Energy Production and Metabolism Practice Questions

Q: Which molecule acts as the primary electron carrier in the Krebs cycle?

NADH. ***NADH*** - **NADH** (reduced nicotinamide adenine dinucleotide) is the **primary electron carrier** produced during the Krebs cycle. - **Three molecules of NADH** are generated per cycle (at isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase steps). - These high-energy electrons are transferred to the **electron transport chain** to generate approximately **7.5 ATP per NADH**. *NAD+* - **NAD+** (oxidized nicotinamide adenine dinucleotide) is the coenzyme that *accepts* electrons during the Krebs cycle. - It is the **oxidized form** that gets reduced to NADH, not the carrier itself. *FADH₂* - **FADH₂** (reduced flavin adenine dinucleotide) is also produced in the Krebs cycle at the **succinate dehydrogenase** step. - However, only **one molecule of FADH₂** is produced per cycle compared to three NADH molecules. - FADH₂ generates approximately **5 ATP** in the electron transport chain, making NADH the quantitatively dominant electron carrier. *NADPH* - **NADPH** (reduced nicotinamide adenine dinucleotide phosphate) is NOT involved in the Krebs cycle. - It is primarily used in **anabolic pathways** such as fatty acid synthesis, cholesterol synthesis, and the **pentose phosphate pathway**. - NADPH serves as a **reducing agent** in biosynthetic reactions and protects against oxidative stress.

Q: Which coenzyme serves as the primary electron acceptor in multiple dehydrogenase reactions of the Kreb's cycle?

NAD. ***NAD⁺ (NAD)*** - **NAD⁺ (Nicotinamide Adenine Dinucleotide)** serves as the primary **electron acceptor** in the Kreb's cycle, being reduced to **NADH** in three key dehydrogenase reactions. - These reactions occur at: **isocitrate dehydrogenase**, **α-ketoglutarate dehydrogenase**, and **malate dehydrogenase** steps. - The **NAD⁺/NADH** coenzyme system is essential for extracting energy from acetyl-CoA, with NADH subsequently donating electrons to the **electron transport chain** for ATP synthesis. *NADP* - **NADP⁺ (Nicotinamide Adenine Dinucleotide Phosphate)** is primarily involved in **anabolic reactions**, such as **fatty acid synthesis** and the **pentose phosphate pathway**. - While structurally similar to NAD⁺ (differing only by a phosphate group), it functions in different metabolic pathways and is not utilized in the **Kreb's cycle**. *NADPH* - **NADPH** is the reduced form of **NADP⁺** and functions as a reducing agent in various **biosynthetic pathways**, including synthesis of **fatty acids**, **cholesterol**, and **nucleotides**. - It also plays a crucial role in **antioxidant defense** (glutathione reduction) and the **respiratory burst** in phagocytes. - NADPH is not involved in the **Kreb's cycle**, which uses the NAD⁺/NADH system instead. *NADH* - **NADH** is the **reduced form** of NAD⁺ produced during the Kreb's cycle reactions. - While NADH and NAD⁺ are two forms of the same coenzyme, the question asks for the **electron acceptor** form, which is **NAD⁺** (oxidized form). - **NADH** carries the extracted electrons to **Complex I** of the electron transport chain, where it is reoxidized back to NAD⁺, generating approximately **2.5 ATP** per NADH molecule.

Q: 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?

Cyanide. ***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.

Q: Which of the following metabolic processes is associated with increased Basal Metabolic Rate (BMR)?

Increased glycolysis. ***Increased glycolysis*** - Among the given options, **increased glycolysis** is the best answer as it represents **active catabolic metabolism** that generates ATP to meet energy demands. - While glycolysis itself doesn't directly increase BMR, **increased glycolytic activity occurs in metabolically active tissues** and reflects higher cellular energy turnover. - Tissues with higher metabolic rates (muscle, brain, liver) have increased glycolysis to meet their ATP demands, making this the most appropriate choice among the options provided. *Increased body fat store* - **Adipose tissue** is metabolically **less active** than lean tissue (muscle, organs). - Increased body fat typically results in a **lower BMR per unit body weight** because fat tissue has minimal metabolic activity compared to muscle. - Greater fat stores are associated with lower, not higher, metabolic rate. *Increased lipogenesis* - **Lipogenesis** (synthesis of fatty acids and triglycerides) is an **anabolic storage process**. - This process occurs during energy surplus and represents a state of **reduced energy expenditure** relative to energy intake. - Storage processes like lipogenesis are associated with **lower overall metabolic activity**, not increased BMR. *Increased glycogenesis* - **Glycogenesis** (synthesis of glycogen from glucose) is an **anabolic storage process** occurring primarily in liver and muscle. - This represents **energy storage**, not energy expenditure, and occurs during fed states when energy demands are being met. - Storage processes do not increase BMR; they indicate surplus energy being stored for later use.

Q: What is the total number of dehydrogenases involved in the Krebs cycle?

4. ***4*** - There are four major **dehydrogenase enzymes** that catalyze oxidation-reduction reactions in the Krebs cycle. - These enzymes are **isocitrate dehydrogenase**, **α-ketoglutarate dehydrogenase complex**, **succinate dehydrogenase**, and **malate dehydrogenase**. *3* - This count is incorrect as it omits at least one key dehydrogenase involved in the Krebs cycle's oxidative steps. - A count of three would exclude one of the enzymes responsible for generating **NADH** or **FADH2**. *2* - This number is significantly underestimated, as the Krebs cycle involves multiple steps where a substrate is oxidized and a coenzyme is reduced. - Such a low number would fail to account for the multiple points of **NADH** and **FADH2** generation. *5* - This count is incorrect, as there are specifically four well-established dehydrogenase enzymes within the Krebs cycle responsible for the production of **NADH** or **FADH2**. - No additional dehydrogenase beyond the four listed plays a primary role in the canonical Krebs cycle.

Q: In the malate shuttle, how many ATPs are produced from one NADH?

2.5 ATP. ***2.5 ATP*** - In the **malate-aspartate shuttle**, mitochondrial **NADH** is regenerated from cytosolic NADH, and then enters the electron transport chain at **Complex I**. - **Complex I** entry means that NADH contributes to the pumping of enough protons to generate approximately **2.5 ATP** through oxidative phosphorylation. *1 ATP* - **1 ATP** is not the direct equivalent produced from the reoxidation of one NADH via the malate shuttle into the electron transport chain. - This value is typically associated with the direct hydrolysis of **ATP** or the energy equivalent of **GTP** produced in the citric acid cycle. *3 ATP* - Historically, **3 ATP** was the accepted stoichiometry for one NADH, but more accurate measurements of proton pumping and ATP synthase activity have revised this. - The value of 3 ATP per NADH does not reflect the most current understanding of mitochondrial bioenergetics. *2 ATP* - **2 ATP** is the approximate yield for **FADH2** entering the electron transport chain at **Complex II**, bypassing Complex I, and thus pumping fewer protons. - This value is not applicable to NADH transferred via the malate-aspartate shuttle, as NADH enters at Complex I.

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: Which of the following is the most reactive free radical?

Hydroxyl radical. ***Hydroxyl radical*** - The **hydroxyl radical (•OH)** is the most reactive free radical in biological systems due to its extremely high oxidation potential and short half-life. - It readily reacts with virtually all cellular macromolecules, including **DNA, proteins, and lipids**, causing widespread damage. *Peroxide radical* - The **peroxide radical (ROO•)**, or more specifically the peroxyl radical, is less reactive than the hydroxyl radical, but still significant in lipid peroxidation. - It plays a role in propagating chain reactions of **lipid damage** in cell membranes. *Alkyl radical* - **Alkyl radicals (R•)** are generally formed as intermediates during the abstraction of hydrogen atoms from saturated compounds. - While reactive, they are typically less reactive and less frequently encountered in biological systems compared to oxygen-centered radicals like the hydroxyl radical. *Superoxide radical* - The **superoxide radical (O₂•−)** is a relatively less reactive free radical compared to the hydroxyl radical, but it is the precursor to many other reactive oxygen species (ROS). - It is primarily involved in **initiation of oxidative stress** and can lead to the formation of more damaging species through reactions like the Haber-Weiss reaction.

Q: The mechanism of action of uncouplers of oxidative phosphorylation involves:

Disruption of proton gradient across the inner membrane. ***Disruption of proton gradient across the inner membrane*** - Uncouplers such as **2,4-dinitrophenol** increase the permeability of the **inner mitochondrial membrane** to protons. - This dissipates the **proton motive force** that is normally used by ATP synthase to produce ATP, leading to the uncoupling of electron transport from ATP synthesis. *Inhibition of ATP synthase* - Inhibitors of ATP synthase directly block the enzyme's activity, preventing the synthesis of ATP while the **proton gradient** remains intact. - This mechanism is distinct from uncouplers, which allow electron transport to continue while dissipating the proton gradient. *Stimulation of ATP synthase* - Uncouplers do not stimulate ATP synthase; rather, their action prevents ATP synthase from effectively utilizing the **proton gradient** for ATP production. - Stimulation of ATP synthase would lead to increased ATP synthesis, which is contrary to the effect of uncouplers. *Blocking electron transport chain complexes* - Inhibitors of the **electron transport chain** (e.g., cyanide, rotenone) directly prevent the flow of electrons, thereby preventing the pumping of protons and the formation of a **proton gradient**. - Uncouplers, in contrast, allow electron transport to proceed but dissipate the proton gradient after it has been established.

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.

Question 1

Which molecule acts as the primary electron carrier in the Krebs cycle?

Accepted Answer

NADH

Answer

NAD

Answer

FADH₂

Answer

NADPH

Question 2

Which coenzyme serves as the primary electron acceptor in multiple dehydrogenase reactions of the Kreb's cycle?

Accepted Answer

NAD

Answer

NADP

Answer

NADPH

Answer

NADH

Question 3

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?

Accepted Answer

Cyanide

Answer

Carbon dioxide

Answer

Oligomycin

Answer

Ouabain

Question 4

Which of the following metabolic processes is associated with increased Basal Metabolic Rate (BMR)?

Accepted Answer

Increased glycolysis

Answer

Increased body fat store

Answer

Increased lipogenesis

Answer

Increased glycogenesis

Question 5

What is the total number of dehydrogenases involved in the Krebs cycle?

Accepted Answer

4

Answer

3

Answer

2

Answer

5

Question 6

In the malate shuttle, how many ATPs are produced from one NADH?

Accepted Answer

2.5 ATP

Answer

1 ATP

Answer

3 ATP

Answer

2 ATP

Question 7

Most abundant source of fuel in starvation -

Accepted Answer

Adipose tissue

Answer

Liver glycogen

Answer

Muscle glycogen

Answer

Blood glucose

Question 8

Which of the following is the most reactive free radical?

Accepted Answer

Hydroxyl radical

Answer

Alkyl radical

Answer

Superoxide radical

Answer

Peroxide radical

Question 9

The mechanism of action of uncouplers of oxidative phosphorylation involves:

Accepted Answer

Disruption of proton gradient across the inner membrane

Answer

Inhibition of ATP synthase

Answer

Stimulation of ATP synthase

Answer

Blocking electron transport chain complexes

Question 10

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)

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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