Glycolysis is the breakdown of glucose, the ring-shaped sugar molecule that serves as a fuel source for every type of cell in nature. Its chemical formula can be summed up by the following net reaction:
C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3(C=O)COOH + 2 ATP + 2 NADH + 4 H+ + 2 H2O
In words, this translates to: A six-carbon molecule of glucose is converted to two molecules of pyruvate, which contains three carbons; two molecules of ATP; and four hydrogen ions. This is accomplished with the help of ADP, free phosphate and the electron-accepting molecule NAD+, which is converted NADH during the reaction.
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The Biochemical Purpose of Glycolysis
In prokaryotes, single-celled organisms that belong to either the Archaea domain or the Bacteria domain, this series of 10 reactions occurring in the cell cytoplasm is the only game in town for synthesizing adenosine triphosphate (ATP), the "energy currency" that all cells use to drive their various functions. In eukaryotes, which belong to the domain Eukaryota_, glycolysis merely sets the stage for the series of reactions in mitochondria that are collectively known as _aerobic respiration.
While you may not be required to memorize all of the reactants, products and enzymes in each of the 10 steps of glycolysis, a few tricks can help you keep a solid picture of the whole process firmly in mind.
Executive Summary of Glycolysis
Glycolysis includes an "investment" phase in which glucose is phosphorylated, rearranged and phosphorylated again, with the two phosphate groups coming from ATP (represented by ADP and P in the reaction above). This is followed by a splitting of the doubly phosphorylated sugar molecule into two identical singly phosphorylated three-carbon molecules, and a "payoff" phase.
In this "payoff" phase, each of the identical molecules is phosphorylated again before before both phosphates on each three-carbon molecule are used to make ATP, yielding 4 ATP in all in this phase. Along the way, the two molecules are rearranged into pyruvate.
Thus, with the investment phase requiring 2 ATP and the payoff phase providing 4 ATP, a total of 2 ATP are generated per glucose molecule undergoing glycolysis.
The Glycolysis Cycle Made Easy
Because the reactions of glycolysis follow a logical sequence, one fairly easy way to learn glycolysis is to simply remember the names of the products formed in each step. This is made simpler by dividing the process into four "investment" molecules and six "payoff" molecules, as follows:
Glucose → Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-biphosphate →
Glyceraldehyde-3-phosphate → 1,3-Biphosphoglycerate → 3-Phosphoglycerate → 2-Phosphoglycerate → Phosphoenolpyruvate → Pyruvate
Note that phosphorylations occur in every other step (creating the second, fourth and sixth products overall), while dephosphorylations occur right after the last phosphorylation and in the final step.
Your Own Glycolysis Mnemonic
Some students find it helpful to create their own mnemonic, or memory device, to remember the steps of glycolysis. One way to go about this is to write the molecules in shorthand form and associate them with a catchy phrase. For example:
Here, "P" always represents a phosphate group in some way. "Gla" and "Gly" stand for "glyceraldehyde" and "glycerate" respectively. You can think of the last two products as "Peppy Pie." But again, get creative and come up with your own scheme if you like.
In eukaryotic cells, the pyruvate moves into organelles called mitochondria, where it undergoes the Krebs cycle and then the electron transport chain reactions. These processes together yield approximately 34 to 36 molecules of ATP per glucose molecule (up to 38 in some situations) entering glycolysis far "upstream," or about 17 to 18 times the energy output of glycolysis alone.