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📝 Exergonic vs endergonic reactions (14 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 14 questions available

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What is Exergonic vs endergonic reactions?

Definition:
Exergonic reactions are chemical reactions that release free energy ( $\Delta G < 0$ ) and are spontaneous, such as catabolic pathways like glycolysis and cellular respiration, while endergonic reactions absorb free energy ( $\Delta G > 0$ ) and are non-spontaneous, requiring energy input, such as anabolic pathways like protein synthesis, and in living systems, energy from exergonic reactions (e.g., ATP hydrolysis) drives endergonic reactions.

Working:
Exergonic reactions work by decreasing the free energy of the system, often releasing heat and doing work, and they are thermodynamically favorable; endergonic reactions require an external energy source to proceed, and they are coupled to exergonic reactions in cells to make them proceed, where the coupling is achieved through shared intermediates like ATP, and the overall $\Delta G$ of a coupled reaction is the sum of the individual $\Delta G$ values.

Example:
A simple example is cellular respiration, where glucose oxidation has $\Delta G \approx -686 \text{ kcal/mol}$ (exergonic), and the energy released is used to synthesize ATP from ADP and phosphate, which is endergonic ( $\Delta G \approx +7.3 \text{ kcal/mol}$ per ATP), and these reactions are coupled in mitochondria, demonstrating how energy is transferred and used in cells.

Reason:
Understanding exergonic and endergonic reactions is essential for grasping metabolism, as it explains how cells capture and use energy, and it is critical for understanding enzyme function, metabolic regulation, and the design of energy-efficient biotechnological processes.

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Medium

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📝 All Exergonic vs endergonic reactions MCQs

Q1. Which statement best distinguishes an exergonic reaction from an endergonic reaction under a specified set of conditions?

A.An exergonic reaction always requires an external energy source, whereas an endergonic reaction never does

B.An exergonic reaction has

\Delta G < 0

, whereas an endergonic reaction has

\Delta G > 0

✅

C.An exergonic reaction always releases heat, whereas an endergonic reaction always absorbs heat

D.An exergonic reaction must be fast, whereas an endergonic reaction must be slow

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The key distinction is the sign of Gibbs free-energy change. An exergonic reaction has $\Delta G < 0$ and is thermodynamically favorable in the stated direction, whereas an endergonic reaction has $\Delta G > 0$ and requires energy input or coupling to proceed favorably.

Q2. A student argues that every reaction that releases heat must be exergonic. Which response most accurately evaluates this reasoning?

A.Correct, because heat release directly determines

\Delta G

B.Correct, because exergonic means exactly the same as exothermic

C.Incorrect, because

\Delta G

depends on both enthalpy and entropy, not heat release alone ✅

D.Incorrect, because exergonic reactions can never release heat

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The reasoning confuses two different thermodynamic quantities. Heat release is associated primarily with enthalpy change, whereas spontaneity is determined by Gibbs free energy, $\Delta G = \Delta H - T\Delta S$ . Thus, heat release alone cannot establish whether a reaction is exergonic.

Q3. A reaction has $\Delta H = -40\ \mathrm{kJ/mol}$ and $\Delta S = -0.10\ \mathrm{kJ/(mol\cdot K)}$ . At $300\ \mathrm{K}$ , what is the most appropriate classification?

A.Exergonic, because

\Delta G = -10\ \mathrm{kJ/mol}

✅

B.Endergonic, because

\Delta G = +10\ \mathrm{kJ/mol}

C.Exergonic, because negative

\Delta H

guarantees a negative

\Delta G

D.Endergonic, because negative entropy always makes reactions unfavorable

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Using $\Delta G = \Delta H - T\Delta S$ , the value is $-40 - 300(-0.10) = -10\ \mathrm{kJ/mol}$ . Since $\Delta G < 0$ , the reaction is exergonic at this temperature. The negative enthalpy helps, while negative entropy opposes spontaneity.

Q4. An endergonic biosynthetic reaction is observed to proceed efficiently inside a cell. Which explanation best accounts for this observation?

A.The reaction becomes exergonic simply because enzymes are present

B.The cell can couple the endergonic reaction to an exergonic reaction so the combined process has favorable

\Delta G

✅

C.Endergonic reactions become spontaneous whenever their activation energy is low

D.The reaction must actually be exothermic, so it cannot be endergonic

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: An endergonic reaction can proceed when it is coupled to a sufficiently exergonic process. The Gibbs free-energy changes of coupled reactions are additive, so a strongly negative contribution can make the overall process favorable even though one individual reaction has positive $\Delta G$ .

Q5. Two reactions have $\Delta G_1 = -25\ \mathrm{kJ/mol}$ and $\Delta G_2 = +18\ \mathrm{kJ/mol}$ . If they are obligatorily coupled in a single pathway, what is the thermodynamic outcome?

A.The combined process has

\Delta G = +43\ \mathrm{kJ/mol}

and is unfavorable

B.The combined process has

\Delta G = -7\ \mathrm{kJ/mol}

and is favorable ✅

C.Only the first reaction occurs because positive

\Delta G

prevents coupling

D.Both reactions become zero-energy processes when coupled

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: For coupled reactions, the Gibbs free-energy changes add algebraically. Therefore, $-25 + 18 = -7\ \mathrm{kJ/mol}$ . Because the overall $\Delta G$ is negative, the coupled pathway is thermodynamically favorable even though the second reaction alone is endergonic.

Q6. A metabolic pathway contains an endergonic step with $\Delta G = +12\ \mathrm{kJ/mol}$ . A nearby reaction releases $30\ \mathrm{kJ/mol}$ and is tightly coupled to it. What conclusion is best supported?

A.The combined process has

\Delta G = +42\ \mathrm{kJ/mol}

B.The endergonic step remains impossible because positive

\Delta G

can never be overcome

C.The combined process can have

\Delta G = -18\ \mathrm{kJ/mol}

, making the coupled pathway favorable ✅

D.The coupled reaction must have

\Delta G = +18\ \mathrm{kJ/mol}

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: When the reactions are genuinely coupled, their free-energy changes are summed. The combined value is $+12 + (-30) = -18\ \mathrm{kJ/mol}$ . Thus, the favorable exergonic reaction can drive the otherwise unfavorable endergonic step through thermodynamic coupling.

Q7. A researcher compares two reactions. Reaction X has a large negative $\Delta G$ but a high activation barrier, while reaction Y has a smaller negative $\Delta G$ and a low activation barrier. Which prediction is most scientifically justified?

A.X must always occur faster because its

\Delta G

is more negative

B.Y may proceed faster even though X is more thermodynamically favorable ✅

C.X cannot occur because high activation energy means positive

\Delta G

D.Y must release more energy because it has a lower activation barrier

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Thermodynamic favorability and reaction rate describe different aspects of a reaction. A more negative $\Delta G$ indicates greater thermodynamic driving force, but activation energy controls kinetics. Therefore, reaction Y can be faster despite having a less negative $\Delta G$ .

Q8. A student calculates $\Delta G = +8\ \mathrm{kJ/mol}$ and concludes, 'The reaction cannot occur under any circumstances.' What is the most accurate correction?

A.Positive

\Delta G

means the reaction cannot occur at all

B.Positive

\Delta G

means the reaction is unfavorable in that direction under those conditions, but coupling or changed conditions can make it favorable ✅

C.Positive

\Delta G

means the reaction must be extremely fast

D.Positive

\Delta G

proves the reaction is exothermic

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: A positive Gibbs free-energy change means the reaction is thermodynamically unfavorable in the specified direction under the stated conditions. It does not mean the reaction is impossible. Coupling, altered concentrations, temperature, or other changes can shift the thermodynamic balance.

Q9. A graph of Gibbs free energy versus reaction progress starts at $80\ \mathrm{kJ/mol}$ and ends at $35\ \mathrm{kJ/mol}$ , with a peak at $120\ \mathrm{kJ/mol}$ . Which interpretation is correct?

A.The reaction is endergonic because the peak is higher than the starting point

B.The reaction is exergonic because the final state has lower free energy than the initial state, although an activation barrier exists ✅

C.The reaction is impossible because the graph contains a peak

D.The reaction is exergonic only if the peak is below

35\ \mathrm{kJ/mol}

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: The overall free-energy change is determined by the difference between final and initial states, $35 - 80 = -45\ \mathrm{kJ/mol}$ , so the reaction is exergonic. The higher peak represents an activation barrier, which affects rate rather than the sign of overall $\Delta G$ .

Q10. Reaction A has $\Delta G = -5\ \mathrm{kJ/mol}$ , while Reaction B has $\Delta G = -50\ \mathrm{kJ/mol}$ . Which conclusion is valid without additional kinetic information?

A.Reaction B is necessarily ten times faster than Reaction A

B.Reaction B is thermodynamically more favorable in the stated direction, but its rate cannot be determined from

\Delta G

alone ✅

C.Reaction A must require an enzyme while Reaction B cannot

D.Reaction A is endergonic because its

\Delta G

is closer to zero

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Both reactions are exergonic because their $\Delta G$ values are negative. Reaction B has a greater thermodynamic driving force because its free-energy change is more negative, but reaction speed depends on kinetic factors such as activation barriers and catalysts.

Q11. A cell converts a nutrient through an exergonic reaction and uses the released free energy to drive an endergonic transport process. Which model best represents the overall strategy?

A.The cell reverses the sign of the nutrient reaction without changing anything else

B.The cell stores or transfers free energy through coupling so the combined reactions have a favorable

\Delta G

✅

C.The cell violates conservation of energy because the transport step gains energy

D.The transport process becomes exergonic simply because nutrients are present

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Cells do not create free energy. Instead, they couple favorable and unfavorable processes so that the sum of their Gibbs free-energy changes is negative. Energy released by an exergonic reaction can therefore be transferred through an appropriate coupling mechanism to support an endergonic process.

Q12. Consider a reaction with $\Delta H = +20\ \mathrm{kJ/mol}$ and $\Delta S = +0.10\ \mathrm{kJ/(mol\cdot K)}$ . At which temperature would it first become thermodynamically favorable as temperature increases?

A.100 K

B.150 K

C.200 K ✅

D.250 K

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The reaction becomes favorable when $\Delta G < 0$ . Setting $\Delta G = 0$ gives $0 = 20 - T(0.10)$ , so $T = 200\ \mathrm{K}$ . Above this threshold, the positive entropy term outweighs the positive enthalpy term and $\Delta G$ becomes negative.

Q13. An experiment reports that an endergonic reaction proceeds after an enzyme is added. The investigator concludes that the enzyme changed the reaction's $\Delta G$ from positive to negative. Which evaluation is best?

A.Correct, because enzymes supply free energy directly

B.Correct, because every enzyme makes its reaction exergonic

C.Incorrect, because enzymes generally lower activation energy without changing the overall

\Delta G

between initial and final states ✅

D.Incorrect, because enzymes prevent reactions from reaching equilibrium

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: Enzymes accelerate reactions by lowering the activation-energy barrier. They do not normally change the overall Gibbs free-energy difference between reactants and products. Therefore, an enzyme can make an unfavorable reaction proceed faster, but it does not by itself make its $\Delta G$ negative.

Q14. A hypothetical biochemical system contains an endergonic reaction with $\Delta G = +24\ \mathrm{kJ/mol}$ . Three potential driving reactions release $10$ , $18$ , and $30\ \mathrm{kJ/mol}$ , respectively. Assuming complete and effective coupling, which driving reaction is sufficient by itself?

A.Only the

10\ \mathrm{kJ/mol}

reaction

B.Only the

18\ \mathrm{kJ/mol}

reaction

C.Only the

30\ \mathrm{kJ/mol}

reaction ✅

D.All three reactions are sufficient because any exergonic reaction can drive any endergonic reaction

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: The combined Gibbs free-energy change must be negative for effective thermodynamic driving. Coupling the $+24\ \mathrm{kJ/mol}$ reaction with $-30\ \mathrm{kJ/mol}$ gives $-6\ \mathrm{kJ/mol}$ , which is favorable. The $10$ and $18\ \mathrm{kJ/mol}$ releases are insufficient by themselves.

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