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📝 Energy coupling in biological reactions (15 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 15 questions available

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What is Energy coupling in biological reactions?

Definition:
Energy coupling is the process by which cells use the energy released from exergonic reactions (such as ATP hydrolysis) to drive endergonic reactions (such as biosynthesis), through the transfer of a phosphoryl group or electrons, and this vital mechanism links catabolic and anabolic pathways, ensuring that energy-requiring cellular processes can proceed despite being thermodynamically unfavorable.

Working:
Energy coupling works by sharing a common intermediate, typically ATP, where the exergonic reaction (e.g., ATP → ADP + $P_i$ , with $\Delta G \approx -30.5 \text{ kJ/mol}$ ) provides the free energy needed for an endergonic reaction (e.g., glucose + $P_i$ → glucose-6-phosphate, with $\Delta G \approx +13.8 \text{ kJ/mol}$ ), and the overall coupled reaction is exergonic, making it spontaneous, often through the formation of a phosphorylated intermediate that changes the shape or reactivity of a molecule, enabling otherwise impossible reactions to occur in cells.

Example:
A simple example is the first step of glycolysis, where the phosphorylation of glucose to glucose-6-phosphate is endergonic ( $\Delta G = +13.8 \text{ kJ/mol}$ ), but it is coupled to ATP hydrolysis (exergonic, $\Delta G = -30.5 \text{ kJ/mol}$ ), giving a net $\Delta G = -16.7 \text{ kJ/mol}$ , which is spontaneous, and this coupling allows glucose to be trapped inside the cell and metabolized.

Reason:
Understanding energy coupling is central to bioenergetics and metabolism because it explains how cells perform work and maintain order, and it is essential for grasping how life manages energy, with applications in metabolic engineering, drug design, and understanding diseases like cancer where energy coupling is disrupted.

Easy

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📝 All Energy coupling in biological reactions MCQs

Q1. A cell links an energetically unfavorable reaction to ATP hydrolysis. What is the primary purpose of this coupling?

A.To make the unfavorable reaction disappear from the metabolic pathway

B.To use energy released by a favorable reaction to drive the unfavorable reaction ✅

C.To increase the temperature of the cell so both reactions proceed faster

D.To convert every reactant directly into ATP

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Energy coupling allows a favorable reaction, such as ATP hydrolysis, to provide the energetic driving force needed for an unfavorable reaction. The linked process can proceed when the combined free-energy change becomes favorable.

Q2. Which statement best describes why ATP is useful as an energy-coupling molecule in cells?

A.ATP stores unlimited energy that can be released whenever needed

B.ATP can participate in reactions whose hydrolysis provides a favorable energy change that can be coupled to other processes ✅

C.ATP directly supplies heat to enzymes during every metabolic reaction

D.ATP makes all biochemical reactions spontaneous regardless of their free-energy changes

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: ATP is useful because its hydrolysis can release usable free energy that is coupled to cellular work. It does not make every reaction spontaneous; coupling works only when the combined energetic balance is favorable.

Q3. Reaction X has $\Delta G = +18$ kJ/mol, while reaction Y has $\Delta G = -25$ kJ/mol. If the reactions are effectively coupled, what is the most reasonable prediction?

A.The combined process has

\Delta G = +43

kJ/mol and cannot proceed

B.The combined process has

\Delta G = -7

kJ/mol and can be thermodynamically favorable ✅

C.Only reaction X contributes to the combined free-energy change

D.The reactions cancel each other's reactants and therefore require no energy

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: When coupled reactions occur as one overall process, their free-energy changes are additive. Here $+18 + (-25) = -7$ kJ/mol, so the combined reaction is thermodynamically favorable even though reaction X alone is unfavorable.

Q4. A metabolic pathway contains an unfavorable step followed immediately by ATP hydrolysis, but the two reactions occur in separate compartments. Which conclusion is most justified?

A.They must still be coupled because ATP is present somewhere in the cell

B.Physical separation can prevent effective coupling because energy transfer requires an appropriate mechanistic connection ✅

C.Compartment separation always makes both reactions favorable

D.ATP hydrolysis automatically transfers its energy across membranes

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Effective energy coupling requires more than simply having ATP and an unfavorable reaction in the same organism. The reactions generally need an appropriate mechanistic connection so that energy released by one process can drive the other.

Q5. An enzyme catalyzes reaction A, which has $\Delta G = +10$ kJ/mol. A second reaction releases $-14$ kJ/mol, and an enzyme complex links the two reactions. What does the combined system most likely accomplish?

A.It produces a net

\Delta G

+24

kJ/mol

B.It produces a net

\Delta G

-4

kJ/mol, allowing the overall coupled process to be favorable ✅

C.It makes reaction A have

\Delta G = 0

regardless of the second reaction

D.It prevents the second reaction from releasing energy

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The free-energy changes of coupled reactions are combined, giving $+10 + (-14) = -4$ kJ/mol. The unfavorable reaction remains intrinsically unfavorable, but the linked overall process becomes thermodynamically favorable.

Q6. A researcher observes that adding ATP allows a previously slow biosynthetic reaction to proceed rapidly. Which interpretation is strongest?

A.ATP necessarily changes the equilibrium constant of the biosynthetic reaction

B.ATP may be hydrolyzed through a coupled mechanism that makes the overall process energetically favorable ✅

C.ATP increases the concentration of every reactant involved

D.ATP functions only as a heat source for the biosynthetic enzyme

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: ATP does not simply heat the reaction or universally change its equilibrium constant. Instead, ATP hydrolysis can be mechanistically coupled to the biosynthetic reaction, making the combined process favorable under cellular conditions.

Q7. A cell must synthesize a molecule whose formation is energetically unfavorable. It couples synthesis to ATP hydrolysis through an intermediate that can react with the substrate. Why is this strategy more effective than merely adding ATP to the solution?

A.The intermediate provides a mechanistic route for transferring energy from ATP hydrolysis into the synthetic process ✅

B.The intermediate prevents ATP from being hydrolyzed

C.The intermediate converts all ATP molecules into heat before synthesis begins

D.The intermediate eliminates the need for enzymes

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Effective coupling requires a mechanism that connects the favorable and unfavorable reactions. An activated intermediate can transfer chemical potential from ATP hydrolysis into substrate transformation, rather than relying on ATP being present nonspecifically.

Q8. A student claims, 'Because ATP hydrolysis releases energy, any reaction in a cell can be driven forward simply by increasing ATP concentration.' What is the best critique?

A.Correct, because ATP concentration alone determines whether every reaction is spontaneous

B.Incorrect, because ATP hydrolysis must be mechanistically coupled to the unfavorable reaction and the overall free-energy balance must be favorable ✅

C.Correct, because enzymes convert all ATP energy into useful work

D.Incorrect, because ATP never participates in cellular energy coupling

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The student's reasoning confuses the presence of a favorable reaction with effective coupling. ATP must interact through an appropriate biochemical mechanism, and the summed free-energy changes must favor the overall process.

Q9. A researcher argues that an enzyme can make an energetically unfavorable reaction proceed because enzymes lower activation energy. Which response identifies the error?

A.The statement is correct because lowering activation energy always makes

\Delta G

negative

B.The statement is incorrect because enzymes lower activation barriers but do not by themselves change the overall free-energy difference between reactants and products ✅

C.The statement is incorrect because enzymes only work during ATP hydrolysis

D.The statement is correct because enzymes change equilibrium concentrations permanently

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Enzymes accelerate reactions by lowering activation barriers, but they do not fundamentally change the free-energy difference between reactants and products. An unfavorable reaction requires coupling or another thermodynamic strategy to become favorable.

Q10. A graph shows the free energy of a coupled process decreasing from 120 kJ/mol at the start to 85 kJ/mol at the end. A student's report says the process is unfavorable because the final free energy is still positive. What is wrong with the report?

A.A positive final free-energy value always proves that the reaction is favorable

B.The relevant quantity is the change in free energy, which is negative because the system decreases by 35 kJ/mol ✅

C.Free energy cannot be represented on a graph

D.The process must consume 85 kJ/mol because that is the final value

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Thermodynamic favorability depends on the change in free energy, not whether an absolute plotted value is positive or negative. The decrease from 120 to 85 kJ/mol gives $\Delta G = -35$ kJ/mol for the process shown.

Q11. Consider two experimental pathways. Pathway A shows a free-energy change of $+12$ kJ/mol, while Pathway B combines the same reaction with another reaction having $-20$ kJ/mol. Which graph pattern would best support successful coupling?

A.Pathway B should show a greater overall decrease in free energy than Pathway A ✅

B.Both pathways must have identical free-energy profiles because coupling changes nothing

C.Pathway A should become more favorable without any additional reaction

D.Pathway B should have a positive overall free-energy change larger than

+12

kJ/mol

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Pathway A is unfavorable because its free-energy change is positive. Adding a reaction with a sufficiently negative free-energy change can make the combined process favorable, so the coupled pathway should show a larger net decrease.

Q12. A graph compares ATP concentration with the rate of a coupled biosynthetic process. The rate rises as ATP increases, then reaches a plateau. Which interpretation is most defensible?

A.The graph proves that ATP always changes the equilibrium constant

B.The plateau may indicate that ATP is no longer the only limiting factor, such as enzyme capacity or availability of another reactant ✅

C.The plateau proves that ATP hydrolysis has stopped completely

D.The graph demonstrates that the biosynthetic reaction cannot be coupled to ATP

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A rising rate followed by a plateau can indicate saturation or another limiting factor. ATP availability may initially constrain the coupled reaction, but once sufficient ATP is available, enzyme capacity or another substrate can become limiting.

Q13. Two proposed mechanisms can drive an unfavorable reaction. Mechanism I directly couples substrate modification to ATP hydrolysis, while Mechanism II simply increases ATP concentration without a physical connection. Which should be preferred?

A.Mechanism II, because ATP concentration is the only requirement for coupling

B.Mechanism I, because it provides a direct mechanistic pathway for transferring energy between the reactions ✅

C.Both are equally effective because ATP has the same energy in every situation

D.Mechanism II, because physical coupling prevents reactions from reaching equilibrium

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Mechanism I is more plausible because energy coupling requires a functional connection between the favorable and unfavorable reactions. Merely increasing ATP concentration does not guarantee that its hydrolysis will drive the desired transformation.

Q14. A pathway contains three reactions with $\Delta G$ values of $+8$ , $-13$ , and $+2$ kJ/mol. The cell couples all three reactions into one coordinated sequence. Which conclusion follows from the combined thermodynamics?

A.The overall process has

\Delta G = -3

kJ/mol and is thermodynamically favorable ✅

B.The overall process has

\Delta G = +23

kJ/mol and must stop

C.Only the first positive value determines the direction of the pathway

D.The negative reaction cancels all reactants in the pathway

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: For a coordinated sequence, the individual free-energy changes can be considered together. Here $+8 - 13 + 2 = -3$ kJ/mol, giving a negative overall value, so the complete coupled sequence is thermodynamically favorable.

Q15. An artificial metabolic system couples reaction P with ATP hydrolysis. P has $\Delta G = +30$ kJ/mol, while ATP hydrolysis contributes $-31$ kJ/mol. A researcher concludes that P will always proceed rapidly because the net $\Delta G$ is negative. What important qualification is missing?

A.A negative net

\Delta G

establishes thermodynamic favorability but does not guarantee a rapid rate; activation barriers and enzyme mechanisms still matter ✅

B.A negative net

\Delta G

means enzymes are unnecessary

C.A negative net

\Delta G

guarantees that all substrates are immediately consumed

D.A negative net

\Delta G

means ATP cannot participate in the process

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: The net value $+30 - 31 = -1$ kJ/mol indicates that the coupled process is thermodynamically favorable. However, thermodynamics does not determine reaction speed; activation energy, enzyme catalysis, concentrations, and mechanism also influence kinetics.

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