Thus, when electrons are transferred from atoms with low electronegativity like carbon and hydrogen to atoms with high electronegativity a redox reaction also occurs. Think of where the electrons are located in a hydrocarbon bond like those in glucose relatively evenly distributed between hydrogen and carbon. Then think about where the electrons are in water and carbon dioxide molecules relatively close to the oxygens. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one just as a ball loses potential energy by rolling down a hill closer to the center of the earth. In the catabolism of glucose the glucose is oxidized and oxygen is reduced .
Glucose acts as a reducing agent and oxygen acts as an oxidizing agent.
Rather, glucose is broken down gradually in a series of steps catalyzed by enzymes. A few of those reactions are coupled directly to the synthesis of ATP from ADP and Pi but the vast majority of the ATP synthesis occurs indirectly through the transfer of hydrogen atoms to oxygen to form water. The enzymes that accomplish that transfer are all called dehydrogenases to reflect that fact. That transfer is accomplished by way of the electron transport chain. Again, not all of the ATPs made in the breakdown of glucose are made by oxidation.