Final answer:
Glycolysis and gluconeogenesis have different energy requirements and directionalities, making it essential for them to have different steps. Specific enzymes in gluconeogenesis bypass the irreversible steps of glycolysis, allowing independent regulation and avoiding futile cycles.
Step-by-step explanation:
It is crucial that not all steps between glycolysis and gluconeogenesis are the same because of the energy requirements and the directionality of each pathway. Glycolysis is an exergonic pathway, which means it releases energy by breaking down glucose into pyruvate and generating ATP. In contrast, gluconeogenesis is an endergonic pathway; it requires an input of energy to convert precursors like pyruvate back into glucose. The three irreversible steps in glycolysis are bypassed by alternative reactions and enzymes in gluconeogenesis, such as pyruvate carboxylase, phosphoenolpyruvate carboxykinase (PEPCK), fructose-1,6-bisphosphatase, and glucose-6-phosphatase. These bypass enzymes are essential, as they allow the cell to independently and tightly regulate glycolysis and gluconeogenesis based on the energetic needs and availability of substrates, avoiding a futile cycle where both processes would counteract each other. Furthermore, these differences allow gluconeogenesis to be an energetically favorable process in certain cells like those of the liver and kidneys, even though it consumes ATP and GTP, because the release of glucose into the blood and the cellular energy state drive the reaction forward.