Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 81: Which of the following high energy phosphate compounds acts as a reservoir for ATP formation?

A. Creatine phosphate (Correct Answer)
B. 1,3 bisphosphoglycerate
C. Succinyl CoA
D. Phosphoenolpyruvate

Explanation: ### Explanation **1. Why Creatine Phosphate is Correct:** Creatine phosphate (Phosphocreatine) is the primary **energy reservoir** in tissues with high and fluctuating energy demands, such as skeletal muscle, heart, and brain. While it is a high-energy compound, it is not used directly for cellular work. Instead, it acts as a "buffer" to maintain constant ATP levels. When ATP consumption is high, the enzyme **Creatine Kinase (CK)** transfers a phosphate group from creatine phosphate to ADP to rapidly regenerate ATP (the Lohmann reaction). This provides an immediate source of energy during the first few seconds of muscular contraction before metabolic pathways like glycolysis kick in. **2. Why the Other Options are Incorrect:** * **Phosphoenolpyruvate (PEP) & 1,3-Bisphosphoglycerate (1,3-BPG):** These are high-energy intermediates of **Glycolysis**. While they have higher negative free energy of hydrolysis than ATP, they are metabolic intermediates used for "substrate-level phosphorylation" to *generate* ATP, rather than serving as long-term storage reservoirs. * **Succinyl CoA:** This is a high-energy intermediate of the **TCA cycle**. Its cleavage provides energy for the synthesis of GTP (or ATP), but it does not function as a storage form of high-energy phosphate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Energy Hierarchy:** Phosphoenolpyruvate has the highest energy bond (~ -14.8 kcal/mol), followed by 1,3-BPG and Creatine Phosphate (~ -10.3 kcal/mol). ATP is considered "intermediate" (~ -7.3 kcal/mol). * **Creatine Synthesis:** It is synthesized from three amino acids: **Glycine, Arginine, and Methionine** (as S-adenosylmethionine). * **Clinical Marker:** Creatinine (the waste product of creatine) is excreted in the urine at a constant rate proportional to muscle mass, making it a key marker for renal function. * **CK Isoenzymes:** Remember the diagnostic significance of CK-MB (Heart), CK-MM (Skeletal Muscle), and CK-BB (Brain).

Question 82: All of the following are increased in starvation except:

A. Lipolysis
B. Ketogenesis
C. Gluconeogenesis
D. Glycogenesis (Correct Answer)

Explanation: **Explanation:** In starvation, the body transitions from an exogenous glucose supply to endogenous fuel mobilization to maintain blood glucose levels and provide energy to vital organs. This metabolic shift is primarily driven by a **low Insulin-to-Glucagon ratio.** **Why Glycogenesis is the Correct Answer:** **Glycogenesis** is the process of synthesizing glycogen from glucose for storage. It is an **anabolic** process stimulated by insulin in the well-fed state. During starvation, the body needs to break down glycogen (Glycogenolysis) rather than store it. Therefore, glycogenesis is inhibited to prevent a futile cycle, making it the only process in the list that decreases. **Analysis of Incorrect Options:** * **Lipolysis (A):** Increased. Low insulin levels activate Hormone-Sensitive Lipase (HSL) in adipose tissue, breaking down triglycerides into glycerol and free fatty acids (FFAs) to be used as alternative fuel. * **Ketogenesis (B):** Increased. As FFAs flood the liver, they undergo β-oxidation, producing excess Acetyl-CoA. This Acetyl-CoA is diverted to synthesize ketone bodies (acetoacetate, β-hydroxybutyrate), which serve as a critical fuel source for the brain during prolonged fasting. * **Gluconeogenesis (C):** Increased. Once hepatic glycogen stores are exhausted (usually within 12–18 hours), the liver (and later the kidneys) synthesizes glucose de novo from non-carbohydrate precursors like lactate, glycerol, and glucogenic amino acids (primarily alanine). **High-Yield Clinical Pearls for NEET-PG:** * **The Metabolic Switch:** The primary hormone driving starvation metabolism is **Glucagon**, while **Insulin** is the primary hormone of the fed state. * **Brain Fuel:** In early starvation, the brain relies on glucose; in prolonged starvation (>3 days), it adapts to use **ketone bodies** for up to 75% of its energy needs. * **Key Enzyme:** The rate-limiting enzyme for ketogenesis is **HMG-CoA Synthase** (mitochondrial).

Question 83: Substrate level phosphorylation is catalysed by which enzyme?

A. Succinate dehydrogenase
B. Thio kinase (Correct Answer)
C. Malate dehydrogenase
D. Hexokinase

Explanation: ### Explanation **Substrate-level phosphorylation (SLP)** is a metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphoryl group to ADP or GDP from a high-energy intermediate, independent of the electron transport chain (ETC) and oxygen. **Why Option B is Correct:** In the Citric Acid Cycle (TCA cycle), the enzyme **Succinate Thiokinase** (also known as Succinyl-CoA synthetase) catalyzes the conversion of Succinyl-CoA to Succinate. This reaction involves the cleavage of a high-energy thioester bond, which provides the energy to phosphorylate GDP to GTP (or ADP to ATP). This is the **only** step in the TCA cycle where SLP occurs. **Analysis of Incorrect Options:** * **A. Succinate dehydrogenase:** This enzyme catalyzes the oxidation of Succinate to Fumarate. It is part of the ETC (Complex II) and produces $FADH_2$, which generates ATP via oxidative phosphorylation, not SLP. * **C. Malate dehydrogenase:** This enzyme converts Malate to Oxaloacetate, producing $NADH$. Like Option A, it contributes to ATP production through the ETC. * **D. Hexokinase:** This enzyme catalyzes the first step of glycolysis (Glucose to Glucose-6-Phosphate). It actually **consumes** one molecule of ATP rather than producing it. **High-Yield Clinical Pearls for NEET-PG:** * **Total SLP sites in metabolism:** There are three major sites: 1. **Phosphoglycerate kinase** (Glycolysis: 1,3-BPG → 3-Phosphoglycerate) 2. **Pyruvate kinase** (Glycolysis: PEP → Pyruvate) 3. **Succinate Thiokinase** (TCA Cycle: Succinyl-CoA → Succinate) * **Arsenic Poisoning:** Arsenite inhibits the pyruvate dehydrogenase complex, while **Arsenate** can bypass the SLP step in glycolysis by substituting for inorganic phosphate, resulting in zero net ATP gain. * **Tissue Specificity:** In the liver and kidneys, Succinate Thiokinase prefers GDP, while in muscle tissues, it prefers ADP.

Question 84: All the following reactions occur inside the mitochondria except?

A. EM pathway (Correct Answer)
B. Krebs cycle
C. Urea cycle
D. Electron transfer

Explanation: **Explanation:** The correct answer is **A. EM pathway**. The **Embden-Meyerhof (EM) pathway**, commonly known as **Glycolysis**, occurs exclusively in the **cytosol** of the cell. It is the metabolic sequence that converts glucose into pyruvate (in aerobic conditions) or lactate (in anaerobic conditions). Since it does not require oxygen or specialized mitochondrial machinery to generate ATP (via substrate-level phosphorylation), it is the primary energy source for cells lacking mitochondria, such as mature erythrocytes. **Analysis of Incorrect Options:** * **B. Krebs cycle (TCA Cycle):** This occurs in the **mitochondrial matrix**. It is the central hub for the oxidation of Acetyl-CoA derived from carbohydrates, fats, and proteins. * **C. Urea cycle:** This is a "split" pathway. The first two reactions (catalyzed by CPS-I and Ornithine Transcarbamoylase) occur in the **mitochondria**, while the remaining steps occur in the cytosol. Since a significant portion starts in the mitochondria, it is considered a mitochondrial-linked process. * **D. Electron transfer (ETC):** The Electron Transport Chain and Oxidative Phosphorylation are located on the **inner mitochondrial membrane**. **High-Yield Clinical Pearls for NEET-PG:** * **Exclusively Mitochondrial:** Krebs cycle, Beta-oxidation of fatty acids, Ketogenesis, and Pyruvate Dehydrogenase (PDH) complex. * **Exclusively Cytosolic:** Glycolysis, HMP Shunt, Fatty acid synthesis, and Cholesterol synthesis. * **Both (Mitochondria + Cytosol):** **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis (Mnemonic: **HUG**). * **RBCs** depend entirely on the EM pathway for energy because they lack mitochondria.

Question 85: Which of the following inhibits Complex IV of the electron transport chain?

A. Amylobarbital
B. Aconitase
C. Secobarbitone
D. Cyanide (Correct Answer)

Explanation: **Explanation:** The Electron Transport Chain (ETC) is the final stage of aerobic respiration where electrons are transferred through complexes to create a proton gradient for ATP synthesis. **Complex IV (Cytochrome c Oxidase)** is the terminal enzyme that transfers electrons to oxygen. **Cyanide (Option D)** is a potent inhibitor of Complex IV. It binds to the ferric iron ($Fe^{3+}$) in the heme $a_3$ component of Cytochrome c oxidase, halting the reduction of oxygen to water. This stops the entire ETC, leading to a rapid drop in ATP production and cellular asphyxiation. Other inhibitors of Complex IV include **Carbon Monoxide (CO)**, **Hydrogen Sulfide ($H_2S$)**, and **Azide**. **Analysis of Incorrect Options:** * **Amylobarbital (Option A) and Secobarbitone (Option C):** These are barbiturates. Barbiturates specifically inhibit **Complex I** (NADH-Q oxidoreductase) by blocking the transfer of electrons from Fe-S centers to ubiquinone (CoQ). * **Aconitase (Option B):** This is not an inhibitor; it is an **enzyme** of the TCA cycle that catalyzes the isomerization of citrate to isocitrate. (Note: It is inhibited by Fluoroacetate). **High-Yield Clinical Pearls for NEET-PG:** * **Complex I Inhibitors:** Rotenone, Piericidin A, and Barbiturates. * **Complex II Inhibitors:** Malonate (competitive inhibitor of Succinate Dehydrogenase) and Carboxin. * **Complex III Inhibitors:** Antimycin A and British Anti-Lewisite (BAL). * **Complex V (ATP Synthase) Inhibitor:** Oligomycin (blocks the $F_0$ subunit). * **Uncouplers:** 2,4-Dinitrophenol (DNP) and Thermogenin (brown fat). These increase oxygen consumption but decrease ATP synthesis by dissipating the proton gradient.

Question 86: In oxidative phosphorylation, how are ATP production and the respiratory chain linked?

A. Chemical methods
B. Physical methods
C. Chemiosmotic methods (Correct Answer)
D. Conformational changes

Explanation: ### Explanation **1. Why Chemiosmotic Methods is Correct:** The link between the respiratory chain (Electron Transport Chain) and ATP production is explained by **Peter Mitchell’s Chemiosmotic Theory**. As electrons flow through Complexes I, III, and IV, protons ($H^+$) are pumped from the mitochondrial matrix into the intermembrane space. This creates an **electrochemical gradient** (proton motive force). The potential energy stored in this gradient is harnessed when protons flow back into the matrix through **Complex V (ATP Synthase)**, driving the phosphorylation of ADP to ATP. **2. Why Other Options are Incorrect:** * **Chemical methods:** This refers to "Substrate-Level Phosphorylation" (e.g., in Glycolysis or the TCA cycle), where a high-energy phosphate is transferred directly from a substrate to ADP without an electron transport chain or membrane gradient. * **Physical methods:** While mechanical rotation occurs within ATP Synthase, "physical methods" is not a recognized biochemical term for the coupling mechanism. * **Conformational changes:** While the **Boyer’s Binding Change Mechanism** describes how ATP Synthase changes shape to catalyze ATP synthesis, it is a *component* of the process, not the overarching method that links the respiratory chain to ATP production. **3. NEET-PG High-Yield Pearls:** * **Uncouplers:** Substances like **2,4-Dinitrophenol (DNP)** and **Thermogenin** (in brown fat) dissipate the proton gradient as heat, allowing respiration to continue without ATP synthesis. * **Inhibitor of Complex V:** **Oligomycin** acts by blocking the $F_0$ fraction of ATP synthase, preventing the inflow of protons and stopping both ATP synthesis and the ETC. * **P:O Ratio:** For every NADH oxidized, ~2.5 ATP are produced; for every $FADH_2$, ~1.5 ATP are produced. * **Location:** The ETC components are located on the **inner mitochondrial membrane**.

Question 87: What is the energy released by the oxidation of 1 mole of acetyl CoA?

A. 6 ATP
B. 8 ATP
C. 10 ATP (Correct Answer)
D. 15 ATP

Explanation: **Explanation:** The oxidation of **1 mole of acetyl CoA** occurs via the **Citric Acid Cycle (TCA cycle)** in the mitochondrial matrix. To determine the total ATP yield, we must account for the high-energy phosphates produced both directly and through the electron transport chain (ETC). **Breakdown of ATP Production per Acetyl CoA:** 1. **3 NADH:** Each NADH yields approximately **2.5 ATP** via oxidative phosphorylation (3 × 2.5 = **7.5 ATP**). 2. **1 FADH₂:** Each FADH₂ yields approximately **1.5 ATP** via oxidative phosphorylation (1 × 1.5 = **1.5 ATP**). 3. **1 GTP:** Produced via substrate-level phosphorylation (equivalent to **1 ATP**). * **Total:** 7.5 + 1.5 + 1 = **10 ATP**. *(Note: Older textbooks used the 3:2 ratio for NADH:FADH₂, totaling 12 ATP, but current medical standards and NEET-PG follow the 2.5:1.5 ratio, totaling 10 ATP).* **Analysis of Incorrect Options:** * **A (6 ATP):** This does not correspond to any single stage of acetyl CoA oxidation. * **B (8 ATP):** This is the net yield of **aerobic glycolysis** (1 glucose to 2 pyruvate). * **D (15 ATP):** This is the yield of **1 mole of Pyruvate** (12.5 ATP) rounded up, or based on outdated ratios. Pyruvate yields 12.5 ATP (10 from acetyl CoA + 2.5 from the pyruvate dehydrogenase reaction). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Isocitrate dehydrogenase. * **Substrate-level phosphorylation:** Occurs at the conversion of Succinyl CoA to Succinate (catalyzed by Succinate thiokinase). * **Inhibitors:** Fluoroacetate (inhibits aconitase) and Arsenite (inhibits alpha-ketoglutarate dehydrogenase). * **Amphibolic nature:** The TCA cycle is both catabolic (energy production) and anabolic (provides precursors for heme and gluconeogenesis).

Question 88: Which of the following inhibits oxidative phosphorylation by inhibiting the transport of ADP into and ATP out of the mitochondrion?

A. Atractyloside (Correct Answer)
B. 2,4-dinitrophenol
C. Carbon monoxide
D. Rotenone

Explanation: ### Explanation **Correct Answer: A. Atractyloside** **Mechanism:** Oxidative phosphorylation requires a continuous supply of ADP inside the mitochondrial matrix and the export of synthesized ATP to the cytosol. This exchange is mediated by the **Adenine Nucleotide Translocase (ANT)**, an antiporter located in the inner mitochondrial membrane. **Atractyloside** (a plant glycoside) and **Bongkrekic acid** (a respiratory toxin) specifically inhibit this translocase. By blocking the entry of ADP, the substrate for ATP synthase is depleted, effectively halting both phosphorylation and the electron transport chain (ETC) due to tight coupling. **Analysis of Incorrect Options:** * **B. 2,4-dinitrophenol (DNP):** This is an **uncoupler**. It increases the permeability of the inner mitochondrial membrane to protons, dissipating the proton gradient as heat. It inhibits ATP synthesis but actually *increases* oxygen consumption and ETC activity. * **C. Carbon monoxide (CO):** This is an **ETC inhibitor** that binds to the heme iron of **Complex IV** (Cytochrome c oxidase), preventing the final transfer of electrons to oxygen. * **D. Rotenone:** This is an **ETC inhibitor** that blocks **Complex I** (NADH dehydrogenase), preventing the transfer of electrons from NADH to Coenzyme Q. **High-Yield Clinical Pearls for NEET-PG:** * **Oligomycin:** Inhibits the $F_0$ fraction of ATP synthase (Complex V), directly blocking the proton channel. * **Ionophores:** Valinomycin is a mobile ion carrier that disrupts the membrane potential by transporting $K^+$ ions. * **Brown Adipose Tissue:** Contains **Thermogenin (UCP1)**, a physiological uncoupler used for non-shivering thermogenesis in newborns. * **Mnemonic for ETC Inhibitors:** **R**otten **A**ntimony **A**te **C**yanide (**R**otenone-CI, **A**ntimycin A-CIII, **A**zide/CO-CIV).

Question 89: In which of the following metabolic pathways is ATP NOT produced, EXCEPT?

A. Urea cycle (Correct Answer)
B. Electron transport chain
C. Tricarboxylic acid cycle
D. Anaerobic glycolysis

Explanation: ### Explanation The question asks to identify the pathway that **does not** produce ATP (i.e., it is an energy-consuming process), using a double-negative phrasing ("NOT produced, EXCEPT"). In metabolic biochemistry, pathways are categorized as **exergonic** (energy-producing) or **endergonic** (energy-consuming). **1. Why Urea Cycle is Correct:** The Urea Cycle is a purely **energy-consuming (endergonic)** pathway. It requires the input of **4 high-energy phosphate bonds** to synthesize one molecule of urea: * 2 ATP are used by *Carbamoyl Phosphate Synthetase I (CPS-I)*. * 1 ATP (cleaved to AMP and PPi, equivalent to 2 ATP) is used by *Argininosuccinate Synthetase*. Since it consumes ATP rather than producing it, it fits the criteria of the question. **2. Analysis of Incorrect Options:** * **Electron Transport Chain (ETC):** This is the primary site of ATP production via oxidative phosphorylation. It generates the majority of cellular ATP (approx. 2.5 per NADH and 1.5 per FADH₂). * **Tricarboxylic Acid (TCA) Cycle:** Produces energy in the form of **1 GTP** (equivalent to 1 ATP) per turn via substrate-level phosphorylation (Succinyl-CoA to Succinate), along with reducing equivalents (NADH/FADH₂) that enter the ETC. * **Anaerobic Glycolysis:** Despite the absence of oxygen, it produces a **net gain of 2 ATP** per glucose molecule through substrate-level phosphorylation. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step of Urea Cycle:** CPS-I (requires N-acetylglutamate as an activator). * **Link to TCA Cycle:** The "Bicycle" link is **Fumarate**, which is produced in the urea cycle and can enter the TCA cycle. * **ATP Accounting:** While the Urea Cycle consumes 4 ATP, the conversion of Fumarate to Malate and then Oxaloacetate generates 1 NADH (2.5 ATP), partially offsetting the energy cost.

Question 90: NADP is not needed in:

A. Steroid synthesis
B. Urea synthesis (Correct Answer)
C. Fatty acid synthesis
D. Reduced glutathione synthesis

Explanation: **Explanation:** The correct answer is **B. Urea synthesis**. The primary role of **NADPH** (the reduced form of NADP) is to serve as a reducing equivalent for **reductive biosynthesis** and to maintain cellular antioxidant defenses. 1. **Why Urea Synthesis is the correct answer:** Urea synthesis occurs in the liver via the Urea Cycle. This process is **energy-consuming** but requires **ATP**, not NADPH. The cycle involves the conversion of toxic ammonia into urea using enzymes like Carbamoyl Phosphate Synthetase I (CPS-I), which requires 2 ATP molecules. There is no reductive step in this pathway that necessitates NADP/NADPH. 2. **Why the other options are incorrect:** * **Steroid & Fatty Acid Synthesis:** Both are major reductive biosynthetic pathways. Fatty acid synthesis (via Fatty Acid Synthase complex) and steroidogenesis (in adrenal cortex/gonads) require large amounts of NADPH to reduce double bonds and incorporate carbon units. * **Reduced Glutathione Synthesis:** NADPH is the essential cofactor for the enzyme **Glutathione Reductase**. This enzyme converts oxidized glutathione (GSSG) back to its reduced form (GSH), which is critical for protecting red blood cells against reactive oxygen species (ROS). **High-Yield Clinical Pearls for NEET-PG:** * **Sources of NADPH:** The **Hexose Monophosphate (HMP) Shunt** (via G6PD enzyme) is the most significant source. Another key source is the **Malic Enzyme**. * **G6PD Deficiency:** Lack of NADPH leads to an inability to maintain reduced glutathione, resulting in hemolysis under oxidative stress (e.g., Fava beans, Primaquine). * **NAD vs. NADP:** Remember: **NAD+** is generally used for **catabolic** pathways (oxidation/energy production), while **NADP+** is used for **anabolic** pathways (synthesis/reduction).

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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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