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Martin Ray A. Arcibal AP Biology November 27, 2011 Mrs. Delgado Free Response ATP Adenosine triphosphate (ATP) is one of the most important chemicals in an organism. Its structure allows it to become the most important source of energy. ATP is made up of a nitrogenous base (adenine), a sugar ribose, and three phosphate groups. To release energy needed by the body, it is necessary to break the bonds of one of the phosphate groups. This can be done through hydrolysis, the addition of water molecules to break the bonds monomers have on one another. This process will yield an inorganic phosphate (HOPO42-, abbreviated at Pi) and an ADP (adenosine diphosphate) molecule. This process is an exergonic reaction. It releases 7.3 kcal per mole of ATP. This free-energy change is measured under standard conditions (1 M, 25°C, 1 atm). The cell does not conform with this rule, primarily because reactants and products concentration differ from 1 M. Under cellular conditions, the free energy output is 38% greater than the output under standard conditions (13 kcal per mole). ATP is responsible for mediating the energy coupling in cells. Energy coupling refers to the use of an exergonic process to drive an endergonic one. Exergonic reactions are catabolic reactions, meaning that they are processes that break down large, complex molecules into smaller, simpler ones. These processes often yield energy and are spontaneous. Cellular respiration is an exergonic reaction. Endergonic reactions are anabolic reactions, meaning that they build complex molecules from simple ones. These processes require energy, so they are nonspontaneous. The formation of proteins from amino acids is an example of an endergonic reaction. To couple these reactions, one of the phosphate bonds is broken from ATP. When the bond is broken, energy is released, which powers the endergonic reaction that comes afterwards. The phosphate group that was released forms a bond with one of the molecules that will undergo the reaction, making it phosphorylated. This is the key to the energy coupling of the reactions because the additional phosphate group makes the molecule more reactive than the original unphosphorylated molecule. After this molecule goes through the desired reaction, the phosphate group is released, to be used once again for the formation of ATP in the cellular respiration cycle. A cell undergoes chemical, transport, and mechanical processes in order to survive. These processes are made possible by an ATP’s ability to store and transfer energy. ATP is produced from a series of endergonic reactions, more specifically Glycolysis, the Electron Transport Chain, and the Citric Acid Cycle (Krebs Cycle), which transforms the glucose acquired from food. Glycolysis produces two NADH2, two ATP molecules, and two pyruvic acid molecules. The NADH2 produced is then used in the electron transport chain, along with four carbon dioxide molecules, six NADH2, and two FADH2 produced from the two molecules of pyruvic acid by the Krebs Cycle, to create 34 ATP molecules. These processes combined yield a total of 36 ATP molecules, which is in accordance to the cellular respiration equation: C6H12O6 + 6O2 6H2O + 6CO2 + 36 ATP. To create an ATP molecule, it is necessary to use the phosphate released by the previously phosphorylated molecule and an ADP molecule. The phosphate will be bonded with an ADP molecule through the dehydration (condensation) reaction, the removal of water molecules to create bonds within monomers. The bonding of three phosphate groups, all of which have negative charges, is the cause of the ATP’s usefulness to a cell. The force of repulsion between the three phosphate groups are compacted together, which contributes to the instability of the third phosphate group. The release of this compacted energy is the reason why the phosphate bonds of ATP are often referred to as high-energy phosphate bonds.