Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 291: Energy source used by brain in later days of starvation is

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

Explanation: ***Ketone bodies*** - During **prolonged starvation**, the liver produces **ketone bodies** (acetoacetate and β-hydroxybutyrate) from fatty acid breakdown. - The brain adapts to utilize these ketone bodies as a primary energy source, reducing its reliance on **glucose**. *Glucose* - While **glucose** is the primary energy source for the brain under normal conditions, its availability diminishes significantly during prolonged starvation. - The brain attempts to conserve glucose for essential functions by switching to alternative fuels. *Glycogen* - The brain stores very limited amounts of **glycogen**, which are rapidly depleted within minutes of glucose deprivation. - It is not a sustainable or significant energy source during extended periods of starvation. *Fatty acids* - **Fatty acids** cannot directly cross the **blood-brain barrier** to a significant extent, thus they are not a direct fuel source for brain cells. - They are, however, used by the liver to synthesize ketone bodies, which then serve as brain fuel.

Question 292: Which metabolic pathway is least active during 12 days of fasting?

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

Explanation: ***Correct: Glycogenolysis*** - **Glycogenolysis**, the breakdown of glycogen stores, is very active during the **initial hours of fasting** (first 24-48 hours) to maintain blood glucose levels. - However, after **12 days of fasting**, liver and muscle **glycogen stores are completely depleted**, making this pathway **essentially inactive** or the least active of all the metabolic pathways. - Once glycogen is exhausted, this pathway cannot contribute further to energy metabolism. *Incorrect: Gluconeogenesis* - This pathway becomes **increasingly active** during prolonged fasting to **synthesize new glucose** from non-carbohydrate precursors (amino acids, lactate, glycerol). - Essential for maintaining blood glucose for **glucose-dependent tissues** like red blood cells and parts of the brain that haven't fully adapted to ketones. - Remains a **crucial and active pathway** throughout prolonged fasting. *Incorrect: Ketogenesis* - **Ketogenesis** is **highly active** during prolonged fasting, producing **ketone bodies** (acetoacetate, β-hydroxybutyrate) from fatty acids in the liver. - Provides the **primary alternative fuel** for the brain (up to 70% of brain energy needs) and other tissues. - This is a **key metabolic adaptation** to preserve protein and glucose during starvation. *Incorrect: Lipolysis* - **Lipolysis** (breakdown of triglycerides into fatty acids and glycerol) is **highly active** during fasting to mobilize stored energy. - Provides **fatty acids** for direct oxidation by most tissues and **glycerol** as a gluconeogenic substrate. - A **fundamental process** for energy supply during nutrient deprivation.

Question 293: Which of the following enzyme activity decreases in fasting?

A. Hormone sensitive lipase
B. Glycogen phosphorylase
C. Acetyl CoA Carboxylase
D. Phosphofructokinase I (Correct Answer)

Explanation: ***Phosphofructokinase I*** - **Phosphofructokinase I (PFK-1)** activity **decreases** during fasting due to **decreased insulin-to-glucagon ratio**, which reduces **fructose-2,6-bisphosphate (F-2,6-BP)** levels, a powerful allosteric activator of PFK-1. - This reduction in activity slows down **glycolysis**, conserving glucose for critical tissues like the brain and redirecting metabolism toward **gluconeogenesis**. - **PFK-1 is the rate-limiting enzyme of glycolysis**, making its regulation particularly significant in the fasted state. *Hormone sensitive lipase* - **Hormone sensitive lipase (HSL)** activity **increases** during fasting due to elevated **glucagon** and **epinephrine** levels, which stimulate its phosphorylation via **protein kinase A (PKA)**. - This increased activity promotes the breakdown of stored **triglycerides** in adipose tissue, releasing **fatty acids** for β-oxidation and energy production. *Glycogen phosphorylase* - **Glycogen phosphorylase** activity **increases** during fasting, primarily stimulated by **glucagon** and **epinephrine**, leading to the breakdown of **glycogen** stores. - This enzyme is crucial for **glycogenolysis**, providing glucose to maintain blood sugar levels when dietary intake is absent. *Acetyl CoA Carboxylase* - **Acetyl CoA Carboxylase (ACC)** activity also **decreases** during fasting, as it is inhibited by **phosphorylation** mediated by **AMP-activated protein kinase (AMPK)** and **protein kinase A (PKA)**. - This reduction in activity inhibits **fatty acid synthesis**, shifting metabolism towards fatty acid **oxidation** for energy production. - **Note:** While ACC activity does decrease during fasting, **PFK-1** is considered the primary answer as it represents the key regulatory point for **glucose metabolism** (glycolysis vs. gluconeogenesis), which is the central metabolic shift during fasting.

Question 294: Which of the following is not a free radical?

A. Superoxide anion
B. Hydrogen peroxide (H2O2) (Correct Answer)
C. Nitric oxide (NO·)
D. Hydroxyl radical (.OH)

Explanation: ***Hydrogen peroxide (H₂O₂)*** - **Hydrogen peroxide** is a **reactive oxygen species (ROS)** but is not a free radical because it has **no unpaired electrons** in its outermost shell. - While it can be converted into the highly reactive hydroxyl radical via the **Fenton reaction**, it is stable enough to be transported across membranes. *Superoxide anion (O₂⁻)* - The **superoxide anion (O₂⁻)** is a free radical because it has an **unpaired electron** in its outer shell. - It is one of the primary **reactive oxygen species** formed during cellular metabolism and can damage cellular components. *Nitric oxide (NO·)* - **Nitric oxide** is an important **free radical** with a single **unpaired electron** in its molecular structure. - It functions as a vital signaling molecule in vascular biology, regulating blood pressure and neurotransmission, despite being a free radical. *Hydroxyl radical (·OH)* - The **hydroxyl radical (·OH)** is one of the most reactive and damaging **free radicals** in biological systems. - It has a single **unpaired electron**, making it highly unstable and able to react indiscriminately with virtually all types of biomolecules.

Question 295: Most abundant source of fuel in starvation -

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

Explanation: ***Adipose tissue*** - **Adipose tissue** stores **triglycerides**, which are hydrolyzed into fatty acids and glycerol to serve as the body's primary energy source during prolonged starvation. - The energy reserve in adipose tissue is significantly larger than glycogen stores, providing **sustained fuel** for days or weeks. *Liver glycogen* - **Liver glycogen** is a readily available source of glucose but is rapidly depleted within **12-24 hours** during starvation. - Its primary role is to maintain **blood glucose levels** for glucose-dependent tissues like the brain. *Muscle glycogen* - **Muscle glycogen** is used primarily for **muscle contraction** and cannot be directly released into the bloodstream to maintain blood glucose levels. - While it's a significant energy reserve for working muscles, it does not contribute to systemic fuel needs during starvation. *Blood glucose* - **Blood glucose** is the immediate circulating fuel, but it is tightly regulated and its levels decrease during starvation as glycogen stores are depleted. - It is not an abundant stored source of fuel but rather a transport form of energy.

Question 296: Which is the primary energy molecule that gives approximately 7.3 kcal/mol?

A. ATP (Correct Answer)
B. GTP
C. Glucose-6-phosphate
D. Creatine phosphate

Explanation: ***ATP*** - **Adenosine triphosphate (ATP)** is the primary energy currency of the cell, providing approximately **7.3 kcal/mol** upon hydrolysis of its terminal phosphate group. - This energy is released when ATP is converted to **ADP (adenosine diphosphate)** and an inorganic phosphate (Pi), driving various cellular processes. *GTP* - **Guanosine triphosphate (GTP)** is another nucleotide triphosphate that carries energy, but it is primarily involved in specific processes like **protein synthesis** and **signal transduction**, not as the ubiquitous primary energy molecule like ATP. - While it also releases energy upon hydrolysis, its standard free energy change is similar to ATP but it's not the main universal energy carrier. *Glucose-6-phosphate* - **Glucose-6-phosphate** is an important intermediate in **glycolysis** and **gluconeogenesis**, but it is not an energy-storing molecule in the same way as ATP. - Its high-energy phosphate bond is used in metabolic pathways, but it doesn't directly release 7.3 kcal/mol as a direct energy source for cellular work. *Creatine phosphate* - **Creatine phosphate** serves as an energy reserve in muscle and nerve cells, rapidly generating ATP from ADP during periods of intense activity. - While it is a high-energy phosphate compound, it functions to **replenish ATP** rather than being the direct energy molecule that performs cellular work.

Question 297: What is a physiological uncoupler?

A. Thyroxine
B. Free fatty acids
C. Thermogenin (Correct Answer)
D. All of the options

Explanation: ***Correct: Thermogenin*** - **Thermogenin (uncoupling protein 1, UCP1)** is the primary physiological uncoupler found in brown adipose tissue - It directly facilitates the **leak of protons** back into the mitochondrial matrix, bypassing ATP synthase - This dissipates the **proton-motive force as heat** rather than producing ATP, making it the classic example of non-shivering thermogenesis - Essential for **temperature regulation** in neonates and cold adaptation in adults *Incorrect: Free fatty acids* - While free fatty acids can activate UCP1 and act as weak protonophores in some contexts, they are primarily **substrates for β-oxidation** and **activators** of thermogenin - They are not considered the primary physiological uncoupler, though they support uncoupling activity *Incorrect: Thyroxine* - **Thyroid hormone** increases metabolic rate and can upregulate the **expression of uncoupling proteins** - However, it does **not directly uncouple** oxidative phosphorylation - It acts as a metabolic regulator rather than a true uncoupler *Incorrect: All of the options* - Only thermogenin is the true physiological uncoupler by definition - The other substances play supportive or regulatory roles but are not direct uncouplers

Question 298: Which organelle produces and destroys H2O2?

A. Peroxisome (Correct Answer)
B. Lysosome
C. Golgi body
D. Ribosome

Explanation: ***Peroxisome*** - **Peroxisomes** are organelles that both produce and break down **hydrogen peroxide (H2O2)** during metabolic processes. - They contain **oxidases** (such as D-amino acid oxidase and urate oxidase) that produce H2O2 as a byproduct during oxidation reactions. - They also contain the enzyme **catalase** that converts H2O2 into water and oxygen, protecting the cell from oxidative damage. - This dual function makes peroxisomes unique in H2O2 metabolism. *Lysosome* - **Lysosomes** are responsible for breaking down waste materials and cellular debris through **hydrolytic enzymes**. - They are primarily involved in **cellular digestion** and waste removal, not H2O2 metabolism. *Golgi body* - The **Golgi apparatus** modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles. - It is crucial for **protein trafficking** and glycosylation, but does not produce or destroy H2O2. *Ribosome* - **Ribosomes** are responsible for **protein synthesis** (translation) based on genetic information carried by mRNA. - They are involved in the assembly of amino acids into proteins, not the metabolism of hydrogen peroxide.

Question 299: What is the immediate source of energy for cellular processes?

A. Cori's cycle
B. HMP
C. ATP (Correct Answer)
D. TCA cycle

Explanation: ***ATP*** - **Adenosine triphosphate (ATP)** is the direct and immediate source of energy for almost all cellular processes, including **muscle contraction**, **active transport**, and **biosynthesis**. - Its high-energy phosphate bonds release energy upon hydrolysis, driving various cellular functions. *Cori's cycle* - The **Cori cycle** involves the interconversion of **lactate** and **glucose** between the muscle and the liver to regenerate glucose stores. - It is an important metabolic pathway for glucose homeostasis during anaerobic conditions, but it does not directly provide immediate energy for cellular processes. *HMP* - The **Hexose Monophosphate Pathway (HMP)**, also known as the **pentose phosphate pathway**, primarily produces **NADPH** and **ribose-5-phosphate**. - While it generates NADPH for reductive biosynthesis and protects against oxidative stress, it is not an immediate source of energy. *TCA cycle* - The **Tricarboxylic Acid (TCA) cycle**, or Krebs cycle, is a central metabolic pathway that oxidizes **acetyl-CoA** to produce **ATP**, **NADH**, and **FADH2**. - While it is a major producer of ATP, it is not the *immediate* source; instead, it generates the precursors that fuel oxidative phosphorylation to produce ATP.

Question 300: Chemical process involved in conversion of progesterone to glucocorticoids is

A. Methylation
B. Hydroxylation (Correct Answer)
C. Carboxylation
D. None of the options

Explanation: ***Hydroxylation*** - The conversion of progesterone to glucocorticoids involves several enzymatic steps, with **hydroxylation reactions** being critical for adding hydroxyl groups at specific carbon positions (e.g., C-17, C-21, C-11). - These hydroxylation steps are catalyzed by various **cytochrome P450 enzymes** (e.g., 17α-hydroxylase, 21-hydroxylase, 11β-hydroxylase) within the adrenal cortex, leading to the formation of active glucocorticoids like **cortisol**. *Methylation* - **Methylation** involves the addition of a methyl group (-CH₃) to a molecule, a process more commonly associated with modifying DNA, proteins, or certain neurotransmitters. - While methylation is a vital biological process, it is not the primary chemical reaction involved in the **steroidogenesis pathway** converting progesterone to glucocorticoids. *Carboxylation* - **Carboxylation** is the addition of a carboxyl group (-COOH) to a molecule, a reaction crucial in processes like photosynthesis (carbon fixation) or the synthesis of certain proteins (e.g., clotting factors). - This chemical modification is not directly involved in the series of transformations that convert progesterone into **glucocorticoids**. *None of the options* - This option is incorrect because **hydroxylation** is indeed a fundamental chemical process in the conversion of progesterone to glucocorticoids.

Practice by Chapter

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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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