Explain what a biochemical standard state is and how standard free energy changes are used in metabolic calculations.

• A. Biochemical standard state uses pH 7.0 (ΔG°′) and 1 M concentrations for solutes; ΔG°′ is used to estimate reaction energetics, but actual ΔG depends on in vivo concentrations and conditions.
• B. Standard state uses pH 0 and 1 mM concentrations; ΔG°′ always equals actual ΔG.
• C. Standard state ignores pH; uses 10 M concentrations; ΔG°′ independent of conditions.
• D. Standard state is irrelevant to metabolism.

Answer: (A)