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 synthesis in resting muscle?
(a) -6.01 kcal/mole
(b) 5.88 kcal/mole
(c) 8.72 kcal/mole
(d) 11.1 kcal/mole

Answer 1

The free-energy change (ΔG) for ATP hydrolysis to ADP and Pi is -7.3 kcal/mole, indicating a spontaneous exergonic reaction under standard conditions. In cellular conditions, ΔG is almost double, demonstrating the impact of nonstandard environments on biological reactions. This concept is key for understanding energy transfer and the direction of biochemical reactions.

Step-by-step explanation:

The relationship between free-energy change (ΔG) and the concentrations of reactants and products is crucial for predicting the direction of spontaneous chemical reactions. ATP hydrolysis is a classic example of an exergonic reaction, with a standard free-energy change (ΔG°) of -7.3 kcal/mole. This value indicates that the reaction, which converts ATP to ADP and inorganic phosphate (Pi), releases free energy and therefore occurs spontaneously under standard conditions. However, in a living cell, the ΔG can differ significantly, often being almost double at -14 kcal/mole due to the nonstandard conditions within the cellular environment.

The overall direction and spontaneity of a reaction are often assessed by ΔG°. If ΔG° is negative, the reaction is exergonic and will tend to proceed in the forward direction, releasing free energy. If ΔG° is positive, the reaction is endergonic and requires an input of energy to proceed. When reactions are coupled, such as the hydrolysis of ATP with an energy-requiring process like a sodium-potassium pump, the free energy released by ATP hydrolysis can be transferred to drive the endergonic reaction, demonstrating the principle of energy coupling.