In the cytoplasm (the part of the cell that contains the organelles), glucose molecules are broken down into smaller compounds that can be used for energy. These steps in the metabolic pathway called glycolysis are necessary for cellular respiration.
First Step: A hexokinase converts glucose into glucose 6-phosphate, which is a very large molecule of carbon that has a lot of energy in it. This is the first step of glycolysis and is important because it prevents glucose from leaking out of the cell.
Second Step: Phosphoglycerate kinase phosphorylates the glucose molecule by adding a phosphate group to its sugar backbone, forming ATP and 3-phosphoglycerate. This is a high energy phosphoryl-transfer compound that can be used for more ATP in the next steps of glycolysis.
Third Step: A phosphoglycerate mutase isomerises the 3-phosphoglycerate into 2-phosphoglycerate, which has less energy but can still be metabolized for more ATP. This step is important because it takes up only a small amount of ATP, which is needed for the other substrate-level phosphorylation steps in glycolysis.
Fourth Step: A phosphorylase converts the 2-phosphoglycerate into phosphoenolpyruvate or PEP, another high energy compound that can be used to make more ATP. This is also a substrate-level phosphorylation step that requires ATP and can only be carried out when the cell has sufficient ATP to support it.
Fifth Step: A phosphoribosylphosphate oxidase extracts the high-energy electrons from the sugar, which can be picked up by the electron carrier NAD+. This can be done for an additional ATP molecule, giving the cell a net gain of two ATP molecules and two higher-energy NADH molecules.