MCAT training passage: Thermodynamics of ATP hydrolysis in living cells

The free energy

G

of a biochemical system quantifies the capacity of the system to do mechanical work.

G

changes over the course of a chemical reaction. The change in

G

during a reaction at the biochemical standard state is written

Δ G^{' \circ}

and is proportional to the standard transformed equilibrium constant

K_{eq}^{'}

, according to Equation 1.

$Δ G^{' \circ} = - RT l n K_{eq}^{'}$ ‍

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.

${ATP}^{4 -} + H_{2} O \to {ADP}^{3 -} + P_{i}^{2 -} + H^{+}$ ‍

$Δ G^{' \circ} = - 30.5 \frac{kJ}{mol}$ ‍

Equation 2

The actual change in free energy for ATP hydrolysis, symbolized by

Δ G

, varies according to cell type and prevailing physiological conditions. Table 1 reports the variation in

Δ 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])

Δ G

of ATP hydrolysis in various cell types

	[ATP]	[ADP]	$Δ G$ ‍ of ATP hydrolysis
Hepatocyte	$3.38 mM$ ‍	$1.32 mM$ ‍	$- 34.9 \frac{kJ}{mol}$ ‍
Myocyte	$8.05 mM$ ‍	$0.93 mM$ ‍	$- 41.2 \frac{kJ}{mol}$ ‍
Neuron	$2.59 mM$ ‍	$0.73 mM$ ‍	$- 37.7 \frac{kJ}{mol}$ ‍
Lymphocyte	$2.62 mM$ ‍	$0.70 mM$ ‍	?

Which of the following does not represent a possible explanation for the large negative $Δ G^{' \circ}$ ‍ for ATP hydrolysis?

(Choice A)
Reduction of repulsive charge-charge interactions in ADP as compared to ATP
(Choice B)
Solvation effects of $H_{2} O$ ‍ on ADP and $P_{i}$ ‍ as compared to ATP
(Choice C)
A large negative change in entropy for the hydrolysis reaction
(Choice D)
More total resonance forms for ADP and $P_{i}$ ‍ as compared to ATP alone

MCAT

Course: MCAT > Unit 5

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