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# Cellular Respiration

Introduction: All living organisms need energy. This energy is provided through a molecule called ATP and is created through a process called cellular respiration. This should not be confused with respiration, which involves the exchange of gases in the lungs. While related (because it does require oxygen to operate normally), cellular respiration involves the breakdown of glucose into ATP (or, in other words, food into energy).

Whether a bacterium or an elephant, all organisms require energy. Plants make their own energy through photosynthesis. However, the sugars created at the end of this process still need to be broken down. They accomplish this by cellular respiration.

The process of cellular respiration can be summarized into the following biochemical pathway:
$C_6H_12O_6 + O_2 -> CO_2 + H_2O + "ATP"$

or

glucose + oxygen ---> carbon dioxide + water + energy

When food is consumed, it goes through a series of very complicated steps in order to become energy. The first step is called glycolysis. This involves the breakdown of glucose into a molecule called pyruvate. Next, the pyruvate moved into the Kreb's (or Citric Acid) cycle where it is further changed. Finally, the products of the Kreb's cycle enter the electron transport chain, where a majority of the ATP is produced.

During this entire process, some ATP is made and electrons are being shuffled all over the place by a carrier called NAD+. In the end, a total of 32-38 ATP molecules are produced from each molecule of glucose. The amount varies because sometimes some gets used up along the way.

There are occasions when cellular respiration can happen without oxygen. This is called fermentation and usually happens after glycolysis. Fermentation can occur in one direction, called alcoholic fermentation (the process that creates adult beverages and bread) or lactic acid fermentation (when you get a cramp). The organisms that perform this are specially adapted to live in an anaerobic environment.