Energy stored within the chemical bonds of the carbohydrate, fat, and protein molecules contained in food. The process of digestion breaks down carbohydrate molecules into glucose molecules. Glucose serves as your body’s main energy source because it can be converted to usable energy more efficiently than either fat or protein. The only type of energy the cells in your body are able to utilize is the adenosine tri-phosphate molecule (ATP). ATP is made up of one adenosine molecule and three inorganic phosphates. Adenosine di-phosphate (ADP) is an ester of adenosine that contains two phosphates, and it’s used to make ATP. The process of metabolizing glucose to produce ATP is called cellular respiration. There are three main steps in this process.
This first stage in cellular respiration takes place in your cell’s cytoplasm. In the course of this stage, dehydrogenase enzymes interact with the glucose molecule. This interaction oxidizes the molecule, meaning it strips it of some of its electrons, as well as a hydrogen ion. Two electrons and one proton are passed on to a coenzyme called NAD+. The combination of NAD+ with these added electrons and proton forms the NADH molecule. The end products of glycolysis are NADH, two pyruvate molecules and two ATP molecules for each individual glucose molecule that is broken down.
Citric Acid (or Krebs) Cycle Stage
The only products of the glycolysis stage that move on to the citric acid cycle stage are the pyruvate molecules. The citric acid cycle takes place in the cell’s mitochondria, and it will only take place if oxygen is present. When the pyruvate molecules penetrate the cell’s mitochondria, carbon dioxide is released, altering the pyruvate molecules. Enzymes interact with these altered pyruvate molecules, oxidizing them. Again these electrons and proton are transferred to coenzymes, forming NADH and FADH2 molecules. The completed citric acid cycle produces carbon dioxide, NADH molecules, FADH2 molecules and two ATP molecules.
Oxidative Phosphorylation Stage
The energy-rich NADH and FADH2 molecules created in the glycolysis and citric acid cycle stages move on to the oxidative phosphorylation stage. This stage also takes place in the cell’s mitochondria. In it, the electrons in the NADH and FADH2 molecules become a part of what’s known as “the electron transport chain.” As the electrons released from these molecules move from the top of the chain to the bottom of the chain, passing from molecule to molecule, the string of electron transfers generates a type of energy that is used to synthesize ATP. The final outcome of the oxidative phosphorylation, electron transport chain produces the mother lode of 34 ATP molecules for every glucose molecule consumed.
In the Final Analysis
The ATP that is formed during glycolysis and the citric acid cycle is formed as a result of an enzyme passing on a phosphate group to ADP. The combination of this phosphate group with ADP creates ATP.
During the oxidative phosphorylation stage, ATP molecules are synthesized from the energy that is released during the transfer of electrons. The electron transport chain does not generate ATP directly. Rather it generates an energy that activates three catalytic sites in the cell mitochondria that allow ADP to combine with a phosphate group to produce ATP. Glucose is the fuel that drives all of these reactions.