Download File - Martin Ray Arcibal

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.