Bioenergetics and ATP Questions and Answers

Q: The actual free energy change of a given biochemical reaction carried out under standard conditions with 1 M initial concentration of each of the reactants and products will be

The actual free energy change is equal to standard free energy change for the reaction.The actual free energy change is related to the standard value by\[ \Delta G = \Delta G^{\circ} + RT\ln Q \]where Q is the ratio of product to reactant concentrations. With every reactant and product at 1 M, Q = 1 and \(\ln 1 = 0\), so \(\Delta G = \Delta G^{\circ}\) at the start of the reaction.\(\Delta G\) is zero only at equilibrium, when Q equals the equilibrium constant, which these conditions do not describe. It would be less or greater than \(\Delta G^{\circ}\) only if Q were below or above 1.

Q: A swimmer is preparing to swim 'non-stop' across the English channel (a distance of 34 kilometers). Consumption of which of the following category of food/s should the swimmer increase to accomplish this feat?

The swimmer should increase Fats.A non-stop Channel swim lasts many hours in cold water. The body's carbohydrate store (liver and muscle glycogen) holds only about 2000 kcal and is largely used up within the first couple of hours of hard exercise, after which prolonged, moderate-intensity work runs increasingly on fat oxidation. Fat yields about 9 kcal per gram against about 4 for carbohydrate and protein, so it is the most compact fuel reserve, and a layer of body fat also insulates the swimmer against the cold.Protein is used for building and repair rather than as a main fuel, so the options with protein are not the answer. Carbohydrates help in the early part of the swim, but the paper's point is the long-endurance demand, which fat meets.

Q: Intracellular concentrations of ATP, ADP, and inorganic phosphate in four cell types are given below. Which one of these cell types has the most negative ΔG for ATP hydrolysis?Cell typeATP (mM)ADP (mM)Inorganic phosphate (mM)L3.01.85.0K3.91.33.0B2.70.72.7M7.20.98.0

The cell type is B.\[ \begin{aligned} \Delta G &= \Delta G^{\circ\prime} \\ &+ RT \ln\frac{[ADP][P_{i}]}{[ATP]} \end{aligned} \]Since \(\Delta G^{\circ\prime}\) is the same for all four, \(\Delta G\) is most negative where the mass-action ratio is smallest. In mM:L: \(1.8 \times 5.0 / 3.0 = 3.0\)K: \(1.3 \times 3.0 / 3.9 = 1.0\)B: \(0.7 \times 2.7 / 2.7 = 0.7\)M: \(0.9 \times 8.0 / 7.2 = 1.0\)B has the smallest ratio, so the logarithm term is most negative there. M has the most ATP, but its high phosphate level brings its ratio up to that of K; L has the largest ratio and the least negative \(\Delta G\).

Q: Which one of the following redox couples has the highest tendency to donate electrons?

The strongest electron donor is the NAD+/NADH couple.The tendency to donate electrons is measured by the standard reduction potential: the more negative it is, the more readily the reduced form gives up electrons. Approximate values at pH 7:NAD+/NADH: -0.32 VFAD/FADH2 (free): about -0.22 VPyruvate/lactate: -0.19 VFumarate/succinate: +0.03 VNADH has the most negative potential and so the highest tendency to donate electrons, which is why it feeds the respiratory chain at its start. Succinate is the weakest donor of the four.

Q: Which one of the following molecules does NOT contain phosphoanhydride bond(s)?

The molecule without a phosphoanhydride bond is fructose-1,6-bisphosphate.A phosphoanhydride bond links two phosphate groups through an oxygen (P-O-P). In fructose-1,6-bisphosphate the two phosphates are attached separately to C1 and C6 of the sugar through phosphoester bonds (C-O-P); they are never joined to each other.The others all contain the anhydride. ADP has one (between its α and β phosphates), ATP has two (α-β and β-γ), and pyrophosphate is itself two phosphates joined by a single anhydride bond.

Q: The feasibility of a biochemical reaction is decided by the equation ΔG = ΔH-TΔS. If both ΔH and ΔS have negative values, the reaction :

The reaction is exergonic, favoured below T = ΔH/ΔS.A reaction is spontaneous when \(\Delta G \lt 0\). With \(\Delta H \lt 0\) and \(\Delta S \lt 0\), the term \(-T\Delta S\) is positive and grows with temperature:\[ \begin{aligned} \Delta G &= \Delta H - T\Delta S \lt 0 \\ T\Delta S &\gt \Delta H \\ T &\lt \frac{\Delta H}{\Delta S} \end{aligned} \]The inequality flips at the last step because \(\Delta S\) is negative. So the favourable enthalpy wins at low temperature, and the reaction is exergonic and favoured below that temperature. It is not unfavoured at every temperature, it is not spontaneous at every temperature, and 'endergonic above' describes the opposite case of positive \(\Delta H\) and \(\Delta S\).

Q: For the reaction A → B at 298 K, the change in enthalpy is − 7 kJ.mol\(^{-1}\) and the change in entropy is − 25 J.K\(^{-1}\).mol\(^{-1}\). How much is the free energy change and whether the reaction is spontaneous or non-spontaneous ?

\(\Delta G\) is 450 J.mol\(^{-1}\) and the reaction is not spontaneous.Use \(\Delta G = \Delta H - T\Delta S\), with \(\Delta H\) in joules:\[ \begin{aligned} \Delta G &= -7000 - 298 \times (-25) \\ &= -7000 + 7450 \\ &= +450\ \text{J mol}^{-1} \end{aligned} \]A positive \(\Delta G\) means the reaction is not spontaneous at 298 K: the favourable enthalpy change is outweighed by the decrease in entropy. Neither 900 J value follows from these data, and calling the reaction spontaneous ignores the sign of \(\Delta G\).

Q: Which of the following phosphorylated compounds on hydrolysis will yield highest amount of energy ?

The highest energy comes from Phosphoenolpyruvate.Standard free energies of hydrolysis are roughly: phosphoenolpyruvate -61.9 kJ/mol, 1,3-bisphosphoglycerate -49.3 kJ/mol, ATP to ADP -30.5 kJ/mol and glucose 6-phosphate -13.8 kJ/mol.PEP's large value comes from the enol pyruvate it releases, which turns at once into the far more stable keto form. That is why PEP can phosphorylate ADP to ATP in the pyruvate kinase step of glycolysis. 1,3-Bisphosphoglycerate is also a high-energy compound but releases less, and glucose 6-phosphate is a low-energy ester.

Q: ATP can be formed from ADP by direct transfer of a phosphoryl group from phosphoenol pyruvate. Such type of reaction is called as :

Such a reaction is Substrate level phosphorylation.In substrate-level phosphorylation a phosphoryl group is transferred directly from a high-energy substrate to ADP by a soluble enzyme. Pyruvate kinase does this with phosphoenolpyruvate, and phosphoglycerate kinase with 1,3-bisphosphoglycerate, both in glycolysis.Oxidative phosphorylation and photophosphorylation make ATP through ATP synthase, driven by a proton gradient that electron transport (from food or from light) builds; no substrate hands over its phosphate. 'Reductive phosphorylation' is not a recognised route of ATP synthesis.

The actual free energy change of a given biochemical reaction carried out under standard conditions with 1 M initial concentration of each of the reactants and products will be

A.equal to zero
B.equal to standard free energy change for the reaction
C.less than the standard free energy change for the reaction
D.greater than the standard free energy change for the reaction

Exam Year(s)

GATE Life Sciences - 2012,

Answer: B

The actual free energy change is equal to standard free energy change for the reaction.

The actual free energy change is related to the standard value by

\[ \Delta G = \Delta G^{\circ} + RT\ln Q \]

where Q is the ratio of product to reactant concentrations. With every reactant and product at 1 M, Q = 1 and \(\ln 1 = 0\), so \(\Delta G = \Delta G^{\circ}\) at the start of the reaction.

\(\Delta G\) is zero only at equilibrium, when Q equals the equilibrium constant, which these conditions do not describe. It would be less or greater than \(\Delta G^{\circ}\) only if Q were below or above 1.

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A swimmer is preparing to swim 'non-stop' across the English channel (a distance of 34 kilometers). Consumption of which of the following category of food/s should the swimmer increase to accomplish this feat?

A.Proteins
B.Fats
C.Proteins and Carbohydrates
D.Carbohydrates

Exam Year(s)

GATE Life Sciences - 2012,

Answer: B

The swimmer should increase Fats.

A non-stop Channel swim lasts many hours in cold water. The body's carbohydrate store (liver and muscle glycogen) holds only about 2000 kcal and is largely used up within the first couple of hours of hard exercise, after which prolonged, moderate-intensity work runs increasingly on fat oxidation. Fat yields about 9 kcal per gram against about 4 for carbohydrate and protein, so it is the most compact fuel reserve, and a layer of body fat also insulates the swimmer against the cold.

Protein is used for building and repair rather than as a main fuel, so the options with protein are not the answer. Carbohydrates help in the early part of the swim, but the paper's point is the long-endurance demand, which fat meets.

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Intracellular concentrations of ATP, ADP, and inorganic phosphate in four cell types are given below. Which one of these cell types has the most negative ΔG for ATP hydrolysis?

Cell type	ATP (mM)	ADP (mM)	Inorganic phosphate (mM)
L	3.0	1.8	5.0
K	3.9	1.3	3.0
B	2.7	0.7	2.7
M	7.2	0.9	8.0

Exam Year(s)

GATE Life Sciences - 2023,

Answer: C

The cell type is B.

\[ \begin{aligned} \Delta G &= \Delta G^{\circ\prime} \\ &+ RT \ln\frac{[ADP][P_{i}]}{[ATP]} \end{aligned} \]

Since \(\Delta G^{\circ\prime}\) is the same for all four, \(\Delta G\) is most negative where the mass-action ratio is smallest. In mM:

L: \(1.8 \times 5.0 / 3.0 = 3.0\)
K: \(1.3 \times 3.0 / 3.9 = 1.0\)
B: \(0.7 \times 2.7 / 2.7 = 0.7\)
M: \(0.9 \times 8.0 / 7.2 = 1.0\)

B has the smallest ratio, so the logarithm term is most negative there. M has the most ATP, but its high phosphate level brings its ratio up to that of K; L has the largest ratio and the least negative \(\Delta G\).

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Which one of the following redox couples has the highest tendency to donate electrons?

A.Fumarate / succinate
B.NAD\(^{+}\) / NADH
C.FAD / FADH
D.Pyruvate / lactate

Exam Year(s)

GATE Life Sciences - 2024,

Answer: B

The strongest electron donor is the NAD⁺/NADH couple.

The tendency to donate electrons is measured by the standard reduction potential: the more negative it is, the more readily the reduced form gives up electrons. Approximate values at pH 7:

NAD⁺/NADH: -0.32 V
FAD/FADH₂ (free): about -0.22 V
Pyruvate/lactate: -0.19 V
Fumarate/succinate: +0.03 V

NADH has the most negative potential and so the highest tendency to donate electrons, which is why it feeds the respiratory chain at its start. Succinate is the weakest donor of the four.

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Which one of the following molecules does NOT contain phosphoanhydride bond(s)?

A.Adenosine diphosphate
B.Adenosine triphosphate
C.Fructose-1,6-bisphosphate
D.Pyrophosphate

Exam Year(s)

GATE Life Sciences - 2025,

Answer: C

The molecule without a phosphoanhydride bond is fructose-1,6-bisphosphate.

A phosphoanhydride bond links two phosphate groups through an oxygen (P-O-P). In fructose-1,6-bisphosphate the two phosphates are attached separately to C1 and C6 of the sugar through phosphoester bonds (C-O-P); they are never joined to each other.

The others all contain the anhydride. ADP has one (between its α and β phosphates), ATP has two (α-β and β-γ), and pyrophosphate is itself two phosphates joined by a single anhydride bond.

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The feasibility of a biochemical reaction is decided by the equation ΔG = ΔH-TΔS. If both ΔH and ΔS have negative values, the reaction :

A.is not favoured at any temperature
B.happens spontaneously
C.is exergonic, favoured below T = ΔH/ΔS
D.is endergonic, favoured above T = ΔH/ΔS

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

The reaction is exergonic, favoured below T = ΔH/ΔS.

A reaction is spontaneous when \(\Delta G \lt 0\). With \(\Delta H \lt 0\) and \(\Delta S \lt 0\), the term \(-T\Delta S\) is positive and grows with temperature:

\[ \begin{aligned} \Delta G &= \Delta H - T\Delta S \lt 0 \\ T\Delta S &\gt \Delta H \\ T &\lt \frac{\Delta H}{\Delta S} \end{aligned} \]

The inequality flips at the last step because \(\Delta S\) is negative. So the favourable enthalpy wins at low temperature, and the reaction is exergonic and favoured below that temperature. It is not unfavoured at every temperature, it is not spontaneous at every temperature, and 'endergonic above' describes the opposite case of positive \(\Delta H\) and \(\Delta S\).

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For the reaction A → B at 298 K, the change in enthalpy is − 7 kJ.mol\(^{-1}\) and the change in entropy is − 25 J.K\(^{-1}\).mol\(^{-1}\). How much is the free energy change and whether the reaction is spontaneous or non-spontaneous ?

A.ΔG = 450 J.mol\(^{-1}\) and reaction is not spontaneous
B.ΔG = 450 J.mol\(^{-1}\) and reaction is spontaneous
C.ΔG = 900 J mol\(^{-1}\) and reaction is spontaneous
D.ΔG = 900 J mol\(^{-1}\) and reaction is not spontaneous

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: A

\(\Delta G\) is 450 J.mol\(^{-1}\) and the reaction is not spontaneous.

Use \(\Delta G = \Delta H - T\Delta S\), with \(\Delta H\) in joules:

\[ \begin{aligned} \Delta G &= -7000 - 298 \times (-25) \\ &= -7000 + 7450 \\ &= +450\ \text{J mol}^{-1} \end{aligned} \]

A positive \(\Delta G\) means the reaction is not spontaneous at 298 K: the favourable enthalpy change is outweighed by the decrease in entropy. Neither 900 J value follows from these data, and calling the reaction spontaneous ignores the sign of \(\Delta G\).

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Which of the following phosphorylated compounds on hydrolysis will yield highest amount of energy ?

A.Glucose 6-phosphate
B.ATP
C.1, 3 Bisphosphoglycerate
D.Phosphoenolpyruvate

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: D

The highest energy comes from Phosphoenolpyruvate.

Standard free energies of hydrolysis are roughly: phosphoenolpyruvate -61.9 kJ/mol, 1,3-bisphosphoglycerate -49.3 kJ/mol, ATP to ADP -30.5 kJ/mol and glucose 6-phosphate -13.8 kJ/mol.

PEP's large value comes from the enol pyruvate it releases, which turns at once into the far more stable keto form. That is why PEP can phosphorylate ADP to ATP in the pyruvate kinase step of glycolysis. 1,3-Bisphosphoglycerate is also a high-energy compound but releases less, and glucose 6-phosphate is a low-energy ester.

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ATP can be formed from ADP by direct transfer of a phosphoryl group from phosphoenol pyruvate. Such type of reaction is called as :

A.Photo-phosphorylation
B.Oxidative phosphorylation
C.Substrate level phosphorylation
D.Reductive phosphorylation

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: C

Such a reaction is Substrate level phosphorylation.

In substrate-level phosphorylation a phosphoryl group is transferred directly from a high-energy substrate to ADP by a soluble enzyme. Pyruvate kinase does this with phosphoenolpyruvate, and phosphoglycerate kinase with 1,3-bisphosphoglycerate, both in glycolysis.

Oxidative phosphorylation and photophosphorylation make ATP through ATP synthase, driven by a proton gradient that electron transport (from food or from light) builds; no substrate hands over its phosphate. 'Reductive phosphorylation' is not a recognised route of ATP synthesis.

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