ATP is an essential molecule that provides energy for all cellular activities. The molecule’s phosphate bonds store a great deal of energy, which can be saved until needed by the cell. ATP is both used and created during cellular respiration, with the entire respiration process yielding a net of approximately 30 molecules of ATP per molecule of glucose consumed.
Adenosine triphosphate (ATP) is a molecular compound that provides energy for all of the activities that occur inside of cells. The energy stored in ATP’s phosphate bonds serves as a battery, permitting the cell to save and use energy in response to changing metabolic demands. Respiration is the biological process through which cells convert glucose into ATP, yielding the familiar waste products of carbon dioxide (CO2) and water. Part of respiration occurs in the cytoplasm of the cell, while the majority of ATP production occurs inside the mitochondria.
ATP consists of a carbon adenine ring, a ribose sugar, and three phosphorous groups attached to each other with oxygen molecules. Energy is released when enzymes break the third phosphate bond, leaving the free phosphorous group and a molecular of adenosine diphosphate (ADP). The breaking of this bond releases a tremendous amount of energy, up to 7.3 kilocalories per mole, about the same amount of energy found in a peanut.
Expenditure of ATP
ATP features in cellular respiration in two distinct ways. First, ATP is used to provide energy for the first stage of respiration, known as glycolysis, which takes place outside of the cell’s mitochondria. For each molecule of glucose, two molecules of ATP are required to convert the glucose into a different compound, called fructose 6-phosphate.
Production of ATP
Second, ATP is produced in the subsequent stages of respiration. Four molecules of ATP are created in the final steps of glycolysis, as the fructose molecule is converted into two molecules of pyruvate. In the second stage of respiration, called the citric acid or “Krebs” cycle, ATP is produced by the energy derived from the stripping of carbon atoms from a citrate molecule. This takes place inside the mitochondria and is this process that creates the carbon dioxide breathed out by living organisms. The majority of ATP production, however, occurs in the membrane of the mitochondria, where the so-called Electron Transport Chain used energetic hydrogen ions to join ADP and loose phosphate groups, creating ATP.
For each molecule of glucose that undergoes respiration, theory suggests that as many as 38 molecules of ATP are produced. However, at least one recent study suggests that the actual number produced may be closer to 30, accounting for inefficiencies that reduce actual ATP yield. Because ATP is used to drive all of the cell’s internal processes, the amount of ATP that must be produced by each cell is enormous. By one estimate, the average human body produces more than two hundred trillion trillion (2 x 1026) molecules of ATP each day.