Building glucose when carbohydrate runs low
Gluconeogenesis is the synthesis of new glucose, occurring mainly in the liver and to a lesser extent the kidneys. It draws on three main precursors: lactate from active muscle and red blood cells, glycerol released when fat is broken down, and glucogenic amino acids such as alanine derived from protein. These feed into the pathway and are assembled into glucose.
Why it is not simply glycolysis run backward
Although gluconeogenesis shares many enzymes with glycolysis, three glycolytic steps are essentially irreversible. The cell bypasses them with dedicated enzymes: pyruvate carboxylase and PEP carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. This is why the process consumes ATP and GTP rather than producing them, and why hormones like glucagon and cortisol can promote glucose synthesis while insulin suppresses it. General educational information about metabolism, not medical advice.