All life needs energy to carry on life's functions. Even sitting and reading takes energy. Growth, digestion, locomotion: all require an expenditure of energy. Running a marathon takes a lot of energy. So, where does all that energy come from?
Fuel for Energy
The energy required to perform life's functions comes from the breakdown of sugar. Photosynthesis uses the sun's energy to combine carbon dioxide and water to form glucose (sugar), giving off oxygen as a waste product. Plants store this glucose as sugar or as starch. Animals, fungi, bacteria and – sometimes – other plants, feed off these plant resources, breaking down the starch or sugar to release the stored energy.
Comparing Fermentation and Cellular Respiration
Fermentation and cellular respiration differ in one critical factor: oxygen. Cellular respiration uses oxygen in the chemical reaction that releases energy from food. Fermentation occurs in an anaerobic or oxygen-depleted environment. Because fermentation doesn't use oxygen, the sugar molecule doesn't break down completely and so releases less energy. The fermentation process in cells releases about two energy units whereas cellular respiration releases a total of about 38 energy units.
Energy from Cellular Respiration
In cellular respiration, oxygen combines with sugars to release energy. This process begins in the cytoplasm and is completed in the mitochondria. In the cytoplasm, one sugar is broken into two molecules of pyruvic acid, releasing two energy units of adenosine triphosphate or ATP. The two pyruvic acid molecules move into the mitochondria where each molecule is converted into a molecule called acetyl CoA. The hydrogen atoms of the acetyl CoA are removed in the presence of oxygen, releasing an electron each time, until no hydrogen remains. At this point, the acetyl CoA has been broken down, and only carbon dioxide and water remain. This process releases four ATP energy units. Now the electrons pass down the electron transport chain, ultimately releasing about 32 ATP units. So, the process of cellular respiration releases about 38 ATP energy units from each glucose molecule.
Energy from the Fermentation Process
What if the cell doesn't have enough oxygen for cellular respiration? The phrase "feel the burn" results from this anaerobic pathway. If the cell's oxygen level is too low for cellular respiration, usually because the lungs can't keep up with the cell's oxygen need, then fermentation cellular respiration takes place. In this case, the sugar molecule only breaks down in the cell's cytoplasm, releasing about two ATP energy units. The breakdown process doesn't continue in the mitochondria. This partial breakdown of the glucose releases a little bit of energy so the cell can keep working, but the incomplete reaction produces lactic acid which builds up in the cell. This lactic acid fermentation causes the burning sensation when the muscles don't receive enough oxygen for cellular respiration.