What are the reversible and irreversible steps in glycolysis?

The reversible steps in glycolysis are steps 1-5 and 7-10, while the irreversible steps are steps 1, 3, and 10.

Glycolysis is a metabolic pathway that breaks down glucose into two molecules of pyruvate, releasing energy in the form of ATP and NADH. This process consists of ten steps, each catalysed by a specific enzyme. Some of these steps are reversible, meaning they can proceed in both directions, while others are irreversible, meaning they can only proceed in one direction.

The reversible steps in glycolysis are steps 2-5 and 7-10. These steps involve the conversion of glucose-6-phosphate to fructose-6-phosphate (step 2), fructose-6-phosphate to fructose-1,6-bisphosphate (step 3), fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (step 4), dihydroxyacetone phosphate to glyceraldehyde-3-phosphate (step 5), 1,3-bisphosphoglycerate to 3-phosphoglycerate (step 7), 3-phosphoglycerate to 2-phosphoglycerate (step 8), 2-phosphoglycerate to phosphoenolpyruvate (step 9), and phosphoenolpyruvate to pyruvate (step 10). These reactions are reversible because they are near equilibrium, meaning the forward and reverse reactions occur at approximately the same rate.

The irreversible steps in glycolysis are steps 1, 3, and 10. These steps involve the conversion of glucose to glucose-6-phosphate (step 1), fructose-6-phosphate to fructose-1,6-bisphosphate (step 3), and phosphoenolpyruvate to pyruvate (step 10). These reactions are irreversible because they are far from equilibrium, meaning the forward reaction occurs at a much faster rate than the reverse reaction. These steps are also the ones where ATP is either used or produced. In steps 1 and 3, ATP is used to add a phosphate group to the glucose molecule, making these steps energetically unfavourable and therefore irreversible. In step 10, ATP is produced when a phosphate group is transferred from phosphoenol.

IB Biology Tutor Summary: In glycolysis, a key process breaking down glucose for energy, steps 2-5 and 7-10 can go forwards or backwards, making them reversible. However, steps 1, 3, and 10 only move in one direction, making them irreversible. These irreversible steps involve significant energy changes, either using up ATP (energy) or producing it, which helps regulate the process's direction.