Energy Production and Metabolism Practice Questions

Q: Which of the following enzymes does not contribute to the energy production in the citric acid cycle?

Citrate synthase. ***Citrate synthase*** - This enzyme catalyzes the **condensation of acetyl-CoA and oxaloacetate** to form citrate. - While it's the **first enzyme** in the citric acid cycle, this specific reaction does not involve the direct production of ATP, NADH, or FADH2. *Isocitrate dehydrogenase* - This enzyme catalyzes the **oxidative decarboxylation** of isocitrate to $\boldsymbol{\alpha}$-ketoglutarate, producing one molecule of **NADH** and releasing $\text{CO}_2$. - The NADH produced will later contribute to ATP synthesis via the **electron transport chain**. *Succinyl Thiokinase* - This enzyme (also known as succinate-CoA ligase) catalyzes the conversion of **succinyl-CoA to succinate**, resulting in the production of **GTP (or ATP)**. - This is an example of **substrate-level phosphorylation** within the citric acid cycle. *Succinate Dehydrogenase* - This enzyme catalyzes the **oxidation of succinate to fumarate**, producing one molecule of **FADH2**. - **FADH2** will later contribute to ATP synthesis in the **electron transport chain**.

Q: Which of the following is not used in the citric acid cycle?

NADP. ***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.

Q: Which of the following organelles is primarily responsible for cellular respiration?

Mitochondria. ***Mitochondria*** - This organelle is often referred to as the "**powerhouse of the cell**" because it is the primary site of **ATP synthesis** through aerobic respiration. - Cellular respiration involves a series of metabolic reactions that convert nutrients into **adenosine triphosphate (ATP)**, providing energy for cellular functions. *Endoplasmic Reticulum* - The **endoplasmic reticulum** is primarily involved in **protein synthesis** (rough ER) and **lipid synthesis** and detoxification (smooth ER). - It does not play a direct role in the production of bulk cellular energy via cellular respiration. *Lysosome* - **Lysosomes** are responsible for **waste breakdown**, recycling cellular debris, and degrading foreign substances through their digestive enzymes. - They are not involved in the processes of cellular respiration or energy production. *Nucleus* - The **nucleus** contains the cell's **genetic material** (DNA) and controls cell growth, metabolism, and reproduction by regulating gene expression. - While it controls cellular processes, it does not directly perform cellular respiration.

Q: What is the theoretical yield of ATP generated in one TCA cycle?

10. ***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

Q: Which of the following primarily occurs in the mitochondria?

ETC. ***Correct Option: ETC*** - The **electron transport chain (ETC)** is a series of protein complexes located **exclusively in the inner mitochondrial membrane**. - It occurs **solely in mitochondria** with no cytosolic component, making it the process that most "primarily" occurs in this organelle. - Its primary role is to generate a **proton gradient** through electron transfer, ultimately producing ATP via **oxidative phosphorylation**. - This is the definitive mitochondrial process among the options. *Ketogenesis* - **Ketogenesis** does occur entirely in the **mitochondrial matrix** of liver cells during fasting or low carbohydrate intake. - While mitochondrial, it is tissue-specific (primarily liver) and metabolically conditional (occurs during fasting states). - It involves synthesis of **ketone bodies** (acetoacetate, β-hydroxybutyrate) from acetyl-CoA. *Urea cycle* - The **urea cycle** is compartmentalized between the **mitochondrial matrix** and **cytosol** of liver cells. - First two steps (carbamoyl phosphate synthetase I and ornithine transcarbamylase) occur in mitochondria. - Remaining steps occur in cytosol, so it is NOT primarily mitochondrial. - Functions to detoxify **ammonia** by converting it to urea. *Steroid synthesis* - **Steroid synthesis** primarily occurs in the **smooth endoplasmic reticulum**. - Only specific steps (e.g., cholesterol side-chain cleavage by CYP11A1) occur in mitochondria. - Most of the steroidogenic pathway is extra-mitochondrial.

Q: What is the primary role of molecular oxygen in the electron transport chain (ETC)?

Acting as the final electron acceptor. ***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.

Q: NADH CoQ reductase is inhibited by ?

Rotenone. ***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.

Q: Among the following enzymes, which one produces NADH in the citric acid cycle?

Isocitrate dehydrogenase. ***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.

Question 1

Which of the following enzymes does not contribute to the energy production in the citric acid cycle?

Accepted Answer

Citrate synthase

Answer

Isocitrate dehydrogenase

Answer

Succinyl Thiokinase

Answer

Succinate Dehydrogenase

Question 2

Which of the following is not used in the citric acid cycle?

Accepted Answer

NADP

Answer

NAD

Answer

FAD

Answer

GDP

Question 3

Which of the following organelles is primarily responsible for cellular respiration?

Accepted Answer

Mitochondria

Answer

Endoplasmic Reticulum

Answer

Lysosome

Answer

Nucleus

Question 4

Sequence of complexes in the electron transport chain is -

Accepted Answer

NADH dehydrogenase → Q → Cytochrome bc1 → Cytochrome aa3 → O2

Answer

NADH dehydrogenase → Cytochrome aa3 → Q → Cytochrome bc1 → O2

Answer

NADH dehydrogenase → Q → Cytochrome aa3 → Cytochrome bc1 → O2

Answer

NADH dehydrogenase → Cytochrome bc1 → Q → Cytochrome aa3 → O2

Question 5

What is the theoretical yield of ATP generated in one TCA cycle?

Accepted Answer

10

Answer

2

Answer

8

Answer

11

Question 6

Which of the following statements best describes the role of inorganic phosphate in the Electron Transport Chain (ETC)?

Accepted Answer

Inorganic phosphate is essential for ATP synthesis in the ETC.

Answer

Generates ATP

Answer

Occurs in mitochondria

Answer

No role of inorganic phosphate

Question 7

Which of the following primarily occurs in the mitochondria?

Accepted Answer

ETC

Answer

Ketogenesis

Answer

Urea cycle

Answer

Steroid synthesis

Question 8

What is the primary role of molecular oxygen in the electron transport chain (ETC)?

Accepted Answer

Acting as the final electron acceptor

Answer

Transfer of electrons to CoQ

Answer

Transfer of electrons from cytosol to mitochondria

Answer

Facilitating ATP synthesis

Question 9

NADH CoQ reductase is inhibited by ?

Accepted Answer

Rotenone

Answer

Antimycin (inhibits cytochrome bc1 complex)

Answer

Atractyloside (inhibits ATP/ADP translocase)

Answer

Carbon monoxide (inhibits cytochrome c oxidase)

Question 10

Among the following enzymes, which one produces NADH in the citric acid cycle?

Accepted Answer

Isocitrate dehydrogenase

Answer

Succinate thiokinase

Answer

Succinate dehydrogenase

Answer

Fumarase

Energy Production and Metabolism — MCQs

Energy Production and Metabolism — MCQs

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