Review of ATP: The Cellular Energy Source
At the cellular level our bodies are not directly able to utilize the chemical energy from the foodstuffs we ingest. For cells, the energy in the carbon-hydrogen bonds of foodstuffs (crude fuel) has to be converted to a usable compound known as adenosine Triphosphate (ATP) (refined fuel). ATP contains adenine (a nitrogen base), ribose (a five carbon sugar), and three phosphate groups.
ATP plays a pivotal role in cellular metabolism by acting as a “common chemical intermediate” that will function in both exergonic (energy releasing) and endergonic (energy absorbing) reactions.
From looking at the figure, one will notice that the energy in the ATP molecule (its ability to do work) lies in the three negatively charged phosphate bonds. Because of their negative charge, the phosphates are repelling from one another. An enzyme known as ATPase is able to split the terminal phosphate from the compound. When this terminal phosphate group is split or hydrolyzed, energy is released. Roughly 60%-70% of the total energy released is degraded to heat. The cell is able to harness the remaining energy and uses it to perform work.
Notice in this exergonic (energy releasing) reaction the terminal phosphate is split from ATP and results in adenosine diphosphate (ADP), an inorganic phosphate (Pi), and energy.