Bioenergetics MCQs for GPAT, NIPER, AIIMS Pharmacist, Railway Pharmacist, SSC, ESIC, and State Pharmacist Examinations.

Short Notes on Bioenergetics

Bioenergetics

Bioenergetics is the branch of biochemistry that studies how living organisms obtain, store, transfer, and utilize energy for various biological processes such as muscle contraction, active transport, biosynthesis, and nerve conduction.

1. Free Energy (Gibbs Free Energy, G)

Free energy is the amount of energy available to perform useful work in a biological system.

• It determines whether a reaction occurs spontaneously or not.

• Represented by ΔG (change in free energy).

Interpretation of ΔG

• ΔG < 0 → Reaction is spontaneous (Exergonic reaction).

• ΔG > 0 → Reaction requires energy input (Endergonic reaction).

• ΔG = 0 → Reaction is at equilibrium.

2. Relationship Between Free Energy, Enthalpy, and Entropy

The relationship is given by ΔG = ΔH − TΔS

Where:

• ΔG = Change in free energy

• ΔH = Change in enthalpy (heat energy)

• T = Absolute temperature (Kelvin)

• ΔS = Change in entropy (disorder)

Enthalpy (ΔH)

Enthalpy represents the heat content of a system.

• ΔH < 0 → Heat released (Exothermic reaction)

• ΔH > 0 → Heat absorbed (Endothermic reaction)

Entropy (ΔS)

Entropy is the measure of randomness or disorder in a system.

• ΔS > 0 → Disorder increases

• ΔS < 0 → Disorder decreases

Important Examination Point

A reaction is most favorable when:

• ΔH is negative, and

• ΔS is positive

Such reactions are spontaneous at all temperatures.

3. Redox Potential (Reduction Potential, E°)

Redox potential measures the tendency of a substance to gain electrons and become reduced.

• Expressed in volts (V) or millivolts (mV).

• Molecules with more positive redox potential readily accept electrons.

• Molecules with more negative redox potential readily donate electrons.

In the Electron Transport Chain

Electrons flow from:
Lower redox potential → Higher redox potential

The final electron acceptor is oxygen, which has a very high reduction potential.

Memory Trick

Positive E° = Electron Acceptor

4. Energy-Rich Compounds

Energy-rich compounds contain high-energy phosphate bonds that release large amounts of energy upon hydrolysis.

Important High-Energy Compounds

• ATP (Adenosine Triphosphate)

• GTP (Guanosine Triphosphate)

• UTP (Uridine Triphosphate)

• CTP (Cytidine Triphosphate)

• Creatine Phosphate

• Phosphoenolpyruvate (PEP)

• 1,3-Bisphosphoglycerate

5. ATP – The Energy Currency of the Cell

ATP is the universal energy carrier of living cells.

Functions of ATP

• Muscle contraction

• Active transport across membranes

• Biosynthesis of proteins, lipids, and nucleic acids

• Cell signaling

• Nerve impulse transmission

Hydrolysis reaction:

ATP + H₂O → ADP + Pi + Energy

Standard free energy released:

ΔG°' ≈ −30.5 kJ/mol (−7.3 kcal/mol)

Why ATP Releases Energy

Energy is released because ADP and inorganic phosphate are more stable than ATP after hydrolysis.

6. Biological Significance of Other Nucleotide Triphosphates

GTP (Guanosine Triphosphate)

• Required for protein synthesis (translation).

• Involved in signal transduction through G-proteins.

Remember:
GTP = Growth of proteins

UTP (Uridine Triphosphate)

• Required for glycogen synthesis.

• Forms UDP-glucose, the activated intermediate.

Remember:
UTP = Used in glycogen synthesis

CTP (Cytidine Triphosphate)

• Required for phospholipid synthesis.

• Forms activated intermediates such as CDP-choline.

Remember:
CTP = Cell membrane phospholipids

7. High-Yield Examination Facts

• ΔG < 0 → Spontaneous reaction.

• ΔG > 0 → Non-spontaneous reaction.

• ΔG = 0 → Equilibrium.

• Oxygen is the final electron acceptor in the ETC.

• PEP has a higher phosphoryl transfer potential than ATP.

• ATP is the energy currency of the cell.

• GTP is mainly involved in protein synthesis.

• UTP is involved in glycogen synthesis.

• CTP is involved in phospholipid synthesis.

• Electrons flow from lower to higher redox potential.

One-Line Revision

ATP for energy, GTP for proteins, UTP for glycogen, and CTP for cell membranes.

These questions focus on conceptual understanding, numerical reasoning, and tricky applications commonly seen in competitive pharmacy examinations.

MCQs

1. For a biochemical reaction occurring at constant temperature and pressure:

ΔG=ΔH−TΔS

A reaction will proceed spontaneously when:

A. ΔG > 0
B. ΔG = 0
C. ΔG < 0
D. ΔH > 0 and ΔS < 0

Answer: C. ΔG < 0

Explanation:

Negative Gibbs free energy indicates a spontaneous (exergonic) process. Positive ΔG indicates a non-spontaneous reaction requiring energy input.

2. Which combination always guarantees a spontaneous reaction at all temperatures?

A. ΔH > 0 and ΔS > 0
B. ΔH < 0 and ΔS < 0
C. ΔH < 0 and ΔS > 0
D. ΔH > 0 and ΔS < 0

Answer: C. ΔH < 0 and ΔS > 0

Explanation:

A negative enthalpy change and positive entropy change both favor spontaneity, making ΔG negative irrespective of temperature.

3. A reaction has:

• ΔH = +40 kJ/mol

• ΔS = +200 J/mol·K

At what temperature will the reaction become spontaneous?

A. Above 100 K
B. Above 200 K
C. Above 300 K
D. Above 400 K

Answer: B. Above 200 K

Explanation:

Convert entropy:

ΔS=0.2 kJ/mol⋅K

At equilibrium:

ΔG=0

T= ΔH​/ΔS = 40/0.2​ = 200K

The reaction becomes spontaneous above 200 K.

4. At equilibrium, the value of Gibbs free energy change is:

A. Positive
B. Negative
C. Zero
D. Equal to enthalpy

Answer: C. Zero

Explanation:

At equilibrium, there is no driving force for reaction progress.

ΔG=0

5. Which statement regarding ATP hydrolysis is correct?

A. ATP contains unstable covalent bonds that explode on hydrolysis.
B. ATP hydrolysis releases energy because products are more stable than reactants.
C. ATP hydrolysis is endergonic.
D. ATP stores energy in phosphate atoms.

Answer: B. ATP hydrolysis releases energy because products are more stable than reactants.

Explanation:

Energy is released because ADP and inorganic phosphate are more stable due to resonance stabilization and reduced electrostatic repulsion.

MCQ 6 The standard free energy change for ATP hydrolysis under physiological conditions is approximately:

A. −7.3 kcal/mol
B. +7.3 kcal/mol
C. −73 kcal/mol
D. +73 kcal/mol

Answer: A. −7.3 kcal/mol

Explanation: The standard free energy of ATP hydrolysis is approximately −30.5 kJ/mol.

MCQ 7 Which of the following has the highest phosphoryl group transfer potential?

A. Glucose-6-phosphate
B. ATP
C. Phosphoenolpyruvate
D. Glycerol-3-phosphate

Answer: C. Phosphoenolpyruvate

Explanation:

PEP has an extremely high free energy of hydrolysis (~−61.9 kJ/mol), greater than ATP.

Order:

PEP > Creatine phosphate > ATP > Glucose-6-phosphate

MCQ 8. Which compound can directly phosphorylate ADP to ATP during glycolysis?

A. Glucose-6-phosphate
B. Phosphoenolpyruvate
C. Fructose-6-phosphate
D. Ribose-5-phosphate

Answer: B. Phosphoenolpyruvate

Explanation:

PEP transfers phosphate to ADP via the pyruvate kinase reaction.

9. Which nucleotide triphosphate is primarily used in protein synthesis?

A. ATP
B. GTP
C. CTP
D. UTP

Answer: B. GTP

Explanation: GTP provides energy during initiation, elongation, and translocation steps of translation.

10. UTP is mainly involved in:

A. Protein synthesis
B. Glycogen synthesis
C. Oxidative phosphorylation
D. DNA replication

Answer: B. Glycogen synthesis

Explanation:

UTP forms UDP-glucose, the activated intermediate required for glycogen synthesis.

11. Which nucleotide triphosphate is primarily required for phospholipid synthesis?

A. ATP
B. GTP
C. CTP
D. TTP

Answer: C. CTP Explanation:

CTP forms activated intermediates such as CDP-choline and CDP-diacylglycerol in phospholipid synthesis.

12. The oxidizing agent in a redox reaction is the substance that:

A. Loses electrons
B. Gains electrons
C. Donates hydrogen
D. Undergoes oxidation

Answer: B. Gains electrons

Explanation: Oxidizing agents accept electrons and themselves become reduced.

MCQ 13. A redox couple with a more positive reduction potential tends to:

A. Donate electrons readily
B. Accept electrons readily
C. Remain chemically inert
D. Undergo oxidation

Answer: B. Accept electrons readily

Explanation: The more positive the reduction potential, the greater the tendency to accept electrons.

14. Electron flow in the electron transport chain occurs from:

A. Higher reduction potential to lower reduction potential
B. Lower reduction potential to higher reduction potential
C. ATP to NADH
D. Oxygen to NADH

Answer: B. Lower reduction potential to higher reduction potential

Explanation: Electrons move spontaneously toward increasingly positive reduction potentials, ultimately reaching oxygen.

15. Which component has the highest reduction potential in the electron transport chain?

A. NADH
B. FMN
C. Cytochrome c
D. Oxygen

Answer: D. Oxygen

Explanation: Oxygen has the highest reduction potential and acts as the final electron acceptor.

16. If ΔG for a reaction is positive, the reaction can still proceed if:

A. Temperature decreases
B. Coupled to ATP hydrolysis
C. Entropy decreases
D. Enthalpy increases

Answer: B. Coupled to ATP hydrolysis

Explanation:

Cells drive unfavorable reactions by coupling them to highly favorable ATP hydrolysis.

17. Which of the following is an example of an endergonic process?

A. ATP hydrolysis
B. Oxidation of glucose
C. Glycogen synthesis
D. Electron transport

Answer: C. Glycogen synthesis

Explanation: Glycogen synthesis requires energy input and is therefore endergonic.

18. The major reason ATP is considered the "energy currency" of the cell is that:

A. It contains the highest-energy phosphate bond known in biology.
B. It can couple exergonic and endergonic reactions efficiently.
C. It is present only in mitochondria.
D. ATP is more stable than ADP.

Answer: B. It can couple exergonic and endergonic reactions efficiently.

Explanation: ATP acts as an intermediate energy carrier linking catabolism with anabolism.

19. Which statement regarding entropy is correct?

A. Entropy is the measure of energy content.
B. Entropy measures the degree of disorder or randomness.
C. Entropy is always negative in biological systems.
D. Entropy is independent of temperature.

Answer: B. Entropy measures the degree of disorder or randomness.

Explanation: Higher entropy corresponds to greater molecular disorder and energy dispersal.

20. During ATP hydrolysis:

ATP + H₂O → ADP + Pi

The reaction is:

A. Oxidation reaction only
B. Reduction reaction only
C. Hydrolysis reaction with negative free energy change
D. Condensation reaction

Answer: C. Hydrolysis reaction with negative free energy change

Explanation: ATP hydrolysis is an exergonic hydrolysis reaction with a negative ΔG.

21. Which statement is TRUE regarding free energy and equilibrium constant?

A. ΔG°' and K_eq are unrelated.
B. A large K_eq corresponds to a positive ΔG°'.
C. A large K_eq corresponds to negative ΔG°'.
D. K_eq is independent of free energy.

Answer: C. A large K_eq corresponds to negative ΔG°'.

22. Which pair is correctly matched?

A. ATP: Glycogen synthesis
B. GTP: Translation
C. CTP: Muscle contraction
D. UTP: Electron transport chain

Answer: B. GTP: Translation

Explanation:

• ATP → General energy currency

• GTP → Protein synthesis

• UTP → Glycogen synthesis

• CTP → Phospholipid synthesis

23. A reaction has:

• ΔH = −50 kJ/mol

• ΔS = −100 J/mol·K

This reaction is:

A. Spontaneous at all temperatures
B. Never spontaneous
C. Spontaneous only at low temperatures
D. Spontaneous only at high temperatures

Answer: C. Spontaneous only at low temperatures

Explanation:

Since both ΔH and TΔS are negative:

At high temperature, the positive value of −TΔS becomes large and may make ΔG positive.

24. Which statement is FALSE?

A. ATP hydrolysis is exergonic.
B. Oxygen has a high reduction potential.
C. ATP has the highest phosphoryl transfer potential in the cell.
D. GTP participates in protein synthesis.

Answer: C. ATP has the highest phosphoryl transfer potential in the cell.

Explanation:

Several compounds possess higher phosphoryl transfer potential than ATP, including:

• Phosphoenolpyruvate

• Creatine phosphate

• 1,3-Bisphosphoglycerate

25. Which of the following is classified as a high-energy phosphate compound?

A. Glucose
B. AMP
C. Creatine phosphate
D. Ribose

Answer: C. Creatine phosphate

Explanation: Creatine phosphate serves as a rapid reserve of high-energy phosphate in skeletal muscle and can regenerate ATP via the creatine kinase reaction.

Dr. Alok Singh

Exam Booster Box: Bioenergetics Quick Revision for competitive exams

• A reaction with ΔG = 0 is at equilibrium — there is no net energy available to do work.

• A positive ΔS favors spontaneity because nature prefers greater disorder.

• Increasing temperature favors reactions with positive entropy change (ΔS > 0).

• Reduction means gain of electrons; oxidation means loss of electrons (LEO says GER).

• The greater the difference in redox potential between two redox couples, the greater the energy released during electron transfer.

• NADH is a strong electron donor because it has a relatively negative reduction potential.

• Creatine phosphate acts as an immediate energy buffer in skeletal and cardiac muscle.

• ATP is not the highest-energy phosphate compound in the cell; it occupies an intermediate position in the phosphoryl transfer scale.

• Substrate-level phosphorylation produces ATP directly without involvement of the electron transport chain.

• Cells maintain ATP concentration nearly constant by continuously regenerating ATP from ADP and Pi.