Question

The relationship of free-energy change (ΔG) to the concentrations of reactants and products is important because it predicts the direction of spontaneous chemical reactions. In the hydrolysis of ATP to ADP and inorganic phosphate (Pi), the standard free-energy change (ΔG°) is -7.3 kcal/mole. The free-energy change depends on concentrations according to the following equation:

ΔG = ΔG° + 1.42 log10 ([ADP] [Pi]/[ATP])
In a resting muscle, the concentrations of ATP, ADP, and Pi are approximately 0.005 M, 0.001 M, and 0.010 M, respectively. What is the ΔG for ATP hydrolysis in resting muscle?
(a) -11.1 kcal/mole
(b) -8.72 kcal/mole
(c) 6.01 kcal/mole
(d) -5.88 kcal/mole

Answer 1

The free-energy change in ATP hydrolysis under standard conditions is -7.3 kcal/mol, but it can be different in cellular conditions due to variations in local variables. This energy release enables cellular work through energy coupling, exemplified by the action of the sodium-potassium pump, where energy from ATP is transferred to other molecules through phosphorylation.

Step-by-step explanation:

The relationship between free-energy change (ΔG) and the concentrations of reactants and products in the hydrolysis of ATP to ADP and inorganic phosphate (Pi) is crucial in predicting the direction of spontaneous reactions. Under standard conditions—typically pH 7.0, 25 degrees Celsius, and 1 atm pressure—the hydrolysis of ATP releases -7.3 kcal/mol. However, cellular conditions vary and can lead to different ΔG values, such as -14 kcal/mol in the cell. This drop in free energy during ATP hydrolysis facilitates cellular work through energy coupling, where an exergonic reaction like ATP hydrolysis is coupled with endergonic reactions, thereby allowing them to proceed.

For example, the sodium-potassium pump uses the energy released from ATP hydrolysis to transport ions across the cell membrane. This process is a prime illustration of how energy is transferred through phosphorylation, where a phosphate group from ATP is transferred to another molecule, enabling cellular functions and maintaining the balance of sodium and potassium within the cell.