[<< wikibooks] Biology, Answering the Big Questions of Life/Metabolism/Metabolism2
== How are food molecules broken down to make energy? ==
The major food molecule in living organisms is a sugar called glucose. Most carbohydrates (sugars and starches) are converted into glucose before they are broken down in the cytosol to release energy. The  series of steps where glucose is broken down to release energy begins with a metabolic pathway called glycolysis. Glycolysis is the "lysing" or cutting of glucose to release energy. The six carbon sugar, glucose, is cut in half and converted into two three-carbon sugars called pyruvate. What happens next depends on the presence or absence of oxygen.
If oxygen is present, then glucose can be broken all the way down into carbon dioxide and water. This process is called aerobic respiration because it requires air (oxygen). In the absence of oxygen, the cell uses a process called anaerobic fermentation. or simply fermentation.
Fermentation doesn't break the sugar down any further, it simply helps reset the system so that more sugar can be broken down.
Because aerobic respiration breaks the sugar all the way down, it releases much more energy than fermentation.

== What is the equation for aerobic respiration? ==
 C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy 
Glucose + Oxygen → Carbon dioxide + water + energy

== How is energy stored in the cell? ==
In living cells, energy is stored in the form of high energy molecules. The most common is a molecule that we call ATP (Adenosine triphosphate). ATP is a nucleic acid containing three high energy phosphate groups. It breaks off these groups to release measured amounts of energy. When ATP loses one phosphate group, it becomes Adenosine diphosphate (ADP). When ATP loses two phosphate groups it becomes adenosine monophosphate (AMP).

Many reactions in the cell are directly run by ATP. ATP drives reactions as varied as moving flagella, pumping salt through pores, and making muscles move. Because ATP has so many uses, it has been called "the currency of the cell".
There are other energy storing molecules found in cells. For the most part, these molecules must be converted into ATP for the cell to use them. This occurs in the mitochondria by a process called oxidative phosphorylation.

== Why do we need to breathe oxygen to live? ==
Very simple organisms (e.g. bacteria) do not need much energy to live. Humans, however, require large amounts of energy to run their bodies. Fermentation produces two ATP molecules for every glucose molecule consumed. Humans need more energy than can be provided simply by fermentation alone. Therefore, humans must use aerobic respiration which produces 36 or 38 ATP's for each glucose molecule consumed. 

(See the advanced page for more information about why humans need oxygen to live.)