The first stage in the process of converting glucose to ATP is glycolysis. Its name, glycolysis, is descriptive of what it does, the "lysis" or splitting of glucose (blood sugar). A single glucose molecule is split forming two molecules of pyruvate.
Glycolysis is an anaerobic process. Oxygen is not required for this process to work. 2 units of ATP are necessary to provide the energy for glycolysis to do its work. Also input into the process is 2 units of NAD+, and 2 units of ADP.
NAD+ is a potentially limiting ingredient in glycolysis since there is a fixed amount of NAD+ available in the cell. When NAD+ is not available glycolysis will halt until NAD+ becomes available. NAD+ is a recycled ingredient in this process. NAD+, converted to NADH performs the task of carrying electrons and hydrogen ions into the mitochondria. When it completes this delivery it returns to its original form of NAD+ for reuse in other glycolysis operations.
(Note: NAD+ is manufactured in another process in the cell by combining niacinamide and ATP. It is possible that a deficiency of niacinamide or ATP could cause a reduction in NAD+ and this could theoretically impede the processing of glucose. However, given that the cell in the person with fibromyalgia is high in pyruvate, it is logical to assume that there is probably sufficient amounts of NAD+ in the cell.)
Output from glycolysis is 2 units of NADH, 4 units of ATP, and 2 units of pyruvate.
The NADH is formed by combining electrons and hydrogen ions from the cytosol (the cell fluid) to NAD+. The 4 units of ATP are generated by combining inorganic phosphates (Pi) from the cytosol and the ATP and ADP that were input into the process.
The 4 units of ATP generated here is not a large amount compared to the potential ATP available in each molecule of glucose. (There is a potential of 38 units of ATP per molecule of glucose. We will talk more about this later.) The 4 units of ATP are all that is needed to power another glycolysis process (2 ATP) and transport the pyruvate and NADH output from glycolysis into the mitochondria (2 ATP).
Even though glycolysis produces a very small amount of ATP from each glucose molecule, because the process works so rapidly, glycolysis can produce a lot of ATP when it needs to. (More on this will be presented later under anaerobic respiration.)
NOTE: In case you have not been able to identify it, the big orange circular thing is supposed to represent a cell. There are many things in a real cell, organelles and such, that are not included to keep it simple.
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