How does the presence or absence of oxygen affect ATP production? 

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ATP stands for adenosine triphosphate and is the energy source used by cells.  The production of ATP is much greater in the presence of oxygen. In the presence of oxygen, 34-38 ATP are produced per glucose molecule. In the absence of oxygen, the net yield of ATP produced is 2 per glucose molecule.

ATP is formed in the presence of oxygen in the process known as aerobic cellular respiration. This process occurs within mitochondria of eukaryotic cells. There are three main parts of aerobic cellular respiration- glycolysis, Krebs Cycle, and the electron transport chain. Here, the processes that occur during each phase of aerobic cellular respiration. Overall, aerobic cellular respiration converts the sugar called glucose and oxygen gas into carbon dioxide gas, water, and 34-36 ATP.

Anaerobic respiration is sometimes referred to as fermentation. Fermentation is a metabolic process in which organisms convert carbohydrates, such as starch or sugar, into lactic acid or alcohol. Another name for fermentation is anaerobic respiration because the process is done in the absence of oxygen (“an” = not, “aero” = air or atmosphere). Compared to aerobic respiration, anaerobic respiration produces much less ATP.

Ethanol fermentation is the type of fermentation that produces alcohol. It is done by yeast and some strains of bacteria. During ethanol fermentation, pyruvate from glucose metabolism is broken into ethanol and carbon dioxide.  Because ethanol fermentation produces alcohol, it is used to produce beer and wine. The carbon dioxide produced by ethanol fermentation is advantageous in the making of breads.

During lactic acid fermentation, glucose is converted into cellular energy (ATP) and lactic acid. Lactic acid buildup occurs within muscle cells during intense intervals of activity when energy is needed at a faster rate than oxygen can be supplied. The lactic acid produced is the “burn” that athletes feel after an intense workout.

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