Cellular respiration is the process cells use to retrieve energy stored in carbohydrates, fats and proteins. Glucose and other molecules are broken down, and the energy released is used to make another molecule called adenosine triphosphate (ATP), the "energy currency" of the cell. While our cells can use fermentation to make ATP without using oxygen, cellular respiration is far more efficient -- so much so that humans and most other animals quickly die if deprived of oxygen.
How Cellular Respiration Works
Cellular respiration begins with glycolysis, where a molecule of glucose is split in the cytoplasm of the cell. The most important steps in cellular respiration, however, take place in the mitochondria, the power plants of the cell, where electrons pass along a series of membrane-embedded proteins called the electron transport chain. Each protein uses some of the energy from this transfer to pump hydrogen ions into the space between the mitochondrion's inner and outer membranes. By concentrating hydrogen ions in this space, the mitchondrion creates a gradient it can use to make ATP, like pumping water uphill so it can drive a turbine. The ATP is then made available to other processes in the cell for energy.
At the end of the electron transport chain in the mitochondria, electrons are donated to oxygen (O2), which combines with hydrogen ions to form water. Without the O2 molecules to accept the electrons, the electron transport chain couldn't function.
Cellular respiration generally refers to aerobic respiration, where cells use the process described above to make ATP. If oxygen is unavailable, however, our cells can still make a limited amount of ATP through lactic acid fermentation. In this process, the cell uses glycolysis to break up glucose (just as in aerobic respiration) and donates electrons to a sugar molecule called pyruvate, which forms when glucose is broken down. This reaction results in a byproduct called lactic acid.
Fermentation vs. Aerobic Respiration
Aerobic respiration yields much more ATP than lactic acid fermentation. In fermentation, the pyruvate accepts electrons from glycolysis; in aerobic respiration, on the other hand, the pyruvate is broken down still further to make more ATP in the mitochondria. As a result, aerobic respiration can generate up to 19 times more ATP per molecule of glucose than lactic acid fermentation.
Why Oxygen Is Important
Oxygen is important because it makes aerobic respiration possible by accepting electrons from the transport chain in the mitochondria. Sometimes oxygen is unavailable in some of your muscle cells (typically during heavy exercise); at times like these, the muscle cells will temporarily revert to lactic acid fermentation, which yields far less energy.