Glycolysis is the breakdown of glucose into pyruvate, whereas gluconeogenesis is the creation of glucose from pyruvate, lactate or Krebs cycle intermediaries. Both processes are essential components of the body's energy metabolism; and although the two reactions do roughly mirror one another, they are different in more ways than they are the same.
Starting and Ending Products
Glycolysis starts with glucose and ends in pyruvate, whereas gluconeogenesis starts with pyruvate and ends in glucose. As a result of the breakdown of glucose, glycolysis generates two new molecules of adenosine triphosphate (ATP) and two new molecules of nicotinamide adenine dinucleotide (NADH). This makes energy from glucose available for the cell's use, and pyruvate continues to the mitochondria to enter the Krebs cycle, leading to still more energy production. In gluconeogenesis, the cell instead consumes ATP in order to regenerate glucose from pyruvate, so there is a net loss of energy from the cell performing gluconeogenesis. Glycolysis, in contrast, leads to energy gain.
Another key difference between gluconeogenesis and glycolysis is where the reactions take place. Essentially, all of the body's cells are able to perform glycolysis, which is the first step in metabolism of glucose taken up via transporters in the cell membrane. Gluconeogenesis takes place primarily in liver cells and to a smaller degree in the kidney, and its purpose is usually metabolism of pyruvate generated from deaminated amino acids rather than pyruvate resulting from earlier glycolysis. Glycolysis and gluconeogenesis do not occur simultaneously in the same cell; this would be a waste of resources for the cell since no energy would be generated if pyruvate were constantly converted to and from glucose.
Because it results in increased energy availability for the cell, glycolysis increases when the cell is in need of energy and decreases when energy is readily available. This is accomplished via feedback mechanisms that involve the regulatory enzymes in glycolysis. Gluconeogenesis, in contrast, is usually performed to produce glucose for export to cells in the rest of the body. Liver cells do not themselves metabolise the glucose from gluconeogenesis.
Finally, the pancreatic hormones released in response to food intake affect glycolysis and gluconeogenesis differently. Insulin, which the body releases in response to carbohydrates and some proteins, causes many of the body's cells to increase their uptake of glucose and transmission of regulatory enzymes involved in glycolysis. Insulin decreases gluconeogenesis in the liver. Glucagon, which is stimulated by protein and low blood sugar, causes increased gluconeogenesis and decreased glycolysis in liver cells.
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