Energy Production and Metabolism — MCQs
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Which of the following is not used in the citric acid cycle?
Cancer cells preferentially utilize glycolysis for energy production even in the presence of adequate oxygen. What is this phenomenon called?
In starvation, earliest to become depleted -
The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
What is the primary role of molecular oxygen in the electron transport chain (ETC)?
Which of the following is not a cofactor required by pyruvate dehydrogenase?
NADH CoQ reductase is inhibited by ?
What is the theoretical yield of ATP generated in one TCA cycle?
Which of the following statements best describes the role of inorganic phosphate in the Electron Transport Chain (ETC)?
Among the following enzymes, which one produces NADH in the citric acid cycle?
Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs
Question 271: Which of the following is not used in the citric acid cycle?
- A. NAD
- B. FAD
- C. NADP (Correct Answer)
- D. GDP
Explanation: ***NADP*** - **NADP (nicotinamide adenine dinucleotide phosphate)** is primarily involved in anabolic reactions, such as fatty acid synthesis and the **pentose phosphate pathway**. - It is not a direct coenzyme or substrate involved in the oxidative reactions of the **citric acid cycle**. *NAD* - **NAD (nicotinamide adenine dinucleotide)** is a crucial coenzyme in the citric acid cycle, acting as an electron acceptor in several steps. - It is reduced to **NADH**, which then donates electrons to the electron transport chain for ATP production. *FAD* - **FAD (flavin adenine dinucleotide)** is another essential coenzyme in the citric acid cycle, reduced to **FADH2**. - **FADH2** transports electrons to the electron transport chain, contributing to ATP generation. *GDP* - **GDP (guanosine diphosphate)** is converted to **GTP (guanosine triphosphate)** in a substrate-level phosphorylation step during the conversion of succinyl-CoA to succinate in the citric acid cycle. - While not directly involved as an electron carrier, its phosphorylation is a key energy-generating step within the cycle.
Question 272: Cancer cells preferentially utilize glycolysis for energy production even in the presence of adequate oxygen. What is this phenomenon called?
- A. Warburg effect (Correct Answer)
- B. Hypoxic adaptation
- C. Anoxic survival
- D. Oxygen-independent metabolism
Explanation: ***Warburg*** - The **Warburg effect** describes how cancer cells preferentially use glycolysis for energy production even in the presence of oxygen, allowing them to thrive in **hypoxic conditions** [1]. - This metabolic adaptation supports **cell proliferation** and survival in tumor microenvironments where oxygen is limited [1][3]. - Cancer cells upregulate glucose uptake and express specific metabolic enzymes like the M2 isoform of pyruvate kinase that facilitate this altered metabolism [2][4]. *Wanton* - This term typically refers to recklessness or extravagance and is not used in the context of cancer metabolism or hypoxia. - There are no associations with **cancer cell adaptation** under adverse environmental conditions. *Wormian* - **Wormian bones** are extra bone pieces within sutures of the skull, unrelated to cancer cell metabolism or survival mechanisms. - This term does not connect to **hypoxia** or metabolic adaptations in cancer biology. *Wolf* - "Wolf" has no recognized connection to cancer cell biology, particularly regarding metabolic adaptations under **hypoxic stress**. - It does not imply any concept associated with how cancer cells cope with adverse conditions. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 307-308. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 308-310. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 290-291. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 26-27.
Question 273: In starvation, earliest to become depleted -
- A. Carbohydrates (Correct Answer)
- B. Proteins
- C. Fats
- D. None of the options
Explanation: ***Carbohydrates*** - **Glycogen stores** (primarily liver and muscle glycogen) are the body's most readily accessible energy source and are depleted within hours of starvation. - The liver initially maintains blood glucose levels by breaking down **glycogen** before resorting to gluconeogenesis. *Proteins* - **Proteins** are conserved as much as possible during early starvation to preserve vital body functions. - Significant **protein breakdown** for energy (gluconeogenesis) typically occurs in later stages of prolonged starvation, after carbohydrate and most fat reserves are diminished. *Fats* - **Fats** (in the form of triglycerides stored in adipose tissue) become the primary energy source after glycogen stores are depleted. - While they provide a large energy reserve, their mobilization and utilization as fuel take longer than glycogen, and they are not the **earliest to be depleted**. *None of the options* - This option is incorrect because **carbohydrates** are indeed the earliest to be depleted during starvation.
Question 274: The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?
- A. Cyanide (Correct Answer)
- B. Carbon dioxide
- C. Oligomycin
- D. Ouabain
Explanation: ***Cyanide*** - **Cyanide** is a potent inhibitor of **cytochrome c oxidase (Complex IV)** in the electron transport chain, binding to the ferric iron (Fe3+) in the heme group of the enzyme. - This binding prevents the transfer of electrons to **oxygen**, thereby halting cellular respiration and ATP production. *Carbon dioxide* - **Carbon dioxide** is a metabolic waste product and a component of the **bicarbonate buffer system**, but it does not directly inhibit cytochrome complex IV. - While high levels can affect physiological pH and enzyme function, its primary role is not as an electron transport chain inhibitor. *Oligomycin* - **Oligomycin** inhibits **ATP synthase (Complex V)** by binding to its Fo subunit, which blocks the flow of protons through the ATP synthase channel. - This prevents the synthesis of ATP but does not directly affect the electron transfer steps of cytochrome complex IV. *Ouabain* - **Ouabain** is a cardiac glycoside that inhibits the **Na+/K+-ATPase pump** in the cell membrane. - It does not have any direct inhibitory effect on the components of the electron transport chain, including cytochrome complex IV.
Question 275: What is the primary role of molecular oxygen in the electron transport chain (ETC)?
- A. Transfer of electrons to CoQ
- B. Transfer of electrons from cytosol to mitochondria
- C. Acting as the final electron acceptor (Correct Answer)
- D. Facilitating ATP synthesis
Explanation: ***Acting as the final electron acceptor*** - **Molecular oxygen** is the terminal electron acceptor in the **electron transport chain**, combining with electrons and protons (H+) to form **water**. - Without oxygen, electron flow would cease, leading to a build-up of reduced electron carriers and halting ATP production via **oxidative phosphorylation**. *Transfer of electrons to CoQ* - **Coenzyme Q (CoQ)** accepts electrons from Complexes I and II but is an intermediate carrier, not the final destination. - The primary role of molecular oxygen occurs much later in the chain. *Transfer of electrons from cytosol to mitochondria* - This process involves specific shuttle systems (e.g., malate-aspartate, glycerol phosphate shuttle) but is distinct from oxygen's role within the ETC. - Oxygen's function is internal to the electron transport process in the mitochondrial matrix. *Facilitating ATP synthesis* - While oxygen's role as the final electron acceptor indirectly enables **ATP synthesis** by maintaining electron flow and the proton gradient, it does not directly synthesize ATP. - **ATP synthase** uses the proton gradient to produce ATP, a separate but dependent step.
Question 276: Which of the following is not a cofactor required by pyruvate dehydrogenase?
- A. CoA
- B. NAD
- C. FAD
- D. Biotin (Correct Answer)
Explanation: ***NAD*** - **NAD+ (Nicotinamide adenine dinucleotide)** is required by the dihydrolipoyl dehydrogenase (E3) component of the pyruvate dehydrogenase complex to accept electrons and form NADH. - This cofactor is crucial for the regeneration of the oxidized lipoamide, allowing the complex to continue its catalytic cycle. *FAD* - **FAD (Flavin adenine dinucleotide)** is also a cofactor for the dihydrolipoyl dehydrogenase (E3) enzyme, accepting electrons from reduced lipoamide before transferring them to NAD+. - It is tightly bound to the E3 enzyme and undergoes reversible oxidation-reduction during the reaction. *Biotin* - **Biotin** is primarily a cofactor for **carboxylase enzymes**, such as pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate. - It is **not involved** in the pyruvate dehydrogenase complex reaction, which is an oxidative decarboxylation, not a carboxylation. *CoA* - **Coenzyme A (CoA)** is essential for the pyruvate dehydrogenase complex, as it accepts the acetyl group from pyruvate to form **acetyl-CoA**. - Acetyl-CoA is the product of the reaction and serves as the entry molecule into the **Krebs cycle**.
Question 277: NADH CoQ reductase is inhibited by ?
- A. Antimycin (inhibits cytochrome bc1 complex)
- B. Atractyloside (inhibits ATP/ADP translocase)
- C. Rotenone (Correct Answer)
- D. Carbon monoxide (inhibits cytochrome c oxidase)
Explanation: ***Rotenone*** - **Rotenone** is a potent inhibitor of **NADH CoQ reductase**, also known as **Complex I** of the electron transport chain. - It blocks the transfer of electrons from **NADH** to **ubiquinone (CoQ)**, thereby halting oxidative phosphorylation. *Antimycin (inhibits cytochrome bc1 complex)* - **Antimycin A** specifically inhibits **Complex III (cytochrome bc1 complex)**, not **NADH CoQ reductase**. - Its action blocks electron transfer from **ubiquinol** to **cytochrome c**. *Atractyloside (inhibits ATP/ADP translocase)* - **Atractyloside** inhibits the **adenine nucleotide translocase (ATP/ADP translocase)**, which is responsible for exchanging ATP for ADP across the inner mitochondrial membrane. - It does not directly affect the electron transport chain components like **NADH CoQ reductase**. *Carbon monoxide (inhibits cytochrome c oxidase)* - **Carbon monoxide (CO)** is a classic inhibitor of **Complex IV (cytochrome c oxidase)**. - It binds to the **heme iron** of **cytochrome a3** with high affinity, preventing oxygen from acting as the final electron acceptor.
Question 278: What is the theoretical yield of ATP generated in one TCA cycle?
- A. 2
- B. 8
- C. 10 (Correct Answer)
- D. 11
Explanation: ***Correct: 10*** - One turn of the **TCA cycle** produces 3 NADH, 1 FADH₂, and 1 GTP (which is equivalent to ATP) - Using modern **P/O ratios**: 3 NADH yield 7.5 ATP (3 × 2.5 ATP/NADH) and 1 FADH₂ yields 1.5 ATP (1 × 1.5 ATP/FADH₂) - Adding the 1 GTP/ATP from substrate-level phosphorylation gives a **total of 10 ATP** *Incorrect: 2* - This only accounts for **substrate-level phosphorylation** (1 GTP converted to ATP) and ignores the substantial ATP generated from NADH and FADH₂ through **oxidative phosphorylation** - The total theoretical yield including electron transport chain is much higher *Incorrect: 8* - This is based on **outdated calculations** using older P/O ratios (3 ATP/NADH and 2 ATP/FADH₂ = 3×3 + 1×2 - 1 = 10, or miscalculation) - Modern biochemistry uses 2.5 ATP per NADH and 1.5 ATP per FADH₂, yielding 10 ATP total *Incorrect: 11* - This **overestimates** the ATP yield, possibly by using incorrect P/O ratios or miscounting the number of reduced cofactors produced - The standard calculation with 3 NADH, 1 FADH₂, and 1 GTP yields exactly 10 ATP
Question 279: Which of the following statements best describes the role of inorganic phosphate in the Electron Transport Chain (ETC)?
- A. Generates ATP
- B. Occurs in mitochondria
- C. Inorganic phosphate is essential for ATP synthesis in the ETC. (Correct Answer)
- D. No role of inorganic phosphate
Explanation: ***Inorganic phosphate is essential for ATP synthesis in the ETC*** - **Inorganic phosphate (Pi)** serves as a crucial **substrate** in oxidative phosphorylation, combining with ADP to form ATP. - The reaction catalyzed by **ATP synthase** is: ADP + Pi → ATP, powered by the proton motive force generated by the ETC. - Without Pi, the ETC cannot fulfill its primary function of ATP production through **oxidative phosphorylation**. - This represents the **direct and essential role** of inorganic phosphate in the context of the Electron Transport Chain. *Generates ATP* - While Pi is involved in ATP synthesis, it does not itself "generate" ATP. - Pi is a **substrate** (reactant), not an energy source; the energy comes from the **proton gradient** created by the ETC. - This option incorrectly attributes ATP generation to Pi alone rather than recognizing it as one component of the synthesis process. *No role of inorganic phosphate* - This is factually incorrect as inorganic phosphate plays a **direct and essential role** in ATP synthesis. - Without Pi, ADP cannot be phosphorylated to form ATP during oxidative phosphorylation. - Pi is an indispensable substrate for the ATP synthase enzyme. *Occurs in mitochondria* - This statement describes the **location of the ETC**, not the role of inorganic phosphate. - While the ETC does occur in the inner mitochondrial membrane, this does not answer what role Pi plays in the process. - The question specifically asks about the role of inorganic phosphate, not where the ETC is located.
Question 280: Among the following enzymes, which one produces NADH in the citric acid cycle?
- A. Succinate thiokinase
- B. Succinate dehydrogenase
- C. Isocitrate dehydrogenase (Correct Answer)
- D. Fumarase
Explanation: ***Isocitrate dehydrogenase*** - This enzyme catalyzes the conversion of **isocitrate to $\alpha$-ketoglutarate**, a reaction that releases **carbon dioxide** and reduces NAD+ to **NADH**. - This is one of the three **irreversible** (rate-limiting) reactions of the citric acid cycle. *Succinate thiokinase* - This enzyme, also known as **succinyl-CoA synthetase**, catalyzes the conversion of succinyl-CoA to succinate. - This reaction produces **GTP** (which can be readily interconverted to ATP), not NADH. *Succinate dehydrogenase* - This enzyme catalyzes the conversion of **succinate to fumarate**. - This reaction reduces **FAD to FADH2**, not NAD+ to NADH. *Fumarase* - This enzyme catalyzes the **hydration of fumarate to malate**. - This reaction does not involve the production of either NADH or FADH2; it simply adds a water molecule.
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