Glycolysis is a part of our metabolism and is actually a catabolic process. The entire process consists of ten steps and it results in the breakdown of glucose molecule to pyruvate molecule (which is then used as a starting molecule in Kreb's cycle, or fermentation- if oxygen is absent). The initial 3 steps of glycolysis require energy (2 molecules of ATP for breakdown of every molecule of glucose) and the latter 7 steps produce energy (4 molecules of ATP), with a net production of energy. 1 ATP molecule is consumed in the first step and ensures a steady supply of glucose in the cell, while also preventing its leakage. Another ATP molecule is used in the 3rd step and this makes the process irreversible, while also producing two charged groups (which will ultimately cause release of 4 ATP molecules instead of 2). Thus the expenditure of ATP is necessary to destabilize glucose and produce more ATP molecules.
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