in the form of ATP. Cell respiration occurs in the mitochondria in and around the area of the cristae. Cell respiration occurs because oxygen is used to "burn" glucose and the energy from that process is captured in the form of heat and ATP. It is similar to the way fire consumes wood. Fire requires oxygen to burn the plant fibers that wood is composed of. The result of this burning is heat and light. Cell respiration requires many enzymes to make the 36 ATP that are gained from the breakdown of each glucose. The ATP can then be used in other parts of the cell to build other molecules or to breakdown more glucose. ATP can be used by the cell to bring materials into the cell during active transport, or remove wastes from the cell. ATP can also be used for movement of the cell.
What happens if a cell cannot get enough oxygen to respire aerobically? Some cells, like our muscle cells, can switch to anaerobic respiration. This process is less efficient because it only produces 4 ATP per glucose molecule and the end product is Lactic Acid as well as CO2. If this happens in our cells the muscle will feel like it is burning because of the acidic nature of the lactic acid molecule? It is difficult for the cell to deal with this molecule. After oxygen levels in the cell return to normal, the cell will remove the lactic acid by placing it into the krebs cycle and getting as much ATP as it can.
Some cells do not have the luxury of being able to switch from aerobic respiration to anaerobic respiration. Some bacteria respire exclusively by anaerobic respiration. They are small and do not need much ATP and they are able to handle the byproducts of anaerobic respiration. These products may be either lactic acid or Ethyl alcohol (ethanol). These very same bacteria may actually be harmed by the presence of oxygen in their environment. Anaerobic bacteria were the first organisms on Earth as there was not any oxygen in the environment.
Today anaerobic bacteria are used in industry to brew alcohol, both for consumption, and other things like fuels and pharmaceuticals. Anaerobic bacteria are necessary in the production of bread, cheese, yogurt, and other food products.
Below you will find a diagram that describes the process of cell respiration and fermentation.