Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 191: During which steps of the Krebs cycle are ATPs formed?

A. Isocitrate dehydrogenase
B. Succinate dehydrogenase
C. Succinate thiokinase and Malate dehydrogenase
D. All of the above (Correct Answer)

Explanation: In the Krebs cycle (TCA cycle), energy is produced in two forms: **Directly** (Substrate-Level Phosphorylation) and **Indirectly** (Oxidative Phosphorylation via NADH and FADH₂). **Why "All of the above" is correct:** The question asks for steps where ATPs are "formed." In biochemistry, this includes both direct synthesis and the potential ATP yield from reduced coenzymes entering the Electron Transport Chain (ETC). 1. **Isocitrate Dehydrogenase (Option A):** This enzyme catalyzes the oxidative decarboxylation of Isocitrate to α-Ketoglutarate, producing **NADH**. In the ETC, 1 NADH yields approximately **2.5 ATP**. 2. **Succinate Dehydrogenase (Option B):** This enzyme converts Succinate to Fumarate, producing **FADH₂**. In the ETC, 1 FADH₂ yields approximately **1.5 ATP**. 3. **Succinate Thiokinase (Option C):** Also known as Succinyl-CoA Synthetase, this is the only step involving **Substrate-Level Phosphorylation**, directly producing **1 GTP** (energetically equivalent to 1 ATP). 4. **Malate Dehydrogenase (Option C):** This enzyme converts Malate to Oxaloacetate, producing another **NADH** (yielding **2.5 ATP**). Since every step mentioned results in the production of energy carriers that ultimately form ATP, "All of the above" is the most accurate choice. **High-Yield NEET-PG Pearls:** * **Total ATP Yield:** One turn of the Krebs cycle produces **10 ATP** (3 NADH = 7.5; 1 FADH₂ = 1.5; 1 GTP = 1). * **Rate-Limiting Enzyme:** Isocitrate Dehydrogenase is the primary rate-limiting step. * **Unique Enzyme:** Succinate Dehydrogenase is the only enzyme of the TCA cycle located in the **inner mitochondrial membrane** (it is also Complex II of the ETC). * **Inhibitor:** Malonate is a competitive inhibitor of Succinate Dehydrogenase.

Question 192: What is the primary storage form of free energy in the cell?

A. Creatine phosphate
B. ATP (Correct Answer)
C. NADH
D. G-6-P

Explanation: **Explanation:** **ATP (Adenosine Triphosphate)** is considered the "universal energy currency" of the cell. It serves as the primary storage form of free energy because its two high-energy phosphoanhydride bonds can be rapidly hydrolyzed to release energy (~7.3 kcal/mol) to drive endergonic reactions, muscle contraction, and active transport. It acts as a bridge between energy-yielding (catabolic) and energy-requiring (anabolic) pathways. **Why other options are incorrect:** * **Creatine Phosphate:** While it contains a high-energy bond, it acts as a **backup reservoir** (phosphagen) to rapidly replenish ATP in tissues with high demand (like skeletal muscle and brain). It is not the primary currency used by all cellular processes. * **NADH:** This is an **electron carrier** (reducing equivalent). While it holds significant potential energy, that energy must first be processed through the Electron Transport Chain (ETC) to generate ATP. * **G-6-P (Glucose-6-Phosphate):** This is a metabolic intermediate in glycolysis and glycogenesis. It is a "trapped" form of glucose within the cell but is not a direct storage form of high-energy phosphate for cellular work. **Clinical Pearls for NEET-PG:** 1. **Energy Charge:** The ratio of [ATP] to [ADP]/[AMP] regulates key rate-limiting enzymes (e.g., PFK-1 in glycolysis). 2. **ATP Yield:** Complete oxidation of 1 mole of glucose yields **30 or 32 ATP** (depending on the shuttle used: Malate-Aspartate vs. Glycerol-3-Phosphate). 3. **High-Energy Compounds:** ATP is an "intermediate" high-energy compound. Compounds like **Phosphoenolpyruvate (PEP)** and **1,3-Bisphosphoglycerate** have higher phosphate transfer potential than ATP.

Question 193: Which of the following steps in the Krebs cycle directly produces ATP by substrate-level phosphorylation?

A. Succinate thiokinase (Correct Answer)
B. Isocitrate dehydrogenase
C. Succinate dehydrogenase
D. Malate dehydrogenase

Explanation: ***Succinate thiokinase*** - This enzyme (also called **succinyl-CoA synthetase**) directly produces **ATP/GTP** through **substrate-level phosphorylation** by converting succinyl-CoA to succinate. - It is the **only step** in the Krebs cycle that generates ATP directly without requiring the **electron transport chain**. *Isocitrate dehydrogenase* - This enzyme converts isocitrate to α-ketoglutarate and produces **NADH**, not ATP directly. - The NADH generated yields approximately **2.5 ATP** indirectly through the **electron transport chain**. *Succinate dehydrogenase* - This enzyme oxidizes succinate to fumarate and produces **FADH2**, not ATP directly. - The FADH2 generated yields approximately **1.5 ATP** indirectly through the **electron transport chain**. *Malate dehydrogenase* - This enzyme converts malate to oxaloacetate and produces **NADH**, not ATP directly. - Like isocitrate dehydrogenase, the NADH yields approximately **2.5 ATP** indirectly through the **electron transport chain**.

Question 194: Which of the following statements regarding the Electron Transport Chain (ETC) is not true?

A. Occurs in mitochondria
B. Generates ATP
C. Has no role of inorganic phosphate (Correct Answer)
D. Involves transport of reducing equivalents

Explanation: ### Explanation The Electron Transport Chain (ETC) and Oxidative Phosphorylation are fundamentally coupled processes. The correct answer is **C** because inorganic phosphate ($P_i$) plays a critical role in the final step of energy production. #### Why Option C is the Correct Answer (The False Statement) While the ETC itself involves the transfer of electrons, it is physically and functionally coupled with **ATP Synthase (Complex V)**. According to Mitchell’s Chemiosmotic Theory, the proton gradient generated by the ETC drives the phosphorylation of ADP. The reaction is: $$ADP + P_i + \text{Energy (from H}^+ \text{ flux)} \rightarrow ATP$$ Without **inorganic phosphate ($P_i$)**, ATP synthesis cannot occur. Furthermore, the availability of $P_i$ (transported into the matrix via the phosphate translocator) is a regulatory factor for the overall rate of respiration. #### Why Other Options are Incorrect (True Statements) * **A. Occurs in mitochondria:** The ETC components (Complex I-IV) are embedded in the **inner mitochondrial membrane**. * **B. Generates ATP:** The primary physiological purpose of the ETC is to create a proton motive force used by Complex V to generate ATP. * **C. Involves transport of reducing equivalents:** The ETC functions by transferring electrons from reducing equivalents like **NADH** (to Complex I) and **FADH₂** (to Complex II) through a series of redox centers to Oxygen. --- ### High-Yield Clinical Pearls for NEET-PG * **Site of ETC:** Inner Mitochondrial Membrane (IMM). Note that the Citric Acid Cycle occurs in the matrix. * **Final Electron Acceptor:** Molecular Oxygen ($O_2$), which is reduced to water ($H_2O$). * **Inhibitors vs. Uncouplers:** * **Inhibitors** (e.g., Cyanide, CO, Rotenone) stop both electron flow and ATP synthesis. * **Uncouplers** (e.g., 2,4-DNP, Thermogenin) allow electron flow to continue but stop ATP synthesis, dissipating energy as **heat**. * **P:O Ratio:** 2.5 for NADH and 1.5 for FADH₂.

Question 195: Which molecule typically serves as the primary source of reducing power in metabolic pathways?

A. NADPH (Correct Answer)
B. NADH
C. FADH2
D. ATP

Explanation: **Explanation:** The correct answer is **NADPH** (Nicotinamide Adenine Dinucleotide Phosphate). In biochemistry, it is crucial to distinguish between the roles of NADH and NADPH based on the metabolic pathways they serve. **1. Why NADPH is correct:** NADPH serves as the primary **reducing power** for **reductive biosynthesis** (anabolic pathways). While NADH is primarily used to generate ATP via the electron transport chain, NADPH provides the electrons necessary to build complex molecules. It is essential for fatty acid synthesis, cholesterol synthesis, and the regeneration of reduced glutathione to protect cells against reactive oxygen species (ROS). **2. Why the other options are incorrect:** * **NADH:** Primarily functions in **catabolic** pathways. It carries electrons from glycolysis and the TCA cycle to the mitochondria for ATP production (oxidative phosphorylation). * **FADH2:** Similar to NADH, it is an electron carrier used specifically in the electron transport chain (Complex II) to generate energy, not as a general source for biosynthesis. * **ATP:** This is the "energy currency" of the cell, providing chemical energy through phosphate bond hydrolysis, but it does not provide reducing equivalents (electrons). **3. High-Yield Clinical Pearls for NEET-PG:** * **Sources of NADPH:** The **Pentose Phosphate Pathway (PPP/HMP Shunt)** is the major source of NADPH. The rate-limiting enzyme is **Glucose-6-Phosphate Dehydrogenase (G6PD)**. * **Clinical Correlation:** In **G6PD deficiency**, the lack of NADPH leads to an inability to maintain reduced glutathione in RBCs, resulting in oxidative stress, Heinz bodies, and hemolytic anemia. * **Key Mnemonic:** **NADH** is for **D**egradation (Catabolism); **NADPH** is for **P**roduction (Anabolism/Biosynthesis).

Question 196: Which of the following enzymes is affected in cyanide poisoning?

A. G-6-P dehydrogenase
B. Isomerase
C. Cytochrome oxidase (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Why Cytochrome Oxidase is correct:** Cyanide poisoning is a classic high-yield topic in medical biochemistry. Cyanide ($CN^-$) acts as a potent irreversible inhibitor of **Cytochrome oxidase (Complex IV)** in the Electron Transport Chain (ETC). Specifically, it binds to the **ferric ($Fe^{3+}$) iron** in the heme $a_3$ component of the enzyme. This binding halts the final step of the ETC—the transfer of electrons to oxygen—effectively stopping ATP production via oxidative phosphorylation. This leads to cellular hypoxia despite adequate oxygen saturation in the blood (histotoxic hypoxia). **Why the other options are incorrect:** * **A. G-6-P dehydrogenase:** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) shunt, responsible for producing NADPH. It is not involved in the ETC or inhibited by cyanide. * **B. Isomerase:** These are a general class of enzymes (like phosphohexose isomerase) that catalyze structural rearrangements. They are not targets of cyanide. **NEET-PG High-Yield Pearls:** * **Antidote Mechanism:** Amyl nitrite/Sodium nitrite is used to induce **methemoglobinemia**. Methemoglobin contains $Fe^{3+}$, which has a higher affinity for cyanide than cytochrome oxidase, "sequestering" the poison away from the mitochondria. * **Other Inhibitors of Complex IV:** Carbon Monoxide (CO) and Hydrogen Sulfide ($H_2S$). * **Clinical Sign:** Patients often present with a "cherry-red" skin discoloration (due to high venous oxygen content) and a characteristic **bitter almond odor** on the breath. * **Lactic Acidosis:** Since aerobic metabolism is blocked, the body shifts to anaerobic glycolysis, leading to severe metabolic acidosis.

Question 197: Pyruvate can be a substrate for all metabolic pathways except:

A. Fatty acid synthesis
B. TCA cycle
C. Cholesterol synthesis
D. Haemoglobin synthesis (Correct Answer)

Explanation: **Explanation:** Pyruvate serves as a critical metabolic junction, but it does not contribute to the synthesis of **Haemoglobin**. **1. Why Haemoglobin synthesis is the correct answer:** Haemoglobin synthesis requires **Succinyl CoA** (which combines with Glycine in the rate-limiting step catalyzed by ALA synthase). While Succinyl CoA is an intermediate of the TCA cycle, it is derived from the oxidation of Alpha-ketoglutarate or the metabolism of odd-chain fatty acids and certain amino acids. Pyruvate cannot be directly converted into Succinyl CoA for heme synthesis; its primary carbon flux is toward energy production or lipid synthesis. **2. Why the other options are incorrect:** * **TCA Cycle:** Pyruvate is converted into **Acetyl-CoA** by the Pyruvate Dehydrogenase (PDH) complex. Acetyl-CoA then enters the TCA cycle by condensing with oxaloacetate. * **Fatty Acid & Cholesterol Synthesis:** Both pathways require **Acetyl-CoA** as the fundamental building block. Since Pyruvate is the primary precursor for mitochondrial Acetyl-CoA (which is then transported to the cytosol via the Citrate Shuttle), it directly supports the synthesis of long-chain fatty acids and the steroid ring of cholesterol. **High-Yield Clinical Pearls for NEET-PG:** * **Pyruvate Carboxylase:** Converts pyruvate to Oxaloacetate (OAA). This is a key **anaplerotic** reaction (refilling TCA intermediates) and the first step of Gluconeogenesis. * **PDH Complex Deficiency:** Leads to lactic acidosis and neurological decline because the brain cannot oxidize pyruvate to Acetyl-CoA, forcing it into the lactate pathway. * **Heme Synthesis Site:** Occurs partly in the mitochondria and partly in the cytosol. Remember: **"The first and last three steps are mitochondrial."**

Question 198: All of the following processes occur in mitochondria, except:

A. Gluconeogenesis
B. TCA cycle
C. Beta oxidation of fatty acids
D. Fatty acid synthesis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Fatty acid synthesis** because it primarily occurs in the **cytosol**. This is a classic "compartmentalization" question frequently tested in NEET-PG. **1. Why Fatty Acid Synthesis is the Correct Answer:** De novo synthesis of fatty acids (Lipogenesis) occurs in the cytosol of cells, primarily in the liver, lactating mammary glands, and adipose tissue. The key enzyme, **Fatty Acid Synthase (FAS) complex**, is located in the cytoplasm. While the starting material (Acetyl-CoA) is generated in the mitochondria, it must be transported to the cytosol via the **Citrate-Malate Shuttle** because the mitochondrial membrane is impermeable to Acetyl-CoA. **2. Why the other options are incorrect:** * **TCA Cycle (Krebs Cycle):** Occurs entirely within the **mitochondrial matrix**. It is the final common pathway for the oxidation of carbohydrates, lipids, and proteins. * **Beta-oxidation of Fatty Acids:** This is the breakdown of fatty acids to generate energy, which occurs in the **mitochondrial matrix**. Fatty acids enter the mitochondria via the **Carnitine Shuttle**. * **Gluconeogenesis:** This is a **bisegmental** process. It begins in the mitochondria (Pyruvate → Oxaloacetate via Pyruvate Carboxylase) and finishes in the cytosol. Since part of it occurs in the mitochondria, it does not fit the "except" criteria. **High-Yield Clinical Pearls for NEET-PG:** * **Exclusively Mitochondrial:** TCA cycle, Beta-oxidation, Ketogenesis, Urea cycle (partial), and Heme synthesis (partial). * **Exclusively Cytosolic:** Glycolysis, HMP Shunt, Fatty acid synthesis, and Translation. * **Both (Mnemonic: "HUG"):** **H**eme synthesis, **U**rea cycle, **G**luconeogenesis. * **Key Enzyme:** The rate-limiting step of fatty acid synthesis is **Acetyl-CoA Carboxylase (ACC)**, which requires Biotin (B7).

Question 199: Which of the following acts as an uncoupler in the electron transport chain?

A. H2S
B. Antimycin a
C. 2, 4-dinitrophenol (Correct Answer)
D. Barbiturates

Explanation: ### Explanation **Correct Answer: C. 2, 4-dinitrophenol (DNP)** **Mechanism of Action:** Uncouplers are substances that dissociate oxidation from phosphorylation. They function by increasing the permeability of the inner mitochondrial membrane to protons ($H^+$). This allows protons to leak back into the mitochondrial matrix, bypassing the **ATP synthase (Complex V)**. Consequently, the proton gradient is dissipated as **heat** instead of being used to synthesize ATP. Oxygen consumption increases as the ETC works at maximum capacity to restore the gradient, but no ATP is produced. **Analysis of Incorrect Options:** * **A. $H_2S$ (Hydrogen Sulfide):** This is an **ETC inhibitor** that acts on **Complex IV** (Cytochrome c oxidase), similar to cyanide and carbon monoxide. It stops the flow of electrons entirely. * **B. Antimycin A:** This is an **ETC inhibitor** that blocks electron transfer at **Complex III** (between Cytochrome b and c1). * **D. Barbiturates (e.g., Amobarbital):** These are **ETC inhibitors** that act on **Complex I** (NADH dehydrogenase), preventing the transfer of electrons from Fe-S centers to Ubiquinone. **High-Yield Clinical Pearls for NEET-PG:** * **Physiological Uncoupler:** **Thermogenin (UCP1)**, found in the brown adipose tissue of newborns, generates heat to maintain body temperature (non-shivering thermogenesis). * **Aspirin Overdose:** High doses of salicylates act as uncouplers, explaining the hyperpyrexia (fever) seen in toxicity. * **DNP History:** Historically used as a weight-loss drug, it was banned due to fatal hyperthermia and cataract formation. * **Key Distinction:** Inhibitors stop **both** oxygen consumption and ATP synthesis; Uncouplers **increase** oxygen consumption but **stop** ATP synthesis.

Question 200: Phenobarbitone inhibits which complex of the electron transport chain?

A. Complex I (Correct Answer)
B. Complex II
C. Complex III
D. Complex IV

Explanation: **Explanation:** The correct answer is **Complex I (NADH:ubiquinone oxidoreductase)**. The Electron Transport Chain (ETC) consists of several complexes that facilitate the transfer of electrons to generate a proton gradient. **Phenobarbitone** (a barbiturate) acts as a potent inhibitor of Complex I. It binds to the complex and prevents the transfer of electrons from the Iron-Sulfur (Fe-S) centers to Ubiquinone (Coenzyme Q). This blockage halts the entire respiratory chain, as electrons cannot proceed to subsequent complexes, leading to a decrease in ATP production and oxygen consumption. **Analysis of Incorrect Options:** * **Complex II (Succinate dehydrogenase):** Inhibited by **Malonate** (competitive inhibitor) and **Carboxin**. Complex II is unique as it does not pump protons and is also a member of the TCA cycle. * **Complex III (Cytochrome bc1 complex):** Inhibited by **Antimycin A** and **British Anti-Lewisite (BAL)**. These substances block electron flow between Cytochrome b and Cytochrome c1. * **Complex IV (Cytochrome c oxidase):** Inhibited by **Cyanide (CN⁻)**, **Carbon Monoxide (CO)**, **Azide (N₃⁻)**, and **Hydrogen Sulfide (H₂S)**. These bind to the heme iron in the complex, preventing the final reduction of oxygen to water. **High-Yield Clinical Pearls for NEET-PG:** * **Complex I Inhibitors Mnemonic:** Remember **"PAR"** — **P**henobarbitone (Barbiturates), **A**mital, and **R**otenone (a fish poison). * **Uncouplers vs. Inhibitors:** While inhibitors (like Phenobarbitone) stop electron flow, **uncouplers** (like 2,4-DNP or Thermogenin) allow electron flow to continue but dissipate the proton gradient as heat, bypassing ATP synthesis. * **Complex V (ATP Synthase):** Specifically inhibited by **Oligomycin**, which closes the H⁺ channel.

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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