Respiration is a process in which cells utilize energy stored in food and transfers that energy to ATP. This energy is needed to power almost any cell activity. Respiration is composed of three processes: glycolysis, Krebs cycle, and the electron transport chain (ETC). Glycolysis essentially breaks a single glucose molecule into two molecules of pyruvate which will be needed in the Krebs cycle. Glycolysis utilizes the concept of substrate level phosphorylation. Recall that phosphorylation reaction is a reaction in which a phosphate group is transferred to another compound. It is imperative to remember that phosphofructokinase (PFK) acts as an allosteric enzyme, on which ATP acts as an allosteric inhibitor. If there is a large amount of ATP, this inhibits the PFK, and this allows the regulation of ATP production.
The Krebs cycle utilizes pyruvate, which combines with coenzyme A, to form acetyl CoA. For each glucose, two acetyl groups enter the cycle, and forms two ATPs, six NADH, and two FADH2. This forms two ATP, just like glycolysis. Only four ATP molecules have been formed, and the rest of energy needed to produce ATP, is stored in the NADH and FADH2. The electron transport chain basically where the high-energy electrons are sent from one acceptor, to the other. As electrons are passed down the ETC, some of that energy is used to form ATP. This process is referred to as oxidative phosphorylation. The reason that the organisms that use aerobic respiration need oxygen is that oxygen is the final electron acceptor in the ETC. If the organism were to be deprived of oxygen, all the electrons would retain their electrons once it reaches the final acceptor molecule. Chemiosmosis is basically the coupling of ATP synthesis to electron transport. With respect to all three processes involved in respiration, they produce a maximum of 36 to 38 ATPs. Anaerobic respiration is a form of respiration, which does not have oxygen as a final electron acceptor. Alcohol fermentation and lactic acid fermentation are inefficient, producing a mere 2 ATP. They utilize lactate acid fermentation to produce yogurt and sauerkraut. Respiration is a complex process, which is essential in biology.

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