Proteins use energy to catalyze chemical reactions and transport molecules. This is the energy that powers a cell’s metabolism. Without it, a cell would be dead.

Energy in the cell is stored as chemical energy in chemical bonds. All molecules contain chemical energy. However, most metabolic proteins use adenosine triphosphate (ATP) as their energy source. You can think of ATP as a standard form of currency for exchanging energy within a cell.

ATP contains three phosphate groups, and the two bonds holding the three phosphate groups together contain a great deal of energy. When a protein uses energy from an ATP molecule, one of those phosphate-phosphate bonds is broken and ATP is converted into ADP (adenosine diphosphate).

During cellular respiration, the energy from the carbon-carbon bonds in glucose is used to re-form ATP by re-attaching a third phosphate group to ADP. An ATP/ADP molecule is basically like a rechargeable battery. When the molecule is charged up, it is an ATP molecule. When it is depleted, it is an ADP molecule. Unfortunately, like a rechargeable battery, ATP does not hold a charge for long. If energy from an ATP molecule is not used quickly, the ATP molecule will break down on its own, losing its third phosphate group to re-form ADP.

In Petri Dish, we wanted to use ATP as our energy source for proteins. Our metabolic network is actually capable of producing ATP and ADP molecules using phosphoribosyl pyrophosphate (PRPP) as a precursor. However, cellular respiration does not recharge ATP/ADP molecules directly, but through the electron transport chain. And we felt that the electron transport chain introduced too much complexity in our cell model for middle school students. Instead, we are using generic energy units that behave like molecules.

A detailed explanation of chemical bonds and energy states can be found in our interactive, multitouch textbook, Chemistry from the Ground Up.