Glycolysis is the reaction that converts each glucose molecule into two pyruvates to be later used in the following steps of cell respiration. As you can see in the image glycolysis goes through a variety of steps before it is able to reach it's final result of two pyruvates. With the aid of ATP, the glucose molecule is first turned into a glucose 6-phosphate. then it continues to be changed itnto fructose 6-phosphate. Once again, with the use of ATP, it is then changed into fructose 1, 6-biphosphate. Then each of these is converted into a dihydroaxyacetone phosphate and into a glyceraldehyde 3-phosphate. Upto here all the reactions have been mainly endergonic. These reactions are represented in the diagram on the left side, shown with the upward, thin, blue arrow. Later, the two glyceraldehyde 3-phosphate molecules go through a series of reactions that modify them and finally end up as pyruvates. Meanwhile the middle steps have come to produce a NADH + a H ion, in the first step, 2 ATP's in the second step, a water molecule in the fourth step, and finally another two ATP's in the fifth step. All the steps involved in transforming Grlyceraldehyde 3-phosphate into pyruvate are all exergonic. They are represented on the diagram as the downward, thin, cream arrow.

This process is the first step in both aerobic and anaerobic respiration. Although it uses up two ATP molecules in the endergonic half of the process, it also creates four ATP molecules in the exergonic half. Therefore, there is a net production of two ATP molecules throughout glycolysis.

 

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In the case of both animal and plant cells, glycolysis takes place exclusively in the cytosol. The key to glycolysis is that it produces TWO pyruvate molecules for every glucose molecule that it starts with. This is because it converts the 6-carbon glucose into TWO 3-carbon molecules, being pyruvate. This is shown on the bottom half of the picture to the left.