Using ATP as an Energy Source
Two prominent questions remain with regard to the use of ATP as an energy source. Exactly how much free energy is released with the hydrolysis of ATP? Also, how is that free energy used to do cellular work? The calculated ∆G for the hydrolysis of one mole of ATP into ADP and Pi is −7.3 kcal/mole (−30.5 kJ/mol). Since this calculation is true under standard conditions, we would expect that a different value exists under cellular conditions. In fact, the ∆G for the hydrolysis of one mole of ATP in a living cell is almost double the value at standard conditions: –14 kcal/mol (−57 kJ/mol).
ATP is a highly unstable molecule. Unless quickly used to perform work, ATP spontaneously dissociates into ADP + Pi. As a result, the free energy released during this process is lost as heat. The second question posed above, that is, how the energy released by ATP hydrolysis is used to perform work inside the cell, depends on a strategy we refer to as energy coupling.
Energy Coupling Example: Sodium-Potassium Pump
Cells couple the exergonic reaction of ATP hydrolysis with endergonic reactions, allowing them to proceed. One example of energy coupling using ATP involves a transmembrane ion pump that is extremely important for cellular function. This sodium-potassium pump (Na+/K+ pump) drives sodium out of the cell and potassium into the cell (see image below).
A large percentage of a cell’s ATP is spent powering this pump. This is because cellular processes bring a great deal of sodium into the cell and potassium out of the cell. The pump works constantly to stabilize cellular concentrations of sodium and potassium.
In order for the pump to turn one cycle (exporting three Na+ ions and importing two K+ ions), one molecule of ATP must be hydrolyzed. When ATP is hydrolyzed, its gamma phosphate doesn’t simply float away. But it is actually transferred onto the pump protein. Scientists refer to this process of a phosphate group binding to a molecule as phosphorylation.
As with most cases of ATP hydrolysis, a phosphate from ATP is transferred onto another molecule. In a phosphorylated state, the Na+/K+ pump has more free energy. As a result, it is triggered to undergo a conformational change. In fact, this change allows it to release Na+ to the outside of the cell. Then it binds extracellular K+, which, through another conformational change, causes the phosphate to detach from the pump. See this lesson to remind yourself about the Na+/K+ pump.
This release of phosphate triggers the K+ to be released to the inside of the cell. Essentially, the energy released from the hydrolysis of ATP is coupled with the energy required to power the pump and transport Na+ and K+ ions. ATP performs cellular work using this basic form of energy coupling through phosphorylation.
Energy Coupling Example: Glycolysis
Often during cellular metabolic reactions, such as the synthesis and breakdown of nutrients, certain molecules must be altered slightly in their conformation to become substrates for the next step in the reaction series. One example is during the very first steps of cellular respiration, when a molecule of the sugar glucose is broken down in the process of glycolysis.
In the first step of this process, ATP is required for the phosphorylation of glucose. This process creates a high-energy but unstable intermediate. This phosphorylation reaction powers a conformational change that allows the phosphorylated glucose molecule to be converted to the phosphorylated sugar fructose.
Fructose is a necessary intermediate for glycolysis to move forward. Here, the exergonic reaction of ATP hydrolysis is coupled with the endergonic reaction of converting glucose into a phosphorylated intermediate in the pathway. Once again, the energy released by breaking a phosphate bond within ATP was used for the phosphorylation of another molecule, creating an unstable intermediate and powering an important conformational change.
Video: Overview of Glycolysis
This interactive animation by NDSU VCell Production describes the ATP-producing glycolysis process.