Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 321: Why is the citric acid cycle called an amphibolic pathway?

A. Both exergonic and endergonic reactions take place
B. Metabolites are utilized in other pathways. (Correct Answer)
C. It can proceed in both forward and backward directions.
D. The same enzymes can be used in reverse directions.

Explanation: ***Metabolites are utilized in other pathways.*** - The citric acid cycle is termed **amphibolic** because it serves both catabolic (breakdown) and anabolic (synthetic) functions. - Its intermediates are constantly drawn off for biosynthesis of molecules like **amino acids**, **heme**, and **glucose**, meaning it's not solely degradative. *Both exergonic and endergonic reactions take place* - While both types of reactions do occur in many metabolic pathways, this is a general characteristic of metabolism and not specific to the definition of an **amphibolic pathway**. - The amphibolic nature specifically refers to the dual role in both **catabolism** and **anabolism**. *It can proceed in both forward and backward directions.* - This statement typically describes a **reversible pathway** or individual reversible reactions, not necessarily an amphibolic pathway. - The citric acid cycle is primarily an oxidative cycle that proceeds in a forward, cyclic direction under aerobic conditions. *The same enzymes can be used in reverse directions.* - While some individual enzymes within metabolic pathways can catalyze reversible reactions, this is not the defining characteristic of an **amphibolic pathway**. - The amphibolic designation refers to the overall pathway's contribution to both breakdown and synthesis of molecules.

Question 322: Which of the following pairs of compounds has the highest standard reduction potential?

A. NADH/NAD+
B. Succinate/Fumarate
C. Ubiquinone/Ubiquinol
D. Fe³⁺/Fe²⁺ (Correct Answer)

Explanation: ***Fe³⁺/Fe²⁺*** - The **Fe³⁺/Fe²⁺ couple** has a **standard reduction potential (E'0)** of **+0.77 V**, making it the highest among the given options. - A higher positive E'0 indicates a stronger tendency for the oxidized form to accept electrons and be reduced. *NADH/NAD+* - The **NADH/NAD+ couple** has a **standard reduction potential** of **-0.32 V**, indicating it is a strong reducing agent. - Its negative reduction potential means it readily donates electrons during metabolic processes. *Succinate/Fumarate* - The **succinate/fumarate couple** has a **standard reduction potential** of **+0.03 V**. - This pair is involved in the **TCA cycle**, where succinate is oxidized to fumarate, releasing electrons. *Ubiquinone/Ubiquinol* - The **ubiquinone/ubiquinol couple** has a **standard reduction potential** varying around **+0.05 to +0.10 V**, depending on the specific state. - It acts as a mobile electron carrier in the **electron transport chain**, accepting electrons from NADH and FADH2.

Question 323: What metabolic changes occur during overnight fasting?

A. Blood glucose decreases slightly
B. Fat breakdown increases
C. Glucose production increases (Correct Answer)
D. Ketone levels rise slightly

Explanation: ***Glucose production increases*** - During overnight fasting (typically 8-12 hours), the body's **primary metabolic priority** is to maintain **blood glucose homeostasis** to fuel the brain and other glucose-dependent tissues. - As **hepatic glycogen stores** become depleted, the liver significantly increases **gluconeogenesis** (glucose production from non-carbohydrate sources like amino acids, lactate, and glycerol) to supply glucose. - This represents the **most critical metabolic adaptation** during overnight fasting, as the brain requires a constant glucose supply (~120g/day) and cannot initially use alternative fuels. *Blood glucose decreases slightly* - During a normal overnight fast, blood glucose levels remain **relatively stable** (70-100 mg/dL) due to compensatory mechanisms. - The body's homeostatic mechanisms (increased glucose production, decreased glucose utilization by muscles) prevent any significant drop in blood glucose. - A significant decrease would indicate **hypoglycemia**, which is prevented by the metabolic changes described above. *Fat breakdown increases* - **Lipolysis** (fat breakdown) does indeed increase significantly during overnight fasting to provide **fatty acids** as an alternative fuel source for skeletal muscle, cardiac muscle, and liver. - This is an important metabolic change, but is **secondary to glucose production** in terms of priority, as it serves to spare glucose for the brain rather than directly maintaining glucose levels. - Increased fatty acid oxidation provides acetyl-CoA for **ketone body synthesis** and reduces glucose consumption by peripheral tissues (glucose-sparing effect). *Ketone levels rise slightly* - **Ketone body production** (acetoacetate, β-hydroxybutyrate) does begin to increase as fasting progresses beyond 8-12 hours. - However, during an *overnight* fast, ketone levels rise only **modestly** (typically <1 mM); clinically significant ketosis develops during **prolonged fasting** (24-72 hours), when ketone bodies become a major fuel source for the brain. - The overnight period represents the **transition phase** where glucose production remains the dominant metabolic response.

Question 324: In the electron transport chain (ETC), which enzyme does cyanide inhibit?

A. Complex II (Succinate dehydrogenase)
B. Cytochrome c oxidase (Complex IV) (Correct Answer)
C. Complex I (NADH dehydrogenase)
D. Complex III (Cytochrome bc1 complex)

Explanation: ***Cytochrome c oxidase (Complex IV)*** - Cyanide binds to the **ferric iron (Fe3+)** in the heme a3 component of cytochrome c oxidase, blocking the final transfer of electrons to oxygen. - This inhibition effectively halts the entire **electron transport chain** and **oxidative phosphorylation**, leading to rapid cellular energy depletion. *Complex I (NADH dehydrogenase)* - While other toxins can inhibit Complex I (e.g., rotenone, amytal), **cyanide specifically targets Complex IV**. - Inhibition here prevents the entry of electrons from **NADH** into the ETC, but it's not cyanide's primary site of action. *Complex III (Cytochrome bc1 complex)* - Complex III is involved in transferring electrons from **ubiquinol** to cytochrome c, but it is not directly inhibited by cyanide. - Antimycin A is a well-known inhibitor of Complex III. *Complex II (Succinate dehydrogenase)* - Complex II directly receives electrons from **succinate** in the citric acid cycle and passes them to ubiquinone, bypassing Complex I. - Cyanide does not inhibit Complex II; inhibitors of this complex include malonate.

Question 325: Major source of energy for brain in fasting/starvation?

A. Glucose
B. Glycogen
C. Fatty acids
D. Ketone bodies (Correct Answer)

Explanation: ***Ketone bodies*** - During **prolonged fasting or starvation**, the body depletes its **glycogen stores** and begins to break down fatty acids. The liver converts these fatty acids into **ketone bodies**, such as **acetoacetate and beta-hydroxybutyrate**. - These **ketone bodies** can cross the **blood-brain barrier** and be used by the brain as an alternative energy source when glucose becomes scarce, preventing protein breakdown for gluconeogenesis. *Glucose* - While **glucose** is the primary and preferred energy source for the brain under normal physiological conditions, its availability significantly decreases during **prolonged fasting or starvation**. - The brain requires a continuous supply of glucose, but in states of severe caloric restriction, the body must conserve glucose for other critical functions and adapt by using alternative fuels. *Glycogen* - **Glycogen** is a stored form of glucose found predominantly in the **liver and muscles**. - The brain itself has minimal **glycogen stores**, which are rapidly depleted during fasting, and thus cannot be a major long-term energy source. *Fatty acids* - **Fatty acids** are a major energy source for many tissues in the body, especially during fasting, but they **cannot directly cross the blood-brain barrier** in significant amounts to fuel the brain. - Instead, **fatty acids** are metabolized into **ketone bodies** in the liver, which then serve as the brain's alternative fuel.

Question 326: Chemiosmotic coupling of oxidative phosphorylation is related to which of the following?

A. ATP generation by pumping of neutrons
B. Formation of ATP at substrate level
C. ATP generation by pumping of protons (Correct Answer)
D. ATP formation by transport of electrons

Explanation: ***ATP generation by pumping of protons*** - **Chemiosmotic coupling** links the electron transport chain's activity to ATP synthesis through the generation of a **proton gradient** across the inner mitochondrial membrane. - The energy released from the flow of electrons through complexes I, III, and IV is used to pump protons from the mitochondrial matrix to the intermembrane space, creating a **proton motive force** that drives ATP synthase. *Formation of ATP at substrate level* - **Substrate-level phosphorylation** involves the direct transfer of a phosphate group from a high-energy substrate to ADP to form ATP, independently of a proton gradient. - This process occurs in reactions like those in **glycolysis** and the **Krebs cycle**, not in oxidative phosphorylation via chemiosmosis. *ATP generation by pumping of neutrons* - **Neutrons** are subatomic particles with no electric charge and are not involved in biological processes like ATP generation or membrane transport. - Pumping of neutrons has no physiological relevance in cellular energy metabolism. *ATP formation by transport of electrons* - While **electron transport** is an integral part of oxidative phosphorylation, it does not directly form ATP. - The energy released during electron transport is used to create the **proton gradient** (chemiosmotic coupling), which then drives ATP synthesis, rather than ATP being formed directly by electron movement.

Question 327: Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?

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

Explanation: ***Succinate thiokinase*** - This enzyme (also known as **succinyl-CoA synthetase**) catalyzes the conversion of **succinyl-CoA** to **succinate**. - During this reaction, the energy released from breaking the **thioester bond** in succinyl-CoA is directly used to synthesize **GTP** (or ATP in some organisms) from GDP (or ADP) and inorganic phosphate, which is a classic example of **substrate-level phosphorylation**. *Isocitrate dehydrogenase* - This enzyme catalyzes the **oxidative decarboxylation** of isocitrate to $\alpha$-ketoglutarate. - This step produces **NADH** and **CO2** but does not involve substrate-level phosphorylation. *Malate dehydrogenase* - This enzyme catalyzes the oxidation of **L-malate** to **oxaloacetate** in the final step of the TCA cycle. - It produces **NADH** but does not involve the direct synthesis of ATP or GTP. *Aconitase* - This enzyme catalyzes the **isomerization** of **citrate** to **isocitrate** via an aconitate intermediate. - No energy is generated or consumed in the form of ATP/GTP during this rearrangement.

Question 328: In ETC NADH generates -

A. 1 ATPs
B. 4 ATPs
C. 3 ATPs (Correct Answer)
D. 2 ATPs

Explanation: ***3 ATPs*** - Each molecule of **NADH** donates electrons to **Complex I** of the electron transport chain (ETC), resulting in the pumping of enough protons to generate approximately **3 ATP molecules** via **oxidative phosphorylation**. - This high yield is due to NADH's ability to activate multiple proton pumps along the ETC, maximizing the **proton gradient** for ATP synthesis. *1 ATPs* - This is an incorrect yield for NADH; **FADH2** typically generates fewer ATPs (around 2) because it enters the ETC at a later stage, bypassing the initial proton pump. - Generating only 1 ATP from NADH would be very inefficient and is not physiologically accurate for oxidative phosphorylation. *2 ATPs* - While closer, 2 ATPs is the approximate yield for **FADH2**, which enters the ETC at **Complex II**, bypassing Complex I and thus pumping fewer protons. - NADH enters at Complex I, which provides enough energy for a higher ATP yield. *4 ATPs* - 4 ATPs is an overestimation of the ATP yield from NADH in the electron transport chain. - The maximum theoretical yield from NADH via oxidative phosphorylation is typically considered to be 3 ATPs.

Question 329: Organ that can utilize glucose, fatty acids and ketone bodies is:

A. Liver
B. Brain
C. Skeletal muscle (Correct Answer)
D. RBC

Explanation: ***Skeletal muscle*** - Skeletal muscle is highly adaptable and can utilize **glucose**, **fatty acids (FAs)**, and **ketone bodies** as fuel sources, especially during prolonged exercise or starvation. - Its metabolic flexibility allows it to switch between these substrates depending on their availability and the body's energy demands. *Liver* - The liver is central to metabolism but primarily **produces ketone bodies** from fatty acids rather than utilizing them as a major fuel source for its own energy needs. - While it uses glucose and FAs, its role in ketone body metabolism is largely synthetic. *Brain* - The brain preferentially uses **glucose** as its primary fuel. - During prolonged starvation, it can adapt to utilize **ketone bodies** as an alternative fuel source, but it does not significantly use fatty acids directly. *RBC* - Red blood cells (RBCs) lack mitochondria and therefore rely exclusively on **anaerobic glycolysis** for energy, metabolizing only **glucose**. - They cannot utilize fatty acids or ketone bodies.

Question 330: Which of the following vitamins forms a coenzyme that acts as the primary electron acceptor in cellular oxidation-reduction reactions?

A. Vitamin B3 (Niacin) (Correct Answer)
B. Vitamin B1 (Thiamine)
C. Vitamin B6 (Pyridoxine)
D. Vitamin B2 (Riboflavin)

Explanation: ***Vitamin B3 (Niacin)*** - **Niacin (Vitamin B3)** is a precursor to **NAD+** (nicotinamide adenine dinucleotide) and **NADP+**, which function as primary electron acceptors in cellular metabolism. - **NAD+** accepts electrons from various metabolic intermediates during glycolysis, beta-oxidation, and the TCA cycle, becoming **NADH**. - **NADH** then transfers these electrons to Complex I of the electron transport chain to generate ATP. - NAD+/NADH is the most abundant and widely used electron carrier in cellular metabolism. *Vitamin B2 (Riboflavin)* - **Riboflavin (Vitamin B2)** is a precursor to **FAD** (flavin adenine dinucleotide) and **FMN** (flavin mononucleotide), which are also electron carriers. - While FAD and FMN are important electron acceptors (e.g., in succinate dehydrogenase and fatty acid oxidation), **NAD+** is quantitatively more significant and accepts electrons from a greater number of reactions. *Vitamin B1 (Thiamine)* - **Thiamine** acts as a coenzyme, **thiamine pyrophosphate (TPP)**, primarily involved in carbohydrate metabolism (e.g., pyruvate dehydrogenase complex, alpha-ketoglutarate dehydrogenase). - It facilitates decarboxylation reactions but does not function as an electron acceptor. *Vitamin B6 (Pyridoxine)* - **Pyridoxine (Vitamin B6)** is converted to **pyridoxal phosphate (PLP)**, a coenzyme primarily involved in amino acid metabolism, including transamination, decarboxylation, and racemization. - It has no role as an electron acceptor in oxidation-reduction reactions.

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