Energy Production and Metabolism Practice Questions

Q: Pyruvate dehydrogenase requires all cofactors except:

Pyridoxin (Vitamin B6). ***Pyridoxin (Vitamin B6)*** - **Pyridoxin** (vitamin B6) is a coenzyme for many enzymes involved in **amino acid metabolism**, but it is **not directly required** by the pyruvate dehydrogenase complex. - The pyruvate dehydrogenase complex uses **thiamine pyrophosphate**, **lipoic acid**, **FAD**, **NAD+**, and **Coenzyme A** as cofactors. *Thiamin* - **Thiamin pyrophosphate** (TPP), derived from thiamin (vitamin B1), is a crucial coenzyme for the **E1 subunit** of the pyruvate dehydrogenase complex. - It participates in the **decarboxylation** of pyruvate, releasing CO2. *Riboflavin* - **FAD** (flavin adenine dinucleotide), derived from riboflavin (vitamin B2), is a coenzyme for the **E3 subunit** (dihydrolipoyl dehydrogenase) of the pyruvate dehydrogenase complex. - It is involved in the **regeneration of oxidized lipoamide**. *Niacin* - **NAD+** (nicotinamide adenine dinucleotide), derived from niacin (vitamin B3), is a coenzyme for the **E3 subunit** of the pyruvate dehydrogenase complex. - It acts as an **electron acceptor** during the reoxidation of FADH2.

Q: Which of the following is not a free radical?

Hydrogen peroxide (H2O2). ***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.

Q: Most abundant source of fuel in starvation -

Adipose tissue. ***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.

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

Ketone bodies. ***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.

Q: Which of the following enzyme activity decreases in fasting?

Phosphofructokinase I. ***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.

Q: What is a physiological uncoupler?

Thermogenin. ***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

Q: During starvation, muscle uses?

Fatty acids. ***Fatty acids*** - During **early and moderate starvation**, muscle tissue primarily uses **fatty acids** released from adipose tissue as its main energy source. - This preserves **glucose** for essential organs like the brain and red blood cells, which have an obligate need for it. *Ketone bodies* - While muscle can utilize **ketone bodies** during prolonged starvation, they are predominantly a fuel source for the **brain** once fatty acid stores are depleted. - The brain's adaptation to using ketones helps reduce the reliance on gluconeogenesis and preserves muscle protein. *Glucose* - Muscle primarily uses **glucose** as its main energy source in the fed state or during high-intensity exercise. - However, during starvation, muscle significantly reduces its glucose uptake to conserve it for other vital organs. *Proteins* - Muscle protein can be broken down into **amino acids** for gluconeogenesis in the liver to maintain blood glucose levels during prolonged starvation. - However, this is a **catabolic process** and not the primary preferred fuel source for muscle activity itself, as it leads to muscle wasting.

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

ATP. ***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.

Q: What is the theoretical maximum number of ATPs generated from the Krebs cycle per glucose molecule?

20 ATPs. ***20 ATPs*** - Each **glucose molecule** yields two molecules of **acetyl-CoA** which enter the Krebs cycle. - Each turn of the **Krebs cycle** generates **3 NADH, 1 FADH2, and 1 GTP** (equivalent to 1 ATP). - Using modern **P/O ratios**: 3 NADH × 2.5 = 7.5 ATP, 1 FADH2 × 1.5 = 1.5 ATP, 1 GTP = 1 ATP, totaling **10 ATP per acetyl-CoA**. - Since two acetyl-CoA molecules are produced per glucose, the total is **2 × 10 = 20 ATPs** from the Krebs cycle alone. *24 ATPs* - This value is based on **older P/O ratios** (3 ATP per NADH, 2 ATP per FADH2), which have been revised in modern biochemistry. - While historically taught, current understanding of the **electron transport chain** efficiency yields lower ATP values per NADH and FADH2. *12 ATPs* - This represents the ATP yield from **one turn** of the **Krebs cycle** (or one acetyl-CoA molecule) using older P/O ratios, not a complete glucose molecule. - A single glucose molecule produces **two acetyl-CoA** molecules, each initiating a separate turn of the cycle. *30 ATPs* - This value typically reflects the theoretical maximum **total ATP** generated from **complete oxidation** of **one glucose molecule**, including contributions from **glycolysis** and the **electron transport chain**. - The Krebs cycle alone contributes only a portion of this total; 30 ATPs includes ATP from all stages of glucose metabolism.

Q: Chemical process involved in conversion of progesterone to glucocorticoids is

Hydroxylation. ***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.

Question 1

Pyruvate dehydrogenase requires all cofactors except:

Accepted Answer

Pyridoxin (Vitamin B6)

Answer

Thiamin

Answer

Riboflavin

Answer

Niacin

Question 2

Which of the following is not a free radical?

Accepted Answer

Hydrogen peroxide (H2O2)

Answer

Superoxide anion

Answer

Nitric oxide (NO·)

Answer

Hydroxyl radical (.OH)

Question 3

Most abundant source of fuel in starvation -

Accepted Answer

Adipose tissue

Answer

Liver glycogen

Answer

Muscle glycogen

Answer

Blood glucose

Question 4

Energy source used by brain in later days of starvation is

Accepted Answer

Ketone bodies

Answer

Glucose

Answer

Glycogen

Answer

Fatty acids

Question 5

Which of the following enzyme activity decreases in fasting?

Accepted Answer

Phosphofructokinase I

Answer

Hormone sensitive lipase

Answer

Glycogen phosphorylase

Answer

Acetyl CoA Carboxylase

Question 6

What is a physiological uncoupler?

Accepted Answer

Thermogenin

Answer

Thyroxine

Answer

Free fatty acids

Answer

All of the options

Question 7

During starvation, muscle uses?

Accepted Answer

Fatty acids

Answer

Ketone bodies

Answer

Glucose

Answer

Proteins

Question 8

What is the immediate source of energy for cellular processes?

Accepted Answer

ATP

Answer

Cori's cycle

Answer

HMP

Answer

TCA cycle

Question 9

What is the theoretical maximum number of ATPs generated from the Krebs cycle per glucose molecule?

Accepted Answer

20 ATPs

Answer

24 ATPs

Answer

12 ATPs

Answer

30 ATPs

Question 10

Chemical process involved in conversion of progesterone to glucocorticoids is

Accepted Answer

Hydroxylation

Answer

Methylation

Answer

Carboxylation

Answer

None of the options

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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