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# MCAT training passage: Thermodynamics of ATP hydrolysis in living cells

## Problem

The free energy $\text{G}$ of a biochemical system quantifies the capacity of the system to do mechanical work. $\text{G}$ changes over the course of a chemical reaction. The change in $\text{G}$ during a reaction at the biochemical standard state is written $\mathrm{\Delta }{\text{G}}^{\prime \circ }$ and is proportional to the standard transformed equilibrium constant ${\text{K}}_{\text{eq}}^{\prime }$, according to Equation 1.
Equation 1
ATP hydrolysis represents one of the main biochemical mechanisms through which the free energy acquired during catabolic cellular metabolism is liberated to power cellular activities.
${\text{ATP}}^{4-}+{\text{H}}_{2}\text{O}\to {\text{ADP}}^{3-}+{\text{P}}_{\text{i}}^{2-}+{\text{H}}^{+}$
$\mathrm{\Delta }{\text{G}}^{\prime \circ }=-30.5\phantom{\rule{0.167em}{0ex}}\frac{\text{kJ}}{\text{mol}}$
Equation 2
The actual change in free energy for ATP hydrolysis, symbolized by $\mathrm{\Delta }\text{G}$, varies according to cell type and prevailing physiological conditions. Table 1 reports the variation in $\mathrm{\Delta }\text{G}$ values for ATP hydrolysis in various rat cell types under normal physiological conditions, along with cellular ATP and ADP concentrations.
Table 1 Concentrations of ATP and ADP and ([ADP]) $\mathrm{\Delta }\text{G}$ of ATP hydrolysis in various cell types
[ATP][ADP]$\mathrm{\Delta }\text{G}$ of ATP hydrolysis
Hepatocyte$3.38\phantom{\rule{0.167em}{0ex}}\text{mM}$$1.32\phantom{\rule{0.167em}{0ex}}\text{mM}$$-34.9\phantom{\rule{0.167em}{0ex}}\frac{\text{kJ}}{\text{mol}}$
Myocyte$8.05\phantom{\rule{0.167em}{0ex}}\text{mM}$$0.93\phantom{\rule{0.167em}{0ex}}\text{mM}$$-41.2\phantom{\rule{0.167em}{0ex}}\frac{\text{kJ}}{\text{mol}}$
Neuron$2.59\phantom{\rule{0.167em}{0ex}}\text{mM}$$0.73\phantom{\rule{0.167em}{0ex}}\text{mM}$$-37.7\phantom{\rule{0.167em}{0ex}}\frac{\text{kJ}}{\text{mol}}$
Lymphocyte$2.62\phantom{\rule{0.167em}{0ex}}\text{mM}$$0.70\phantom{\rule{0.167em}{0ex}}\text{mM}$?
Which of the following does not represent a possible explanation for the large negative $\mathrm{\Delta }{\text{G}}^{\prime \circ }$ for ATP hydrolysis?
Choose 1 answer: