Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 71: What is the total number of ATP molecules produced by the complete oxidation of one molecule of acetyl CoA in the TCA cycle?

A. 6
B. 8
C. 10 (Correct Answer)
D. 12

Explanation: **Explanation:** The complete oxidation of one molecule of **Acetyl CoA** occurs via the Citric Acid Cycle (TCA cycle). To determine the total ATP yield, we must account for both substrate-level phosphorylation and oxidative phosphorylation (via the Electron Transport Chain). **Breakdown of ATP Production per Acetyl CoA:** 1. **3 NADH molecules:** Produced at the Isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and Malate dehydrogenase steps. (3 × 2.5 = **7.5 ATP**) 2. **1 FADH₂ molecule:** Produced at the Succinate dehydrogenase step. (1 × 1.5 = **1.5 ATP**) 3. **1 GTP (equivalent to ATP):** Produced at the Succinate thiokinase step via substrate-level phosphorylation. (**1 ATP**) **Total:** 7.5 + 1.5 + 1 = **10 ATP**. **Analysis of Incorrect Options:** * **Option A (6) & B (8):** These values are too low and do not account for the full yield of reduced coenzymes (NADH/FADH₂) processed through the ETC. * **Option D (12):** This was the "old" calculation (using 1 NADH = 3 ATP and 1 FADH₂ = 2 ATP). Modern biochemistry (P:O ratios) recognizes the yield as 10 ATP. NEET-PG currently follows the updated 10 ATP yield. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Isocitrate dehydrogenase. * **Substrate-level phosphorylation:** Occurs only at the conversion of Succinyl CoA to Succinate. * **Only membrane-bound enzyme:** Succinate dehydrogenase (also part of Complex II of ETC). * **Total ATP per Glucose:** Complete oxidation of one glucose molecule (2 Acetyl CoA) yields **30 or 32 ATP**, depending on the shuttle used (Glycerol-3-phosphate vs. Malate-aspartate).

Question 72: If 1 NADH provides reducing equivalents to the electron transport chain, how many ATP will be formed?

A. 1.5 ATP
B. 2.5 ATP (Correct Answer)
C. 3 ATP
D. 1.5 ATP

Explanation: ### Explanation The correct answer is **2.5 ATP**. This value is based on the modern **P/O ratio** (Phosphate/Oxygen ratio), which measures the number of ATP molecules synthesized per pair of electrons transferred to oxygen. **1. Why 2.5 ATP is correct:** When NADH enters the Electron Transport Chain (ETC) at **Complex I**, it leads to the pumping of **10 protons ($H^+$)** across the inner mitochondrial membrane into the intermembrane space (4 from Complex I, 4 from Complex III, and 2 from Complex IV). According to current bioenergetic models, it takes approximately **4 protons** to synthesize and export 1 ATP (3 for the ATP synthase rotor and 1 for the phosphate translocator). * Calculation: 10 protons ÷ 4 protons/ATP = **2.5 ATP**. **2. Why the other options are incorrect:** * **3 ATP (Option C):** This is the **older, classical value**. While many older textbooks used 3 ATP for NADH and 2 ATP for $FADH_2$, modern biochemistry (Lehninger, Harper) has revised these to 2.5 and 1.5, respectively, based on the actual proton cost. * **1.5 ATP (Options A & D):** This is the yield for **$FADH_2$**. $FADH_2$ enters at Complex II, bypassing the first proton pump. It only results in **6 protons** being pumped. * Calculation: 6 protons ÷ 4 protons/ATP = **1.5 ATP**. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Glycerol-3-Phosphate Shuttle:** Delivers NADH equivalents from glycolysis to the ETC as $FADH_2$, yielding only **1.5 ATP** per cytosolic NADH. * **Malate-Aspartate Shuttle:** Delivers cytosolic NADH equivalents as mitochondrial NADH, yielding **2.5 ATP**. * **Uncouplers (e.g., 2,4-DNP, Thermogenin):** These dissipate the proton gradient, allowing electron transport to continue without ATP synthesis, releasing energy as heat. * **Cyanide/CO Inhibition:** These inhibit **Complex IV (Cytochrome c oxidase)**, completely halting the proton gradient formation and ATP production.

Question 73: From which fuel sources is Acetyl-CoA produced?

A. Carbohydrate
B. Lipid
C. Amino acids
D. All of the above (Correct Answer)

Explanation: **Explanation:** Acetyl-CoA is the "universal intermediary" in metabolism, serving as the common point where the breakdown products of all major macronutrients converge before entering the Citric Acid Cycle (TCA Cycle) for ATP production. 1. **Carbohydrates:** Through glycolysis, glucose is converted into pyruvate. In the mitochondria, the **Pyruvate Dehydrogenase (PDH) complex** oxidatively decarboxylates pyruvate to form Acetyl-CoA. 2. **Lipids:** Fatty acids undergo **Beta-oxidation** in the mitochondrial matrix. Each cycle of beta-oxidation cleaves a two-carbon unit to produce one molecule of Acetyl-CoA. 3. **Amino Acids:** "Ketogenic" amino acids (like Leucine and Lysine) and "Glucogenic/Ketogenic" amino acids (like Phenylalanine and Tyrosine) are deaminated and their carbon skeletons are converted directly or indirectly into Acetyl-CoA. **Why "All of the above" is correct:** Since carbohydrates, lipids, and proteins all possess metabolic pathways that terminate in or pass through the production of Acetyl-CoA to generate energy, all three options are correct. **High-Yield Clinical Pearls for NEET-PG:** * **Irreversibility:** The conversion of Pyruvate to Acetyl-CoA by PDH is **irreversible**. This is why acetyl-CoA (derived from fats) cannot be used for net glucose synthesis (gluconeogenesis). * **Ketogenesis:** When Acetyl-CoA levels exceed the capacity of the TCA cycle (e.g., in starvation or Diabetes Mellitus), it is diverted to form **Ketone Bodies**. * **Cofactors:** The PDH complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**eversed **N**eck **P**ads **L**oose).

Question 74: Hydrolysis of which of the following yields 10.5 Kcal energy?

A. ATP
B. GTP
C. UTP
D. Creatine phosphate (Correct Answer)

Explanation: **Explanation:** The question focuses on the **standard free energy of hydrolysis ($\Delta G^{0'}$)** of high-energy phosphates. In biochemistry, compounds are classified based on the energy released upon the cleavage of their phosphate bonds. **1. Why Creatine Phosphate is Correct:** Creatine phosphate (Phosphocreatine) is a **high-energy reservoir** found predominantly in muscle and brain tissue. The hydrolysis of its phosphate bond yields approximately **10.3 to 10.5 kcal/mol**. This energy is significantly higher than that of ATP, allowing creatine phosphate to act as a rapid "buffer" to regenerate ATP from ADP via the enzyme **Creatine Kinase** during the first few seconds of intense muscular contraction. **2. Why the Other Options are Incorrect:** * **ATP (Adenosine Triphosphate):** Often called the "energy currency" of the cell, the hydrolysis of ATP to ADP and Pi releases approximately **7.3 kcal/mol**. While vital, it is energetically "lower" than creatine phosphate. * **GTP and UTP:** These are chemically analogous to ATP. The hydrolysis of the terminal phosphate bond in Guanosine triphosphate (GTP) or Uridine triphosphate (UTP) also yields approximately **7.3 kcal/mol**. They are used for specific processes like protein synthesis (GTP) or glycogen synthesis (UTP). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Highest Energy Compound:** **Phosphoenolpyruvate (PEP)** has the highest energy of hydrolysis at approximately **-14.8 kcal/mol**, followed by 1,3-bisphosphoglycerate and Creatine Phosphate. * **Low-energy phosphates:** Compounds like Glucose-6-phosphate yield only ~3.3 kcal/mol. * **Creatine Kinase (CK):** In myocardial infarction, the **CK-MB** isoenzyme is a critical diagnostic marker. * **The "Energy Ladder":** Remember the hierarchy: PEP > 1,3-BPG > Creatine Phosphate > ATP > G-6-P. Compounds above ATP can donate a phosphate to ADP to form ATP (Substrate-level phosphorylation).

Question 75: Which of the following vitamins is involved in the electron transport chain?

A. Thiamine
B. Riboflavin (Correct Answer)
C. Nicotinic acid
D. Vitamin B12

Explanation: **Explanation:** The Electron Transport Chain (ETC) relies on specific coenzymes to transfer electrons through various complexes. **Riboflavin (Vitamin B2)** is the precursor for **FMN (Flavin Mononucleotide)** and **FAD (Flavin Adenine Dinucleotide)**. In the ETC, FMN is a crucial component of **Complex I** (NADH dehydrogenase), while FAD is an integral part of **Complex II** (Succinate dehydrogenase). These flavoproteins act as prosthetic groups that undergo reversible redox reactions to facilitate electron flow toward Oxygen. **Analysis of Options:** * **Riboflavin (B2):** Correct. It forms FMN and FAD, which are essential electron carriers in Complexes I and II. * **Thiamine (B1):** Incorrect. Its active form, TPP, is a cofactor for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase) but does not participate directly in the ETC. * **Nicotinic acid (B3):** While NAD+ (derived from B3) carries electrons *to* the ETC, it is technically considered a substrate that dissociates from enzymes, whereas the question specifically targets the structural/functional vitamins integrated *within* the chain's complexes (flavoproteins). *Note: In some contexts, B3 is also involved, but B2 is the classic answer for "integral" ETC components.* * **Vitamin B12:** Incorrect. It is involved in DNA synthesis and the conversion of propionyl-CoA to succinyl-CoA, but not the ETC. **High-Yield NEET-PG Pearls:** * **Complex II** is the only enzyme of the TCA cycle (Succinate dehydrogenase) that is also a component of the ETC. * **Iron-Sulfur (Fe-S) clusters** are present in Complexes I, II, and III and are essential for single-electron transfers. * **Inhibitors:** Remember Rotenone (Complex I), Antimycin A (Complex III), and Cyanide/CO (Complex IV) for related MCQ patterns.

Question 76: Which of the following is not an inhibitor of oxidative phosphorylation?

A. Carboxin (Correct Answer)
B. Oligomycin
C. Valinomycin
D. Atractyloside

Explanation: **Explanation:** The correct answer is **Carboxin**. To understand this, one must distinguish between inhibitors of the **Electron Transport Chain (ETC)** and inhibitors of **Oxidative Phosphorylation (OxPhos)**. 1. **Why Carboxin is correct:** Carboxin is a specific inhibitor of **Complex II (Succinate Dehydrogenase)** in the ETC. While it stops the flow of electrons from succinate to Coenzyme Q, it does not directly inhibit the process of oxidative phosphorylation (the synthesis of ATP by ATP synthase). In the context of NEET-PG, inhibitors are often classified by their specific site of action; Carboxin is an ETC inhibitor, not an OxPhos inhibitor. 2. **Analysis of Incorrect Options:** * **Oligomycin:** This is a classic inhibitor of oxidative phosphorylation [1]. It binds to the **Fo subunit** of ATP synthase, blocking the proton channel and preventing the phosphorylation of ADP to ATP [1], [2]. * **Valinomycin:** This is an **ionophore (uncoupler)**. It dissipates the proton gradient by transporting potassium ions across the inner mitochondrial membrane. By destroying the electrochemical gradient, it inhibits oxidative phosphorylation. * **Atractyloside:** This is a plant toxin that inhibits the **Adenine Nucleotide Translocase (ANT)** [3]. It prevents the exchange of ATP and ADP across the inner mitochondrial membrane. Without ADP entering the matrix, ATP synthesis ceases [3]. **High-Yield Clinical Pearls for NEET-PG:** * **Complex I Inhibitors:** Rotenone, Amobarbital (Amytal), Piericidin A. * **Complex III Inhibitors:** Antimycin A. * **Complex IV Inhibitors:** Cyanide, Carbon Monoxide (CO), Sodium Azide, Hydrogen Sulfide ($H_2S$). * **Uncouplers:** 2,4-Dinitrophenol (DNP), Thermogenin (in brown adipose tissue), Aspirin (in high doses) [3]. Uncouplers *increase* oxygen consumption and heat production while *decreasing* ATP synthesis [2].

Question 77: What is the largest reserve of energy stored in the body?

A. Liver glycogen
B. Muscle glycogen
C. Adipose tissue (Correct Answer)
D. Blood glucose

Explanation: **Explanation:** The human body stores energy in various forms to maintain metabolic homeostasis during fasting. The **largest reserve of energy** is found in **Adipose tissue** (Triacylglycerols/Fat). **1. Why Adipose Tissue is Correct:** Adipose tissue stores energy as Triacylglycerols (TAGs). This is the most efficient storage form for two reasons: * **Energy Density:** Fat provides ~9 kcal/g, more than double that of carbohydrates or proteins (~4 kcal/g). * **Hydrophobicity:** Unlike glycogen, fat is stored in an anhydrous (water-free) state. This allows the body to pack a massive amount of energy without the added weight of water, making it the primary long-term energy reservoir. In a 70kg man, adipose tissue provides approximately 135,000 kcal. **2. Why the other options are incorrect:** * **Liver Glycogen:** This is a limited reserve (~75-100g) used primarily to maintain blood glucose levels during short-term fasting (12–18 hours). * **Muscle Glycogen:** While the total amount of muscle glycogen (~400g) is greater than liver glycogen, it lacks the enzyme *Glucose-6-Phosphatase*. Therefore, it cannot contribute to blood glucose and is used exclusively for local muscle contraction. * **Blood Glucose:** This is a transient transport form of energy, not a storage reserve. It contains only about 40–60 kcal at any given time. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Depletion:** During starvation, the body uses exogenous glucose first, followed by glycogenolysis, then gluconeogenesis, and finally lipolysis/ketogenesis. * **Protein as Reserve:** While muscle protein is a significant energy source, it is considered "functional" rather than a "storage" reserve, as its utilization leads to structural and metabolic impairment. * **Caloric Value:** Remember the 4-9-4 rule (Carbs: 4, Fats: 9, Proteins: 4 kcal/g).

Question 78: Which of the following is an ionophore?

A. 2,4-dinitrophenol
B. Valinomycin (Correct Answer)
C. Thermogenin
D. Oligomycin

Explanation: ### Explanation **Correct Answer: B. Valinomycin** **Concept:** Ionophores are lipid-soluble molecules that increase the permeability of the inner mitochondrial membrane to specific ions. They act as **uncouplers** because they dissipate the electrochemical gradient required for ATP synthesis. **Valinomycin** is a classic mobile carrier ionophore that specifically binds and transports **Potassium ($K^+$) ions** across the membrane. By shuttling $K^+$ into the mitochondrial matrix, it disrupts the membrane potential, thereby uncoupling electron transport from oxidative phosphorylation. **Analysis of Incorrect Options:** * **A. 2,4-Dinitrophenol (DNP):** While DNP is a potent uncoupler, it is a **protonophore** (shuttles $H^+$ ions), not a metal ionophore. It was historically used for weight loss but caused fatal hyperthermia. * **C. Thermogenin (UCP-1):** This is a **physiological uncoupler** found in brown adipose tissue. It creates a proton leak across the inner mitochondrial membrane to generate heat (non-shivering thermogenesis) rather than ATP. * **D. Oligomycin:** This is an **inhibitor of oxidative phosphorylation**, not an uncoupler. It acts by binding to the $F_o$ subunit of ATP synthase, physically blocking the flow of protons and stopping ATP production. **High-Yield Clinical Pearls for NEET-PG:** * **Gramicidin** is another important ionophore (channel-forming) that allows the flux of monovalent cations ($Na^+, K^+$). * **Uncouplers** increase Oxygen consumption and the Rate of Electron Transport Chain (ETC) but **decrease ATP synthesis**, leading to energy dissipation as heat. * **Inhibitors** (like Cyanide or Oligomycin) decrease both Oxygen consumption and ATP synthesis. * **Aspirin** in toxic doses acts as an uncoupler, explaining the hyperpyrexia seen in salicylate poisoning.

Question 79: What is the final product of the TCA cycle?

A. Oxaloacetate (Correct Answer)
B. Acetyl CoA
C. CO2
D. Pyruvate

Explanation: **Explanation:** The **Tricarboxylic Acid (TCA) Cycle**, also known as the Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. The correct answer is **Oxaloacetate (OAA)** because the TCA cycle is a true "cycle." 1. **Why Oxaloacetate is correct:** The cycle begins when **Acetyl CoA (2C)** condenses with **Oxaloacetate (4C)** to form Citrate (6C). Through a series of decarboxylation and oxidation reactions, the carbon skeleton is eventually restored to Oxaloacetate. Since OAA is regenerated at the end of the final step (catalyzed by Malate Dehydrogenase) to initiate a new turn, it is considered the final product of the pathway. 2. **Why other options are incorrect:** * **Acetyl CoA:** This is the **substrate** (entry point) that fuels the cycle, not the product. * **CO2:** While CO2 is a byproduct of the decarboxylation steps (Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase), it is a waste product rather than the structural "final product" of the cyclic pathway. * **Pyruvate:** This is the end product of **Glycolysis**. It is converted into Acetyl CoA by the Pyruvate Dehydrogenase (PDH) complex before entering the TCA cycle. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Isocitrate Dehydrogenase. * **Energy Yield:** One turn of the TCA cycle produces **10 ATP** (3 NADH = 7.5, 1 FADH2 = 1.5, 1 GTP = 1). * **Amphibolic Nature:** The TCA cycle is both catabolic (energy production) and anabolic (OAA and α-ketoglutarate are precursors for amino acid synthesis). * **Inhibitor:** Fluoroacetate inhibits Aconitase, while Arsenite inhibits the α-ketoglutarate dehydrogenase complex.

Question 80: Which molecule is the final electron acceptor in the electron transport chain?

A. Coenzyme Q
B. FADH2
C. O2 (Correct Answer)
D. Cytochrome C

Explanation: **Explanation:** The Electron Transport Chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane that facilitates oxidative phosphorylation. The primary goal of the ETC is to transfer electrons from reduced coenzymes (NADH and FADH2) to a final acceptor, releasing energy used to pump protons and synthesize ATP. **Why O2 is the correct answer:** Molecular Oxygen (**O2**) has the highest reduction potential in the respiratory chain. It acts as the **terminal electron acceptor** at Complex IV (Cytochrome c oxidase). Here, O2 reacts with four electrons and four protons to be reduced into two molecules of water (H2O). Without oxygen, the entire chain stalls, stopping ATP production (the biochemical basis of hypoxia). **Why other options are incorrect:** * **Coenzyme Q (Ubiquinone):** This is a mobile electron carrier that transfers electrons from Complexes I and II to Complex III. It is an intermediate, not the final acceptor. * **FADH2:** This is an electron **donor** (produced in the TCA cycle) that enters the ETC at Complex II (Succinate dehydrogenase). * **Cytochrome C:** This is a small peripheral membrane protein that serves as a mobile carrier, transferring electrons from Complex III to Complex IV. **High-Yield Clinical Pearls for NEET-PG:** * **Complex IV Inhibitors:** Cyanide, Carbon Monoxide (CO), and Azide inhibit Cytochrome c oxidase, effectively "suffocating" the cell at the molecular level. * **Complex V:** ATP Synthase is the site of ATP production, driven by the proton motive force (chemiosmotic theory). * **Uncouplers:** Substances like 2,4-Dinitrophenol (DNP) or Thermogenin (in brown fat) dissipate the proton gradient as heat instead of ATP.

Practice by Chapter

Bioenergetics and Thermodynamics

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ATP as Energy Currency

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Tricarboxylic Acid Cycle

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Electron Transport Chain

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Oxidative Phosphorylation

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Mitochondrial Diseases

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Uncouplers and Inhibitors of Oxidative Phosphorylation

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Shuttle Systems: Malate-Aspartate and Glycerol-Phosphate

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Energy Yield from Nutrients

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Metabolic Rate and Basal Metabolism

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Brown Adipose Tissue and Thermogenesis

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Oxygen Toxicity and Free Radicals

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