Calculate the actual, physiological ΔG for the reaction ...

Calculate the actual, physiological $\Delta G$ for the reaction $$ \text { Phosphocreatine }+\mathbf{A D P} \rightarrow \text { creatine }+\mathbf{A T P}$$ at $37^{\circ} \mathrm{C},$ as it occurs in the cytosol of neurons, with phosphocreatine at $4.7 \mathrm{mM},$ creatine at $1.0 \mathrm{mM}, \mathrm{ADP}$ at $0.73 \mathrm{mM},$ and $\mathrm{ATP}$ at $2.6 \mathrm{mM}$

Key Concepts

Standard Gibbs Free Energy Change (?G?')

The standard Gibbs free energy change refers to the free energy difference between products and reactants when they are in their standard states, typically 1 M concentration (or 1 atm pressure for gases) and at pH 7 for biological reactions. This value serves as a benchmark, allowing the prediction of reaction spontaneity under standard conditions and providing a starting point for making adjustments to physiological conditions.

Gibbs Free Energy

Gibbs free energy is a fundamental thermodynamic quantity that indicates the maximum amount of work obtainable from a chemical reaction at constant temperature and pressure. It determines whether a reaction can occur spontaneously, with a negative Gibbs free energy meaning the reaction proceeds forward, and is a vital concept in both chemical and biochemical energy analyses.

Reaction Quotient (Q)

The reaction quotient is a ratio calculated from the concentrations (or activities) of products and reactants at any moment in a reaction. It indicates how far the system is from equilibrium and is essential for determining the actual free energy change in a reaction by accounting for the real-time conditions of the reactants and products.

Thermodynamic Equation Relating ?G and ?G?'

The relationship between the actual free energy change and the standard free energy change is given by the equation ?G = ?G?' + RT ln Q, where R is the gas constant and T is the temperature in Kelvin. This equation incorporates the effects of temperature and the reaction quotient, thus allowing the calculation of the free energy change under non-standard, physiological conditions.

Temperature Dependence in Biochemical Reactions

Temperature plays a critical role in biochemical reactions by affecting the distribution of molecular energies and the rates at which reactions occur. The inclusion of temperature in the ?G equation (through the RT ln Q term) ensures that the calculated free energy change accurately reflects the conditions within living organisms, where temperature can influence reaction spontaneity and equilibrium positions.

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